Correct Answer: Anterior interosseous nerve; inability to flex the interphalangeal joint of the thumb and distal interphalangeal joint of the index finger

In extension-type supracondylar humerus fractures, the direction of displacement of the distal fragment dictates the structures at risk. Posterolateral displacement of the distal fragment causes the proximal fragment to displace anteromedially, putting the median nerve—specifically its anterior interosseous nerve (AIN) branch—at the greatest risk. The AIN is a purely motor nerve that innervates the flexor pollicis longus (FPL), the flexor digitorum profundus (FDP) to the index and middle fingers, and the pronator quadratus. Injury results in the inability to make an 'A-OK' sign. Posteromedial displacement of the distal fragment puts the radial nerve at risk. Flexion-type fractures put the ulnar nerve at risk.

Correct Answer: Underlying endocrine disorder, such as hypothyroidism

The risk of developing a contralateral SCFE is approximately 20-40% overall. However, prophylactic pinning of the contralateral hip is generally reserved for patients at exceptionally high risk. Strong indications for prophylactic pinning include underlying endocrine disorders (e.g., hypothyroidism, growth hormone deficiency), renal osteodystrophy, previous radiation therapy to the pelvis, and very young age at presentation (typically less than 10 years old for boys). While obesity (high BMI) is a risk factor for SCFE, it alone is not an absolute indication for prophylactic pinning of the contralateral side. The severity of the initial slip or an acute-on-chronic presentation does not dictate contralateral prophylactic pinning.

Correct Answer: Femoral nerve palsy; discontinue the harness temporarily

The clinical presentation of decreased active knee extension in an infant wearing a Pavlik harness is highly suspicious for a femoral nerve palsy. This complication typically occurs due to hyperflexion of the hip, which compresses the femoral nerve against the inguinal ligament. The appropriate management is to temporarily discontinue the harness or significantly loosen the anterior straps to reduce hip flexion until nerve function returns, which usually happens within a few days to weeks. Sciatic nerve palsy is less common and would present with decreased ankle/toe movement. Avascular necrosis is a severe complication related to excessive abduction, not hyperflexion, and does not present acutely with isolated loss of knee extension.

Correct Answer: Age of the patient at the time of clinical onset

In Legg-Calvé-Perthes disease, the age of the patient at the onset of the disease is the single most important prognostic factor. Children who develop the disease at a younger age (typically under 6 years) have a much better prognosis because they have more time for the femoral head to remodel before skeletal maturity. Patients who present at an older age (especially over 8 years) have a higher risk of developing a permanently deformed femoral head (coxa magna, coxa plana) and subsequent early-onset osteoarthritis. While lateral pillar involvement (Herring classification) is also a critical prognostic radiographic factor, age is the most important demographic/clinical factor.

Correct Answer: Cavus, Adductus, Varus, Equinus

The Ponseti method relies on a specific sequence of correction, easily remembered by the acronym CAVE: Cavus, Adductus, Varus, Equinus. The first step is to correct the cavus deformity by elevating the first ray (supinating the forefoot) to align it with the hindfoot. Once the cavus is corrected, the midfoot is abducted around the head of the talus to correct the adductus and varus deformities simultaneously. The equinus deformity is corrected last, often requiring a percutaneous Achilles tenotomy, as attempting to correct equinus too early can lead to a midfoot break (rocker-bottom deformity).

Correct Answer: Anterior inferior tibiofibular ligament (AITFL)

The patient has a juvenile Tillaux fracture, which is a Salter-Harris III fracture of the anterolateral distal tibial epiphysis. This occurs in adolescents because the distal tibial physis closes in a specific pattern: central, then anteromedial, then posteromedial, and finally lateral. When an external rotation force is applied to the ankle during the period when the medial physis is closed but the lateral physis remains open, the anterior inferior tibiofibular ligament (AITFL) avulses the anterolateral epiphysis. The ATFL and CFL are lateral ankle ligaments involved in sprains, while the PITFL is involved in Volkmann fractures (posterior malleolus).

Correct Answer: Varus derotational osteotomy (VDRO) of the proximal femur and pelvic osteotomy

Hip surveillance is critical in children with cerebral palsy, especially those with higher GMFCS levels (IV and V), as they are at high risk for progressive hip displacement. The Reimers migration percentage (MP) guides treatment. An MP < 30% is typically observed. An MP between 30% and 40% in a young child may be treated with soft tissue releases (adductor/psoas). However, an MP > 40-50%, especially with acetabular dysplasia (acetabular index > 25-30 degrees), indicates significant bony deformity that will not respond to soft tissue release alone. Bony reconstruction with a proximal femoral varus derotational osteotomy (VDRO) and a pelvic osteotomy (e.g., Dega or San Diego) is the standard of care to achieve a stable, concentric reduction.

