A 14-year-old male is brought to the clinic due to a 4-week history of right knee pain and… — USMLE Step 2 CK

A $14$ -year-old male is brought to the clinic due to a $4$ -week history of right knee pain and an altering gait. He has no history of trauma, fever, or recent illnesses. His weight is at the $98^{\text{th}}$ percentile for his age. Vital signs show a temperature of $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), heart rate of $78\text{/min}$ , and blood pressure of $118/76\text{ mm Hg}$ . Physical examination of the right knee reveals no effusion, joint line tenderness, or ligamentous laxity. During the examination of the lower extremities in the supine position, passive flexion of the right hip results in obligatory external rotation of the thigh. Laboratory studies show a leukocyte count of $6,800/\text{mm}^3$ , an erythrocyte sedimentation rate of $8\text{ mm/h}$ , and a C-reactive protein level of $0.3\text{ mg/dL}$ . Which of the following is the most appropriate next step in management?

Bilateral radiography of the hipsAnswer
B
Magnetic resonance imaging of the right knee
C
Reassurance and scheduled ibuprofen
D
Arthrocentesis of the right hip
E
Empiric intravenous antibiotics

Answer

Bilateral radiography of the hips

The clinical vignette describes a classic presentation of slipped capital femoral epiphysis (SCFE). This condition is characterized by the posterior and inferior displacement of the proximal femoral epiphysis relative to the metaphysis. It typically affects obese adolescents during periods of rapid growth. Because the obturator nerve provides sensory innervation to both the hip and the medial knee, patients frequently present with dull, aching pain localized strictly to the knee or distal thigh. The absence of knee pathology on examination, combined with the hallmark sign of obligatory external rotation of the thigh during passive hip flexion, strongly localizes the lesion to the hip. The most appropriate next step in management is to obtain bilateral radiography of the hips (anteroposterior and frog-leg lateral views) to confirm the diagnosis and evaluate the contralateral hip, as bilateral involvement occurs in up to

40\%

of cases.

Step-by-Step Solution

Analyze the patient's demographic and clinical presentation.

The patient is an obese adolescent male (

14

years old,

98^{\text{th}}

percentile weight) presenting with insidious onset of knee pain and a limp without preceding trauma.

Identifying demographic risk factors (obesity, adolescent age) helps narrow the differential diagnosis for pediatric musculoskeletal pain.

Interpret the physical examination and laboratory findings.

The knee examination is completely normal, ruling out local knee pathology. However, passive flexion of the right hip causes obligatory external rotation. Inflammatory markers (ESR, CRP, WBC) are normal.

Normal knee findings combined with abnormal hip mechanics indicate the knee pain is referred from the hip, classically via the obturator nerve. Normal labs rule out infectious etiologies.

Determine the diagnostic next step based on the suspected pathology.

The clinical picture is highly suspicious for Slipped Capital Femoral Epiphysis (SCFE). Bilateral anteroposterior (AP) and frog-leg lateral radiographs of the hips should be obtained.

Radiography is the gold standard initial imaging modality to visualize the posterior and inferior displacement of the proximal femoral epiphysis and to evaluate the contralateral hip, which is frequently affected asymptomatically.

Key Concept

Slipped Capital Femoral Epiphysis (SCFE) often presents in obese adolescents as isolated knee or distal thigh pain (referred via the obturator nerve). Obligatory external rotation during passive hip flexion is the classic physical exam finding, requiring immediate bilateral hip radiography.

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