An 11-year-old boy presents to his pediatrician because his mother is concerned that he has been limping for the past 3 days. He states that he is limping because of a dull, aching pain in the right groin and thigh that extends down to his right knee. He has no history of preceding trauma. The pain is increased by physical activity. On physical exam, there is a shift of the torso over the affected hip with standing. Which of the following explains this physical exam finding?
(A) Weakness in hip abduction
(B) Weakness in hip flexion
(C) Weakness in hip extension
(D) Weakness in knee extension
(E) Weakness in knee flexion