Correct Answer: Infantile Blount disease; Knee-ankle-foot orthosis (KAFO) bracing

The patient presents with infantile Blount disease (tibia vara), characterized by a localized disorder of the posteromedial proximal tibial physis. Risk factors include early walking, obesity, and African American descent. The metaphyseal-diaphyseal angle (Drennan angle) is crucial for distinguishing Blount disease from physiologic bowing. An angle > 16 degrees is highly predictive of Blount disease, whereas < 10 degrees suggests physiologic bowing. For children under the age of 3 to 4 years with early-stage infantile Blount disease (Langenskiöld stages I-II), the initial treatment is full-time bracing with a KAFO. If bracing fails or the child presents at an older age/higher stage, a proximal tibial osteotomy is indicated.

Correct Answer: Type I collagen

The clinical presentation of multiple fractures, blue sclerae, and dentinogenesis imperfecta is classic for Osteogenesis Imperfecta (OI). OI is primarily caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes, which encode the alpha-1 and alpha-2 chains of Type I collagen. Type I collagen is the major structural protein in bone, skin, sclera, and dentin. FGFR3 mutations cause achondroplasia. Type II collagen mutations cause various skeletal dysplasias, including spondyloepiphyseal dysplasia (SED). COMP mutations are associated with pseudoachondroplasia and multiple epiphyseal dysplasia (MED). CBFA1 (RUNX2) mutations cause cleidocranial dysplasia.

Correct Answer: 93%

The Kocher criteria are used to differentiate septic arthritis from transient synovitis in the pediatric hip. The four classic criteria are: 1) Non-weight-bearing on the affected side, 2) Temperature > 38.5°C (101.3°F), 3) ESR > 40 mm/hr, and 4) WBC count > 12,000/mm³. This patient meets three of the four criteria (non-weight-bearing, fever > 38.5°C, and ESR > 40 mm/hr). His WBC count is normal (< 12,000). According to Kocher's original study, the probability of septic arthritis is approximately 3% for 1 criterion, 40% for 2 criteria, 93% for 3 criteria, and 99% for all 4 criteria. Therefore, with 3 criteria met, the probability is 93%.

Correct Answer: B

The status of the triradiate cartilage is a strong radiographic predictor of future contralateral slip in patients presenting with a unilateral SCFE. An open triradiate cartilage indicates significant remaining skeletal growth and correlates with a high risk (up to 80% in some studies) of developing a contralateral SCFE. The modified Oxford bone age score is also used to assess this risk, heavily weighting the triradiate cartilage and iliac apophysis. Prophylactic pinning of the contralateral hip is often considered in patients with open triradiate cartilage, endocrine disorders, or an inability to follow up reliably. The posterior sloping angle and Klein's line are used to diagnose or quantify the severity of the current slip, not predict contralateral occurrence. The alpha angle is used to assess for femoroacetabular impingement (cam morphology), which can be a sequela of SCFE. The metaphyseal blanch sign of Steel is a radiographic sign of SCFE on an AP pelvis radiograph, representing the overlapping of the posteriorly displaced epiphysis and the metaphysis.

Correct Answer: B

The clinical presentation of absent active knee extension with preserved distal motor function in an infant wearing a Pavlik harness is classic for a femoral nerve palsy. This complication is typically caused by excessive hyperflexion of the hip in the harness, which compresses the femoral nerve against the inguinal ligament or pelvic brim. The appropriate management is to temporarily remove the harness or adjust it to decrease the amount of flexion, allowing the nerve palsy to resolve, which it almost always does. Excessive abduction in a Pavlik harness is associated with avascular necrosis (AVN) of the femoral head, not an isolated obturator nerve palsy. Sciatic nerve palsy is not a typical complication of Pavlik harness treatment and would present with distal deficits (ankle/toe movement). Transient synovitis is a cause of hip pain in older children (typically 3-8 years) and does not cause isolated motor nerve palsies.

Correct Answer: B

In the Ponseti method for clubfoot correction, the foot is abducted around the head of the talus, which acts as the fulcrum. The sequence of correction is C-A-V-E: Cavus, Adductus, Varus, Equinus. The cavus is corrected first by supinating the forefoot and elevating the first ray to align it with the hindfoot. Subsequently, the adduction and varus are corrected simultaneously by abducting the foot while applying counter-pressure over the lateral aspect of the head of the talus. A historical error in clubfoot casting (often referred to as Kite's error) was applying pressure over the calcaneocuboid joint, which blocks the abduction of the calcaneus and prevents correction of the heel varus, leading to a midfoot breach (rocker-bottom deformity).

Correct Answer: A

In extension-type supracondylar humerus fractures, the direction of displacement dictates the structures at risk. Posterolateral displacement of the distal fragment causes the sharp proximal fragment to displace anteromedially, putting the median nerve—specifically its anterior interosseous nerve (AIN) branch—and the brachial artery at greatest risk. AIN palsy is the most common nerve injury in extension-type supracondylar fractures overall. It is a purely motor nerve, and injury results in the inability to flex the interphalangeal (IP) joint of the thumb (flexor pollicis longus) and the distal interphalangeal (DIP) joint of the index finger (flexor digitorum profundus), leading to an abnormal 'A-OK' sign. Posteromedial displacement puts the radial nerve at risk. Flexion-type fractures put the ulnar nerve at risk.

