**Core Concept:** Avascular necrosis, also known as osteonecrosis or bone infarction, is a condition characterized by the death of bone tissue due to reduced or absent blood supply. It can result from various factors including trauma, such as fractures, and is more common in bones with high bone mass and activity, like the femoral head (e.g., hip).

**Why the Correct Answer is Right:** Option D, the humerus, is an exception because it has a rich blood supply due to its location and function. Unlike other bones, the humerus receives its blood supply primarily from the radial and ulnar arteries, which have numerous branches supplying the bone. This rich blood supply makes the humerus less prone to avascular necrosis compared to other bones.

**Why Each Wrong Option is Incorrect:**

A. Avascular necrosis can indeed occur in the femoral head (hip), as seen in conditions like Ficat and Arlet staging of avascular necrosis.
B. Avascular necrosis can also affect the vertebrae, particularly the vertebral bodies, as seen in cases of osteonecrosis.
C. Although avascular necrosis can affect the scapula, it is less common compared to the femoral head and vertebrae due to its relatively poor blood supply.

**Clinical Pearl:** A clinical pearl for remembering the bones at risk for avascular necrosis is "FEMORAL head and vertebrae Might Occur, SCAPULA less so". This mnemonic can help students remember the bones at high risk for avascular necrosis, which are the femoral head, vertebrae, and scapula.

**Correct Answer:** Avascular necrosis is less likely to occur in the humerus (D).