**Core Concept**
Leprosy, also known as Hansen's disease, is a chronic bacterial infection caused by Mycobacterium leprae that affects the peripheral nerves, skin, and mucous membranes. The disease leads to demyelination and axonal degeneration in the affected nerves, resulting in sensory and motor deficits.

**Why the Correct Answer is Right**
The peripheral nerves in leprosy are affected due to the direct invasion of M. leprae into the nerve tissue, leading to an immune response that causes demyelination and axonal damage. The nerves are most commonly affected in the cooler, peripheral parts of the body, such as the extremities, where M. leprae has a higher affinity for nerve tissue. The ulnar nerve is particularly susceptible to damage due to its anatomical location and the fact that it has a relatively long course in the arm, making it more prone to compression and damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** The radial nerve is also affected in leprosy, but it is not the most commonly affected nerve.
* **Option B:** The median nerve is not typically mentioned as a commonly affected nerve in leprosy.
* **Option D:** The sciatic nerve is a major nerve that is not typically mentioned as being affected in leprosy.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that leprosy can affect multiple peripheral nerves, leading to a combination of sensory and motor deficits. A thorough neurological examination and history-taking are crucial in diagnosing leprosy and distinguishing it from other causes of neuropathy.

**Correct Answer:** C.