Correct Answer: B

The most significant prognostic factor for the long-term outcome in Legg-Calvé-Perthes disease is the age of the patient at the onset of the disease. Children who develop the disease before the age of 6 generally have a good prognosis because they have more time for the femoral head to remodel before skeletal maturity. Children over the age of 8 at onset have a significantly worse prognosis and a higher likelihood of developing early osteoarthritis, as there is less remaining growth for remodeling. The lateral pillar classification (Herring) is the most important radiographic prognostic factor, but age remains the most critical overall clinical factor. While restricted motion and metaphyseal cysts are part of the clinical picture, they do not supersede age in prognostic value.

Correct Answer: B

Hip displacement is a common and severe complication in children with cerebral palsy, particularly those with higher GMFCS levels (IV and V). The Reimers migration percentage measures the amount of the femoral head that is uncovered by the acetabulum. A migration percentage >50% in a 7-year-old indicates a subluxated/dislocated hip that requires bony reconstruction. Soft tissue releases alone (adductor tenotomies) are indicated for hips with a migration percentage between 30% and 40-50% in younger children (typically <4-5 years old) but have an unacceptably high failure rate once significant bony dysplasia and subluxation (>50%) have occurred. Therefore, a varus derotational osteotomy (VDRO) of the proximal femur combined with a pelvic osteotomy (e.g., Dega or San Diego) is the standard of care to restore hip joint congruity. Proximal femoral resection is a salvage procedure for painful, chronically dislocated hips in older, non-ambulatory patients. THA is rarely indicated in this age group.

Correct Answer: B

The patient's clinical presentation of recurrent fragility fractures, blue sclerae, and dentinogenesis imperfecta is classic for Osteogenesis Imperfecta (OI). OI is primarily caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes, which encode the alpha-1 and alpha-2 chains of Type I collagen. Type I collagen is the major structural protein in bone, skin, sclera, and dentin. Defects in Type II collagen are associated with chondrodysplasias (e.g., achondrogenesis). Mutations in FGFR3 cause achondroplasia. Mutations in COMP cause pseudoachondroplasia and multiple epiphyseal dysplasia (MED). Mutations in CBFA1 (RUNX2) cause cleidocranial dysplasia.

Correct Answer: C

The injury described is a juvenile Tillaux fracture, which is a Salter-Harris III fracture of the anterolateral distal tibial epiphysis. This fracture occurs in adolescents (typically 12-15 years old) due to the asymmetric closure pattern of the distal tibial physis. The physis closes first centrally, then anteromedially, then posteromedially, and finally laterally. When an external rotation force is applied to the foot during this specific window of skeletal maturity, the anterior inferior tibiofibular ligament (AITFL) avulses the anterolateral epiphysis, which is the only portion of the physis that remains open and vulnerable. The ATFL and CFL are lateral ankle ligaments involved in sprains. The PITFL is involved in Volkmann fractures (posterior malleolus). The deltoid is the medial ligament complex.

Correct Answer: C

The patient has infantile Blount disease (tibia vara), characterized by pathologic varus deformity of the proximal tibia. Risk factors include early walking, obesity, and African American descent. Radiographically, a metaphyseal-diaphyseal angle (MDA) of greater than 16 degrees is highly predictive of Blount disease rather than physiologic bowing. For a child under the age of 4 with infantile Blount disease (Langenskiöld stages I or II), the initial treatment of choice is a trial of bracing with Knee-Ankle-Foot Orthoses (KAFOs) worn during weight-bearing activities. If the child is over 4 years old, or if bracing fails to correct the deformity after 1 year, surgical intervention (proximal tibial osteotomy) is indicated. Reassurance is appropriate for physiologic bowing (MDA < 11 degrees). Vitamin D is for rickets. Guided growth is generally reserved for older children or specific cases, but KAFO is the standard initial step for a 3-year-old.

Correct Answer: B

The treatment of pediatric femoral shaft fractures is highly dependent on the age and weight of the child. For children aged 6 months to 4-5 years (preschool age), early spica casting is the standard of care for isolated, closed femur fractures with acceptable shortening (<2 cm). A Pavlik harness is indicated for infants less than 6 months of age. Flexible intramedullary nailing is the treatment of choice for school-aged children (typically 5 to 11 years old) weighing less than 50 kg (110 lbs). Rigid intramedullary nailing is reserved for older adolescents (typically >11 years) with closed physes or using a trochanteric entry to avoid the piriformis fossa and the risk of AVN. Plate fixation is an option but is generally reserved for specific indications such as polytrauma, open fractures, or when flexible nails are not suitable.

