Question

the severe secondary osteoarthritis that follows aseptic necrosis of the femoral head is primarily due to:
pannus - mediated cartilage lysis
immune synovitis
ischemic necrosis of articular cartilage
sensory neuropathy
collapse of underlying subchondral bone

Explanation:

Aseptic necrosis of the femoral head leads to collapse of the underlying subchondral bone. This collapse disrupts the normal joint - surface mechanics, causing severe secondary osteoarthritis. Pannus - mediated cartilage lysis is more typical in rheumatoid arthritis, immune synovitis is an immune - mediated joint inflammation not the main cause here, ischemic necrosis of articular cartilage is not the primary driver, and sensory neuropathy is not related to this osteoarthritis pathogenesis.

Answer:

collapse of underlying subchondral bone