**Core Concept**
Leprosy, also known as Hansen's disease, is a chronic bacterial infection caused by Mycobacterium leprae that primarily affects the skin, peripheral nerves, mucosa of the upper respiratory tract, and eyes. The disease's pathogenesis is characterized by a cell-mediated immune response involving T cells and macrophages.

**Why the Correct Answer is Right**
The correct answer is the organ system that is least affected by leprosy. Leprosy primarily targets the peripheral nerves, skin, and mucosa, leading to symptoms such as numbness, skin lesions, and facial disfigurement. The autonomic nervous system is less affected, allowing for relatively preserved reflexes and autonomic functions.

**Why Each Wrong Option is Incorrect**
**Option A:** The skin is a primary target of leprosy, leading to lesions, nodules, and disfigurement. Leprosy causes damage to the skin and its appendages, including hair follicles and sweat glands.
**Option B:** The peripheral nerves are severely affected in leprosy, leading to numbness, pain, and muscle weakness. Leprosy causes demyelination and axonal degeneration in peripheral nerves.
**Option D:** The eyes are also affected in leprosy, leading to corneal ulcers, dryness, and vision loss. Leprosy can cause damage to the cornea and conjunctiva, leading to vision impairment.

**Clinical Pearl / High-Yield Fact**
Leprosy is a classic example of a disease that affects the peripheral nerves, skin, and mucosa, highlighting the importance of a thorough physical examination and attention to detail in diagnosis.

**Correct Answer: C. Autonomic nervous system. The autonomic nervous system is relatively unaffected in leprosy.**