**Question:** Lichen planus

**Core Concept:** Lichen planus is a chronic inflammatory dermatological disorder caused by an unknown etiology. It is characterized by the presence of a polygonal, violaceous papule or plaque on the skin, oral mucosa, or both. Lichen planus can be classified into four clinical types: eruptive, atrophic, bullous, and eruptive bullous.

**Why the Correct Answer is Right:** Lichen planus is a T-cell-mediated autoimmune disorder that affects the basal layer of the epidermis and keratinocytes. The immune system recognizes keratinocytes as foreign antigens, leading to deposition of immunoglobulins and complement on the cell surface, activating complement-dependent cytotoxicity, and leading to epidermal cell death.

**Why Each Wrong Option is Incorrect:**

A. Lichen planus is a viral infection (e.g., HPV): This is incorrect because lichen planus is an autoimmune disorder, not a viral infection.
B. Lichen planus is a drug-induced reaction: While certain medications can exacerbate or induce lichen planus, the disease itself is not caused by drugs.
C. Lichen planus is a fungal infection: Lichen planus is an autoimmune disorder, not a fungal infection.
D. Lichen planus is a bacterial infection: Similar to options A and B, lichen planus is an autoimmune disorder and not caused by bacteria.

**Clinical Pearl:** Lichen planus is associated with an increased risk of oral cancer, especially in patients with longstanding, severe, or erosive oral lesions. Early diagnosis and management of oral lesions can reduce the risk of cancer development.

**Correct Answer:** C. Lichen planus is an autoimmune disorder affecting keratinocytes, leading to epidermal cell death and the characteristic clinical features.