**Core Concept**
Accessory breasts, also known as supernumerary breasts or polymastia, are rare congenital anomalies that occur when additional breast tissue develops outside the normal breast location. They can be found in both males and females, but are more common in females.

**Why the Correct Answer is Right**
The most common site for accessory breasts is along the milk lines, which are the embryonic pathways that breast tissue follows during development. The milk lines run from the axilla to the groin, with the most common site being near the axilla. Accessory breasts can also be found in the armpit, chest wall, or even along the abdomen. The development of accessory breasts is thought to occur due to incomplete regression of the milk lines during embryonic development.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a location along the milk lines, which is the most common site for accessory breasts.
**Option B:** This option is incorrect because it is too vague and does not provide a specific location.
**Option C:** This option is incorrect because it is not a common site for accessory breasts.

**Clinical Pearl / High-Yield Fact**
Accessory breasts can be bilateral, unilateral, or even ectopic, meaning they can be located in an abnormal position. They can be associated with other congenital anomalies, such as Poland syndrome, which is a rare condition characterized by the absence or underdevelopment of the pectoralis major muscle.

**Correct Answer:** D. Axilla