Correct Answer: Bilateral varus derotational osteotomies (VDRO) and pelvic osteotomies

In children with cerebral palsy, hip displacement is common, particularly in those with higher GMFCS levels (Level IV and V). The Reimers migration percentage (MP) is used to quantify subluxation. An MP > 40% with associated bony changes (acetabular dysplasia) or severe contractures typically requires bony reconstruction. Soft tissue releases (adductor/psoas tenotomies) are generally reserved for younger children (typically < 4-5 years old) with an MP between 30% and 40% and no significant bony deformity. Because this patient has an MP of 45% and early acetabular changes, soft tissue release alone will likely fail to prevent further subluxation. Bilateral VDRO combined with pelvic osteotomies (such as a Dega or San Diego osteotomy) is the gold standard for reconstructing these hips. Proximal femoral resection is a salvage procedure reserved for painful, chronically dislocated hips in non-ambulatory patients who have failed other treatments or present late. Selective dorsal rhizotomy is a neurosurgical procedure to reduce spasticity, primarily indicated for ambulatory diplegic patients, and does not directly treat established hip dysplasia.

Correct Answer: Type I collagen

This patient's clinical presentation is classic for Osteogenesis Imperfecta (OI), a genetic disorder characterized by bone fragility, blue sclerae, dentinogenesis imperfecta, and hearing loss. OI is predominantly caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes, which encode the alpha-1 and alpha-2 chains of Type I collagen. Type I collagen is the major structural protein in bone, skin, dentin, and sclerae. Type II collagen mutations are associated with skeletal dysplasias such as achondrogenesis and spondyloepiphyseal dysplasia. FGFR3 mutations are the cause of achondroplasia, the most common form of short-limb dwarfism. Mutations in COMP cause pseudoachondroplasia and multiple epiphyseal dysplasia. CBFA1 (also known as RUNX2) mutations are responsible for cleidocranial dysplasia, characterized by absent or hypoplastic clavicles and delayed closure of cranial sutures.

Correct Answer: Short leg cast immobilization for 6 weeks

The patient's presentation of a rigid flatfoot, restricted subtalar motion, and a "C sign" on lateral radiographs is highly indicative of a talocalcaneal coalition. The "C sign" is formed by the medial outline of the talar dome and the posteroinferior outline of the sustentaculum tali, indicating a middle facet coalition. The initial management for a symptomatic tarsal coalition is conservative, consisting of immobilization in a short leg cast or controlled ankle motion (CAM) boot for 4 to 6 weeks to reduce inflammation and pain. If conservative management fails, surgical intervention may be considered. Resection of the coalition with interposition (fat, wax, or muscle) is indicated for patients who fail conservative treatment and do not have advanced degenerative changes. Arthrodesis (subtalar or triple) is reserved for patients with advanced degenerative changes or those who fail resection. A medial displacement calcaneal osteotomy is not a primary treatment for tarsal coalition.

Correct Answer: Knee-ankle-foot orthosis (KAFO) during weight-bearing

This patient has infantile Blount disease (tibia vara), characterized by abnormal endochondral ossification of the medial aspect of the proximal tibial physis. The metaphyseal-diaphyseal angle (MDA), described by Levine and Drennan, is crucial for differentiating physiologic bowing from Blount disease. An MDA > 16 degrees is highly predictive of progressive Blount disease. For children under the age of 3 to 4 years with an MDA > 16 degrees or progressive deformity (Langenskiöld stages I-II), the initial treatment of choice is bracing with a KAFO during weight-bearing activities. If bracing fails, or if the child presents at an older age (typically > 4 years) or with a more advanced Langenskiöld stage (III or higher), surgical intervention such as a proximal tibial valgus osteotomy is indicated. Guided growth (hemiepiphysiodesis) is an option for older children with open physes but is not the first-line treatment for a 3-year-old with an MDA of 18 degrees, where bracing is highly effective.

Correct Answer: Early spica casting

The treatment of pediatric femoral shaft fractures is dictated primarily by the patient's age and weight. For children aged 1 to 5 years (or weighing less than 20 kg), early spica casting is the gold standard and provides excellent outcomes with low complication rates. A Pavlik harness is indicated for infants less than 6 months of age. Flexible intramedullary nailing is the treatment of choice for children aged 5 to 11 years (or weighing between 20 kg and 50 kg). Rigid intramedullary nailing is reserved for older adolescents (typically > 11 years or > 50 kg) nearing skeletal maturity, using a lateral trochanteric entry to avoid avascular necrosis of the femoral head. Plate fixation is an option for specific fracture patterns (e.g., highly comminuted, length unstable) or in patients with polytrauma, but is not the standard first-line treatment for an isolated spiral fracture in a 3-year-old.

Correct Answer: Anterior inferior tibiofibular ligament

This patient has a juvenile Tillaux fracture, which is a Salter-Harris III fracture of the anterolateral distal tibial epiphysis. This fracture occurs in adolescents due to the asymmetric closure pattern of the distal tibial physis. The physis closes first centrally, then anteromedially, then posteromedially, and finally laterally. When an external rotation force is applied to the foot, the anterior inferior tibiofibular ligament (AITFL) becomes taut and avulses the anterolateral epiphysis, which is the last portion of the physis to close. The anterior talofibular and calcaneofibular ligaments are lateral ankle ligaments involved in inversion sprains. The posterior inferior tibiofibular ligament is involved in Volkmann fractures (posterior malleolus). The deltoid ligament is the primary medial stabilizer of the ankle.

