A child who limps or has hip or knee pain is a favorite USMLE scenario. These three USMLE Step 1 practice questions cover slipped capital femoral epiphysis (SCFE), Legg-Calvé-Perthes disease and developmental dysplasia of the hip. Pick your answer first, then open the explanation.
Question 1: Obese teenager with knee pain and a limp
A 13-year-old boy has had a limp and dull right knee pain for 3 weeks. There was no trauma. He is at the 98th percentile for BMI. Examination of the knee is normal, but internal rotation of the right hip is limited, and the thigh rotates outward when the hip is flexed. Which of the following is the most important risk factor for his condition?
A. Obesity B. Previous long-bone fracture C. Inhaled corticosteroid use D. Family history of osteoporosis
Correct answer: A. Obesity
This is slipped capital femoral epiphysis (SCFE): the femoral head slips posteriorly off the neck through the growth plate during the adolescent growth spurt. Obesity is the strongest risk factor because it increases shear stress on the physis. Hip pain is often referred to the knee (obturator nerve), so the knee exam is normal.
Why the other options are wrong:
A healed fracture years earlier does not weaken the proximal femoral growth plate.
Inhaled corticosteroids at usual asthma doses do not cause SCFE (high-dose systemic steroids are a risk for avascular necrosis, a different condition).
Family history of osteoporosis is not related to SCFE.
High-yield: Diagnose with bilateral frog-leg lateral hip X-rays (“ice cream slipping off the cone”). Treat with surgical pinning; the patient should not bear weight. SCFE is also linked to hypothyroidism and growth hormone therapy.
Question 2: Young boy with painless limp and avascular necrosis of the femoral head
A 6-year-old boy has had a mild, intermittent limp for 2 months. Hip X-ray shows flattening and increased density of the right femoral head. Which of the following is the most likely diagnosis?
A. Slipped capital femoral epiphysis B. Legg-Calvé-Perthes disease C. Transient synovitis D. Septic arthritis
Correct answer: B. Legg-Calvé-Perthes disease
Legg-Calvé-Perthes disease is idiopathic avascular necrosis of the femoral head in children, usually boys aged 4–8 years. It presents with a gradual, often painless limp; X-ray shows a flattened, dense, fragmented femoral head.
Why the other options are wrong:
SCFE affects obese adolescents (10–16 years) and shows slippage of the epiphysis, not necrosis.
Transient synovitis causes acute hip pain after a viral illness with a normal X-ray and resolves in days.
Septic arthritis causes fever, a very painful hip and refusal to bear weight; it is an emergency.
High-yield: Treatment of Perthes is mostly observation, activity limitation and bracing to keep the femoral head in the socket; prognosis is better in younger children.
Question 3: Newborn hip “clunk” on examination
During a newborn examination, the physician flexes and abducts the hip and feels a “clunk” as the femoral head moves back into the acetabulum. Which of the following is the most appropriate next step?
A. Reassure that this is a normal finding B. Hip ultrasound C. Hip X-ray D. MRI of the pelvis
Correct answer: B. Hip ultrasound
A positive Ortolani maneuver (reduction of a dislocated hip) suggests developmental dysplasia of the hip (DDH). Because the femoral head is still cartilage in infants under about 4–6 months, ultrasound is the best imaging test.
Why the other options are wrong:
Reassurance is wrong: a clunk is abnormal and needs evaluation and treatment.
X-ray is used after about 4–6 months, once the femoral head ossifies.
MRI is not needed and would require sedation.
High-yield: Risk factors: breech presentation, female sex, firstborn, family history and oligohydramnios. Treatment: Pavlik harness. The Barlow maneuver dislocates an unstable hip; Ortolani relocates it.
Key takeaways
Obese adolescent + knee pain + limited internal rotation of the hip → SCFE.
Boy 4–8 years + painless limp + flattened femoral head → Legg-Calvé-Perthes (avascular necrosis).
Newborn with positive Ortolani/Barlow → DDH; hip ultrasound, Pavlik harness.
