**Question:** Regarding Lichen planus all are true except –

A. Lichen planus is a chronic inflammatory disease
B. It is caused by hypersensitivity to external triggers
C. It primarily affects the oral mucosa
D. It is a malignant disease

**Core Concept:** Lichen planus is a chronic inflammatory disease involving the skin, mucous membranes, and nails. It is an immune-mediated disease characterized by the infiltration of T cells into the dermis and mucous membranes. Lichen planus is not caused by hypersensitivity to external triggers, which is why option B is incorrect. While the disease can affect various sites, it primarily involves the skin, mucous membranes, and nails. In particular, the oral mucosa is often involved, which makes option C correct. Finally, lichen planus is not a malignant disease, so option D is also incorrect.

**Why the Correct Answer is Right:**

Lichen planus is not a malignant disease (option D) because it is a chronic inflammatory condition affecting the skin, mucous membranes, and nails. Unlike malignant diseases, lichen planus does not involve uncontrolled cell growth and does not progress to malignancy.

**Why Each Wrong Option is Incorrect:**

**Option B (caused by hypersensitivity to external triggers):** Lichen planus is an immune-mediated disease, not caused by external triggers. Its etiology remains unclear, but evidence suggests that genetic, hormonal, and environmental factors may play a role. The disease is characterized by the infiltration of T cells into the dermis and mucous membranes.

**Option C (primarily affects the skin):** While lichen planus can affect various sites, it is primarily characterized by the involvement of the skin, mucous membranes, and nails. The oral mucosa is often affected, making option C incorrect.

**Option D (a malignant disease):** Lichen planus is a chronic inflammatory condition affecting the skin, mucous membranes, and nails. Unlike malignant diseases, lichen planus does not involve uncontrolled cell growth and does not progress to malignancy.

**Clinical Pearl:** Lichen planus is a complex, multifactorial disease involving immune system dysfunction and genetic, hormonal, and environmental factors. It primarily affects the skin, mucous membranes, and nails, but may also involve other organs like the liver and kidney. Treatment options include topical corticosteroids, retinoids, and immunosuppressive agents, along with conservative management like avoiding irritants like tobacco, alcohol, and stress.