Correct Answer: Neurofibromatosis type 1

Anterolateral bowing of the tibia in a young child, especially with radiographic evidence of a narrowed, sclerotic medullary canal, is highly suspicious for congenital pseudarthrosis of the tibia (CPT). CPT is strongly associated with Neurofibromatosis type 1 (NF1), with approximately 50% of patients with CPT having NF1. The bowing typically progresses to a fracture that fails to heal (pseudarthrosis) due to abnormal periosteum at the site. It is important to distinguish the direction of bowing: anterolateral bowing is associated with CPT and NF1; posteromedial bowing is associated with calcaneovalgus foot deformity and typically resolves spontaneously (though a leg length discrepancy may persist); and anteromedial bowing is associated with fibular hemimelia. Fibrous dysplasia, osteogenesis imperfecta, achondroplasia, and cleidocranial dysplasia do not classically present with isolated anterolateral tibial bowing and CPT.

Correct Answer: Observation

A discoid meniscus is an abnormally thickened, disc-shaped meniscus (most commonly lateral) that is prone to tearing. The Watanabe classification divides them into complete, incomplete, and Wrisberg variant (lacking posterior meniscotibial attachments, leading to hypermobility). While a Wrisberg variant can cause a "snapping knee syndrome," the standard of care for an asymptomatic or pain-free snapping discoid meniscus is observation. Surgical intervention is strictly reserved for patients who develop symptoms such as pain, locking, swelling, or a painful snap indicating a tear or significant instability. When surgery is indicated, arthroscopic saucerization (reshaping the meniscus to a normal crescent) with or without peripheral repair is the treatment of choice to preserve meniscal tissue and prevent early osteoarthritis. Total meniscectomy is avoided due to the high risk of rapid joint degeneration.

Correct Answer: Unrecognized joint penetration by the screw

Chondrolysis is a devastating complication of SCFE characterized by the rapid destruction of articular cartilage, leading to a stiff, painful hip and joint space narrowing on radiographs. The most significant and well-documented risk factor for chondrolysis following surgical fixation of SCFE is unrecognized intra-articular hardware penetration. To prevent this, surgeons must utilize the "approach-withdraw" technique under fluoroscopy to ensure the screw has not breached the articular surface. While severe slip angle and unstable slips increase the risk of avascular necrosis (AVN), hardware penetration is the primary culprit for chondrolysis. Prophylactic pinning of the contralateral hip does not increase the risk of chondrolysis in the operative hip. Delay in surgery for a stable slip does not inherently cause chondrolysis, though it may allow the slip to progress.

Correct Answer: Coronal: Metaphyseal fracture; Sagittal: Epiphyseal fracture; Axial: Physeal separation

A triplane fracture is a complex transitional fracture of the distal tibia that occurs in adolescents as the physis is closing. It typically consists of three distinct fracture planes. On an anteroposterior (AP) radiograph (coronal plane), the fracture appears as a Salter-Harris II injury, with a vertical fracture line extending through the metaphysis. On a lateral radiograph (sagittal plane), it appears as a Salter-Harris III injury, with a vertical fracture line extending through the epiphysis. In the axial plane, the fracture propagates horizontally through the physis, connecting the metaphyseal and epiphyseal fracture lines. Therefore, the correct sequence is Coronal = Metaphysis, Sagittal = Epiphysis, and Axial = Physis. CT imaging is highly recommended to fully characterize the fracture fragments and plan for surgical reduction if displacement is > 2 mm.

Correct Answer: C

In patients with cerebral palsy (CP), the hip typically subluxates in a posterosuperior direction due to the deforming forces of spastic hip flexors and adductors. Therefore, the acetabular deficiency in CP is predominantly posterosuperior. Pelvic osteotomies utilized in this population, such as the Dega or San Diego osteotomies, are incomplete transiliac osteotomies that hinge on the triradiate cartilage and the symphysis pubis. They are specifically designed to hinge open posteriorly and laterally to provide lateral and posterior coverage of the femoral head. This contrasts with developmental dysplasia of the hip (DDH), where the deficiency is typically anterolateral, and osteotomies like the Salter or Pemberton are used to provide anterior and lateral coverage (Option D). Option A and E are incorrect as they do not address the posterosuperior deficiency. Option B is a secondary benefit of some pelvic osteotomies (like the Chiari) but is not the primary goal of a volume-expanding osteotomy in CP.

Correct Answer: E

Prophylactic pinning of the contralateral hip in SCFE is a topic of debate, but there are absolute and relative indications. The strongest indication for prophylactic pinning is the presence of an underlying endocrine disorder (e.g., hypothyroidism, renal osteodystrophy, panhypopituitarism) or prior radiation therapy. Patients with endocrine disorders have a bilateral involvement rate approaching 100%, compared to the 20-40% bilateral rate in idiopathic cases. Other relative indications for prophylactic pinning include an inability to follow up, young age at presentation (males < 12, females < 10), and open triradiate cartilage. While obesity (Option C) and young age (Option A) increase the risk of a contralateral slip, an endocrine disorder is the most definitive and strongest indication among the choices provided. Slip angle (Option D) and sex (Option B) are not primary determinants for prophylactic contralateral pinning.

