Which of the following is NOT a structure of the proximal femur?

The obturator foramen is indeed not a structure of the proximal femur. Instead, it is an opening formed by the ischium and pubis bones of the pelvic girdle. The obturator foramen serves as a passageway for nerves and blood vessels, specifically the obturator nerve and the obturator vessels, but it is not part of the femur itself.

On the other hand, the greater trochanter, lesser trochanter, and femoral neck are all critical components of the proximal femur. The greater trochanter is a large, prominent projection on the lateral aspect of the femur that serves as a muscle attachment point. The lesser trochanter is a smaller, conical projection located on the medial side of the femur, important for muscle attachments as well. The femoral neck connects the head of the femur to the shaft, and its angle and structure play an essential role in hip stability and mobility.