What characterizes avascular necrosis?

Avascular necrosis, also known as osteonecrosis, occurs when there is a loss of blood supply to the bone, leading to the death of bone tissue. This condition is often characterized by a decrease in active range of motion, particularly in the hip joint. As the femoral head becomes necrotic, pain and stiffness develop, which can significantly restrict movement and flexibility in joint function.

In this context, the decrease in active range of motion in hip flexion especially reflects the effects of pain or joint damage caused by avascular necrosis. The lack of blood flow exacerbates these issues, leading to further deterioration of the joint mechanics over time.

The other options do not align with the typical characteristics of avascular necrosis. An excessive blood supply would counteract the condition, while fluid accumulation typically indicates inflammation or other joint pathologies rather than avascular necrosis. Increased mobility of the hip joint is also contrary to what is commonly observed, as the condition usually leads to stiffness rather than enhanced flexibility.