Correct Answer: B

The patient exhibits a classic "OK sign" deficit, indicating an inability to flex the flexor pollicis longus (FPL) and the flexor digitorum profundus (FDP) to the index finger. This is the hallmark of an anterior interosseous nerve (AIN) palsy. The AIN is a purely motor branch of the median nerve, which aligns with the patient's normal sensation in the hand. AIN injury is the most common neurologic deficit associated with extension-type supracondylar humerus fractures (often due to traction or contusion over the proximal fracture fragment). The vast majority of these injuries are neuropraxias that resolve spontaneously. Observation for 3 to 6 months is the standard of care. Immediate exploration (Option A) is not indicated for isolated, closed nerve injuries in this setting. The median nerve proper (Options C and D) would present with sensory deficits in the palmar thumb, index, and middle fingers. Ulnar nerve injuries (Option E) are more commonly associated with flexion-type supracondylar fractures or iatrogenic injury from medial pin placement.

Correct Answer: B

Relapse in clubfoot treated with the Ponseti method is most commonly due to noncompliance with the foot abduction orthosis (bracing). In a toddler presenting with a relapsed clubfoot characterized by dynamic supination (due to an overactive tibialis anterior pulling the foot into supination without the counterbalancing pull of the peroneals) and recurrent equinus, the standard treatment is repeat serial casting to regain passive correction, followed by an anterior tibial tendon transfer (ATTT) to the lateral cuneiform. The ATTT removes the deforming supinatory force and converts it into a dorsiflexion force, preventing further relapse. A SPLATT (Option C) is typically reserved for spastic conditions like cerebral palsy or stroke, not idiopathic clubfoot. Comprehensive posteromedial release (Option A) is a historical procedure that leads to a stiff, painful foot in adulthood and is avoided if possible. Bony procedures (Options D and E) are salvage options for rigid, older, or severe syndromic feet, not for a supple relapse in a 3-year-old.

Correct Answer: B

In Legg-Calvé-Perthes disease, the two most important prognostic factors are the age of onset and the extent of epiphyseal involvement (often measured by the Herring lateral pillar classification). Age at onset > 8 years is a highly significant predictor of a poor outcome because there is less remaining growth potential for the femoral head to remodel into a spherical shape before skeletal maturity. While the "head-at-risk" signs (like the Gage sign, lateral subluxation, calcification lateral to the epiphysis) indicate a higher risk of deformation, age is the overriding prognostic variable. A Herring Lateral Pillar Class B (Option D) in a child > 8 years old actually warrants surgical containment (e.g., femoral or pelvic osteotomy) because the natural history is poor, whereas a child < 8 years with Class B often does well with symptomatic treatment. Male sex (Option C) is more common in Perthes but female sex is actually associated with a worse prognosis because girls mature earlier, leaving even less time for remodeling.

Correct Answer: C

The patient has a juvenile Tillaux fracture, which is a Salter-Harris III fracture of the anterolateral distal tibial epiphysis. This fracture occurs uniquely in adolescents (typically ages 12-15) because of the asymmetric closure pattern of the distal tibial physis. The physis closes first centrally, then anteromedially, then posteromedially, and finally laterally. When an external rotation force is applied to the foot, the anterior inferior tibiofibular ligament (AITFL) becomes taut. Because the anterolateral physis is the last to close and remains open, the AITFL avulses the anterolateral epiphysis rather than tearing the ligament or causing a syndesmotic injury. The anterior talofibular ligament (Option A) and calcaneofibular ligament (Option B) are lateral ankle ligaments involved in inversion sprains. The posterior inferior tibiofibular ligament (Option D) is involved in Volkmann fractures (posterior malleolus). The deltoid ligament (Option E) is medial.

Correct Answer: C

Osteosarcoma is the most common primary malignant bone tumor in children and adolescents. It is strongly associated with mutations in tumor suppressor genes, most notably the RB1 (retinoblastoma) gene and the TP53 gene (associated with Li-Fraumeni syndrome). Patients with hereditary retinoblastoma (germline RB1 mutation) have a significantly increased risk of developing osteosarcoma later in life. Option A, the t(11;22) translocation, results in the EWS-FLI1 fusion protein and is the hallmark of Ewing sarcoma. Option B, EXT1 (and EXT2) mutations, are associated with hereditary multiple exostoses (osteochondromas). Option D, GNAS mutations, are associated with fibrous dysplasia and McCune-Albright syndrome. Option E, the t(X;18) translocation, is characteristic of synovial sarcoma.

