During a well-child visit, the parents of a 6-year-old boy express concern about a change … — USMLE Step 2 CK

During a well-child visit, the parents of a $6$ -year-old boy express concern about a change in their son's walking pattern. They report that for the past $8$ weeks, he has been favoring his right leg, especially after running or playing at the park. When asked if he has any pain, the child points to his right anterior thigh and the area just above his kneecap. The parents have not noticed any fevers, rashes, or recent illnesses. He has met all developmental milestones and has no significant past medical history. His temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), heart rate is $90/\text{min}$ , and blood pressure is $98/60\text{ mm Hg}$ . Physical examination reveals a well-appearing boy who walks with a right-sided antalgic gait and a noticeable drop of the left hemipelvis during the stance phase on his right leg. There is no visible swelling or erythema over the right knee or hip. Palpation of the knee elicits no tenderness, and knee range of motion is full and painless. However, placing the right hip in internal rotation and abduction provokes mild discomfort and is noticeably restricted compared to the left side. Blood tests reveal an erythrocyte sedimentation rate of $9\text{ mm/h}$ and a leukocyte count of $6,800/\text{mm}^3$ . Which of the following is the most appropriate next step in the management of this patient?

Plain radiography of the pelvisCevap
B
Diagnostic aspiration of the right hip joint
C
Magnetic resonance imaging of the right knee
D
Initiation of oral ibuprofen and rest for $1$ week
E
Reassurance and routine follow-up in $6$ months

Cevap

The correct answer is plain radiography of the pelvis.

The correct answer is plain radiography of the pelvis. This patient's clinical presentation—an

8

-week history of an antalgic limp, referred anterior thigh/knee pain, restricted hip mobility (specifically decreased internal rotation and abduction), and a positive Trendelenburg sign (pelvic drop)—is highly characteristic of Legg-Calvé-Perthes disease (idiopathic avascular necrosis of the femoral head). It classically affects boys between

4

and

10

years of age. Because the condition develops insidiously and does not involve systemic infection or inflammation, patients are afebrile with normal inflammatory markers (erythrocyte sedimentation rate and leukocyte count). The most appropriate initial diagnostic step to evaluate for the characteristic flattening, fragmentation, and sclerosis of the femoral head is bilateral plain radiography of the pelvis (anteroposterior and frog-leg lateral views).

Adım Adım Çözüm

Analyze the clinical timeline and patient demographic.

6

-year-old boy presents with an insidious,

8

-week history of a limp and activity-related pain without systemic symptoms (no fever, normal inflammatory markers).

This establishes a chronic, non-infectious etiology typical for the age group.

Correlate subjective symptoms with physical examination findings.

Despite the patient reporting pain in the anterior thigh and knee, the physical examination reveals a normal knee but a positive Trendelenburg sign (pelvic drop) and restricted internal rotation/abduction of the right hip.

This confirms the pathology is localized to the hip, with pain referred distally via the obturator nerve.

Formulate the most likely diagnosis.

The combination of age, chronic timeline, restricted hip mobility, and normal laboratory values points directly to Legg-Calvé-Perthes disease (idiopathic avascular necrosis of the capital femoral epiphysis).

Alternative diagnoses like transient synovitis (too brief) or septic arthritis (acute, febrile) do not match the clinical picture.

Determine the initial diagnostic test.

Bilateral plain radiography of the pelvis (anteroposterior and frog-leg lateral views) is required to visualize the femoral heads.

Radiographs can confirm the diagnosis by revealing characteristic findings such as flattening, fragmentation, or sclerosis of the femoral head.

Anahtar Kavram

Legg-Calvé-Perthes disease typically presents in boys aged

4

10

years with an insidious-onset limp, restricted hip internal rotation/abduction, referred thigh/knee pain, and normal inflammatory markers. The best initial diagnostic step is plain radiography of the pelvis.

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