presents with a limp for the past 2 days. He has been otherwise
well without any fever. On examination, he is lying on the examining
table with his left hip flexed, abducted, and externally rotated.
You are able to move his hip without much difficulty although he
does resist extreme internal rotation. His WBC count, ESR, and X-ray
are normal. Of the following, which is the most likely etiology
of his limp?