Correct Answer: B

Infantile Blount disease is characterized by pathologic varus deformity of the proximal tibia due to disordered endochondral ossification of the medial aspect of the proximal tibial physis. A metaphyseal-diaphyseal angle > 16 degrees is highly predictive of progressive Blount disease rather than physiologic bowing. For children under the age of 3 with early-stage infantile Blount disease (Langenskiöld stages I and II), the initial treatment of choice is bracing with Knee-Ankle-Foot Orthoses (KAFOs) worn during weight-bearing activities. Bracing is effective in unloading the medial compartment and allowing the physis to recover. Observation (Option A) is inappropriate given the high Drennan angle and metaphyseal beaking. Surgical intervention, such as a proximal tibial valgus osteotomy (Option C), is indicated if bracing fails, if the child presents after age 4, or if they have advanced Langenskiöld stages (III or higher). Guided growth (Option D) is more commonly used in older children or adolescents with late-onset Blount disease.

Correct Answer: D

The clinical presentation of rigid flatfeet, restricted subtalar motion, and recurrent ankle sprains in an adolescent is classic for a tarsal coalition. The "C-sign" on a lateral radiograph is a continuous C-shaped arc formed by the medial outline of the talar dome and the posteroinferior outline of the sustentaculum tali. This sign is highly indicative of a talocalcaneal (subtalar) coalition, specifically involving the middle facet. The gold standard imaging modality to confirm the diagnosis, determine the extent of the coalition (osseous, cartilaginous, or fibrous), and plan surgical resection is a CT scan. Calcaneonavicular coalitions (Options A and B) are best seen on a 45-degree internal oblique radiograph and present with an "anteater nose" sign, not a C-sign. MRI can be used for fibrous or cartilaginous coalitions but CT remains the primary advanced imaging modality for surgical planning of bony coalitions.

Correct Answer: B

The treatment of pediatric femoral shaft fractures is highly dependent on the patient's age and weight. For a 4-year-old child weighing less than 20 kg with an isolated, length-stable fracture (< 2-3 cm of shortening), early spica casting is the gold standard and most appropriate definitive treatment. A Pavlik harness (Option A) is indicated for infants less than 6 months of age. Flexible intramedullary nailing (Option C) is the treatment of choice for children aged 5 to 11 years, or those weighing more than 20 kg, as they are too large for spica casting to be easily managed and have a higher risk of unacceptable shortening. Rigid antegrade intramedullary nailing (Option D) is contraindicated in young children due to the risk of avascular necrosis of the femoral head from injury to the ascending cervical branches of the medial femoral circumflex artery at the piriformis fossa. Plate fixation (Option E) is an option for length-unstable fractures or polytrauma but is not the first-line treatment for a stable fracture in an 18 kg 4-year-old.

Correct Answer: Excessive flexion of the hip in the harness

The patient is presenting with a femoral nerve palsy, which is a known complication of Pavlik harness treatment for DDH. Femoral nerve palsy presents as a loss of active knee extension and is caused by hyperflexion of the hip in the harness, which compresses the femoral nerve against the inguinal ligament. If this occurs, the harness should be temporarily removed or adjusted to decrease the amount of hip flexion until nerve function returns. Excessive abduction of the hip (Option A) is associated with avascular necrosis (AVN) of the femoral head, not femoral nerve palsy. Inadequate flexion (Option D) or excessive adduction (Option E) would lead to failure of reduction or posterior dislocation of the hip, rather than a nerve palsy.

Correct Answer: Presence of primary hypothyroidism

Prophylactic pinning of the contralateral hip in SCFE is a debated topic, but there are clear absolute and relative indications. The strongest indication for prophylactic pinning is the presence of an underlying endocrine disorder, such as hypothyroidism, panhypopituitarism, or renal osteodystrophy, as these patients have a significantly higher risk (up to 100% in some studies) of developing a contralateral slip. Other indications include patients undergoing radiation therapy, and chronologic age less than 10 years (or open triradiate cartilage). While obesity (Option B) and male sex (Option E) are risk factors for SCFE in general, they are not absolute indications for prophylactic pinning on their own. The severity of the slip on the affected side (Option D) does not dictate the need for contralateral prophylaxis.

Correct Answer: Cavus, Adductus, Varus, Equinus

The Ponseti method is the gold standard for the treatment of idiopathic clubfoot. The correction follows a specific sequence, easily remembered by the acronym CAVE: Cavus, Adductus, Varus, and Equinus. The first step is to elevate the first ray to correct the cavus deformity, which aligns the forefoot with the hindfoot. Subsequent casts gradually abduct the supinated foot around the head of the talus, which simultaneously corrects the adductus and the varus deformities due to the kinematic coupling of the subtalar joint. The equinus deformity is corrected last; attempting to correct it too early can lead to a rocker-bottom foot deformity. If equinus cannot be fully corrected with casting, a percutaneous Achilles tenotomy is performed.

Correct Answer: Radial nerve

In extension-type supracondylar humerus fractures, the direction of displacement of the distal fragment dictates the structures at risk. When the distal fragment is displaced posteromedially, the proximal fragment is driven anterolaterally, putting the radial nerve at the greatest risk of tethering or injury. Conversely, if the distal fragment is displaced posterolaterally, the proximal fragment is driven anteromedially, placing the anterior interosseous nerve (AIN) and the brachial artery at greatest risk. The AIN is the most commonly injured nerve overall in extension-type supracondylar fractures. Ulnar nerve injuries (Option C) are most commonly associated with flexion-type supracondylar fractures or iatrogenic injury from medial pin placement during operative fixation.

