**Core Concept**
The accessory spleen, also known as splenunculus, is a small, separate mass of splenic tissue located outside the main spleen. It is a congenital anomaly that occurs during embryonic development. Accessory spleens are usually found in close proximity to the main spleen.

**Why the Correct Answer is Right**
The hilum of the spleen is the most common location for accessory spleens due to the presence of splenic tissue during fetal development. The hilum is the area where the splenic artery, vein, and nerves enter and exit the spleen. During embryonic development, splenic tissue can become separated and form an accessory spleen in this region. This is often due to incomplete fusion of splenic tissue during fetal development.

**Why Each Wrong Option is Incorrect**
**Option B:** The greater omentum is a large fold of peritoneum that hangs from the stomach and small intestine, and is not a common location for accessory spleens. While it is possible for accessory spleens to be found in the omentum, it is not the most common location.

**Option C:** The lesser omentum is a smaller fold of peritoneum that connects the liver to the stomach and small intestine. It is not a common location for accessory spleens.

**Option D:** The duodenum is the first part of the small intestine, and is not a common location for accessory spleens.

**Clinical Pearl / High-Yield Fact**
Accessory spleens are often found incidentally during imaging studies or surgery for other conditions. It is essential to recognize the presence of an accessory spleen to avoid mistaking it for a separate pathology.

**✓ Correct Answer: A. Hilum of spleen**