**Core Concept**
The postcentral gyrus, also known as the primary somatosensory cortex, is a crucial area in the brain responsible for processing sensory information from the body. This includes touch, temperature, pain, and proprioception. The organization of the postcentral gyrus follows a somatotopic map, where different body parts are represented in a specific order.

**Why the Correct Answer is Right**
The lower limb is represented superiorly and medially within the postcentral gyrus due to the somatotopic organization of the brain. This means that the brain maps the body in a specific order, with the lower limb being located towards the top and medial (towards the midline) part of the postcentral gyrus. This is because the sensory information from the lower limb is projected to the brain in a specific pattern, with the medial and superior parts of the postcentral gyrus corresponding to the lower limb.

**Why Each Wrong Option is Incorrect**
**Option A:** The upper limb is actually represented in the lateral (outer) part of the postcentral gyrus, not the superior and medial part.

**Option C:** The abdomen is not represented in the postcentral gyrus, as it is primarily involved in processing sensory information from the skin and not from internal organs.

**Option D:** The genitalia are also not represented in the postcentral gyrus, as they are not part of the somatosensory cortex that processes touch, temperature, and pain.

**Clinical Pearl / High-Yield Fact**
The somatotopic organization of the brain is a fundamental concept in neurology and is often tested in exams. Remember that the lower limb is represented superiorly and medially in the postcentral gyrus, while the upper limb is represented laterally.

**✓ Correct Answer: B. Lower limb**