Correct Answer: Age at the onset of disease

Legg-Calvé-Perthes disease is an idiopathic avascular necrosis of the proximal femoral epiphysis in children. The two most critical prognostic factors for long-term outcomes (i.e., development of premature osteoarthritis and sphericity of the femoral head at skeletal maturity) are the age of the patient at the onset of the disease and the extent of epiphyseal involvement (often measured by the Herring Lateral Pillar classification). Children who develop the disease before age 6 generally have a good prognosis regardless of treatment because they have more time for remodeling. Children over the age of 8 have a worse prognosis and are more likely to benefit from surgical containment (e.g., femoral or pelvic osteotomy) if they fall into Herring Lateral Pillar group B or B/C. Gender, BMI, and initial pain are not the primary determinants of long-term joint survival.

Correct Answer: Anterior inferior tibiofibular ligament; closure occurs medial to lateral

The patient has a Tillaux fracture, which is a Salter-Harris III fracture of the anterolateral distal tibial epiphysis. This injury occurs uniquely in adolescents due to the asymmetrical closure pattern of the distal tibial physis. The physis closes first centrally, then medially, and finally laterally. During this transitional period (typically ages 12-15), an external rotation force on the foot causes the anterior inferior tibiofibular ligament (AITFL) to avulse the anterolateral epiphysis, which is the only portion of the physis that remains open and vulnerable. The AITFL is the deforming force, and the closure pattern is central to medial to lateral.

Correct Answer: Varus derotational osteotomy (VDRO) of the proximal femur with or without pelvic osteotomy

Hip displacement is highly prevalent in children with cerebral palsy, particularly those with higher GMFCS levels (IV and V). The Reimers migration percentage (MP) is used to quantify subluxation. An MP > 40-50% indicates significant subluxation that is unlikely to respond to soft tissue releases alone and typically requires bony reconstruction to prevent painful dislocation and facilitate perineal care. The standard of care for a spastic hip with an MP > 40% in a 6-year-old is a proximal femoral varus derotational osteotomy (VDRO), often combined with a pelvic osteotomy (e.g., Dega or San Diego) if there is significant acetabular dysplasia. Soft tissue releases (Option C) are generally reserved for younger children (age < 4) with an MP between 25% and 40%. Observation (Option A) is inappropriate as the hip will likely progress to dislocation.

Correct Answer: Knee-ankle-foot orthosis (KAFO) during weight-bearing

The patient presents with infantile Blount disease (tibia vara), characterized by abnormal endochondral ossification of the medial aspect of the proximal tibial physis. Risk factors include early walking, obesity, and African American descent. The metaphyseal-diaphyseal angle (Drennan angle) is crucial for differentiating Blount disease from physiologic bowing. An angle > 16 degrees is highly predictive of progression to Blount disease. For a child under the age of 3 with Langenskiöld stage I or II disease, the initial treatment of choice is bracing with a KAFO during weight-bearing hours. If the child is older than 3 or 4 years, or if bracing fails, surgical intervention (proximal tibial osteotomy) is indicated. Reassurance (Option A) is appropriate for physiologic bowing (angle < 10 degrees). Vitamin D (Option B) is the treatment for rickets, which would present with systemic physeal widening and cupping.

Correct Answer: Chondrolysis

Chondrolysis is a devastating complication of SCFE characterized by the rapid, progressive loss of articular cartilage. It presents clinically with severe global stiffness, pain, and flexion contractures. Radiographically, it is identified by concentric joint space narrowing (typically defined as a joint space < 3 mm). While unrecognized hardware penetration is a known cause of chondrolysis, the vignette explicitly states there is no evidence of hardware penetration, meaning this is idiopathic chondrolysis associated with the SCFE itself. Avascular necrosis (Option A) typically presents with segmental collapse and sclerosis of the femoral head, not isolated concentric joint space narrowing. Septic arthritis (Option C) would present more acutely with systemic signs of infection. Cam impingement (Option E) causes activity-related groin pain and restricted internal rotation, but not global stiffness and concentric joint space loss.

Correct Answer: Talocalcaneal joint at the middle facet

The patient's clinical presentation (rigid flatfoot, recurrent sprains, decreased subtalar motion) is classic for a tarsal coalition. The two most common types are calcaneonavicular and talocalcaneal coalitions. The radiographic "C-sign" on a lateral view is formed by the medial outline of the talar dome and the inferior outline of the sustentaculum tali, and it is highly indicative of a talocalcaneal coalition. Talocalcaneal coalitions most frequently occur at the middle facet of the subtalar joint. Calcaneonavicular coalitions (Option A) are best seen on a 45-degree internal rotation oblique radiograph and are associated with the "anteater nose" sign (tubular elongation of the anterior process of the calcaneus). The posterior facet (Option D) is rarely the primary site of a talocalcaneal coalition.