**Question:** Lichenoid drug eruption is caused by -

A. Hypersensitivity reaction
B. Cutaneous vasculitis
C. Eosinophilic granuloma
D. Cutaneous vasculitis

**Correct Answer:** A. Hypersensitivity reaction

**Core Concept:** Lichenoid drug eruption is a type of cutaneous hypersensitivity reaction caused by the immune response to drugs. It presents as a series of interconnected, superficial, and reticulated lesions with a characteristic "lichen planus-like" appearance. This reaction is characterized by the infiltration of lymphocytes, histiocytes, and eosinophils into the dermis, leading to tissue damage and inflammation.

**Why the Correct Answer is Right:** Lichenoid drug eruption is a hypersensitivity reaction, which is a type of immune response triggered by the activation of T cells and the release of cytokines and other immune mediators. In this case, the drug is the foreign antigen that stimulates the immune system to produce a specific immune response. The immune response leads to the infiltration of lymphocytes, histiocytes, and eosinophils into the dermis, resulting in tissue damage and inflammation. This results in the characteristic reticulated and interconnected lesions seen in lichenoid drug eruption.

**Why Each Wrong Option is Incorrect:**

B. Cutaneous vasculitis: Cutaneous vasculitis refers to inflammation of blood vessels in the skin and is not the primary mechanism behind lichenoid drug eruption.

C. Eosinophilic granuloma: Eosinophilic granuloma is a specific type of granuloma caused by eosinophilic infiltration. It is distinct from lichenoid drug eruption and does not explain the pathogenesis of this condition.

D. Cutaneous vasculitis: Similar to option B, cutaneous vasculitis is not the primary mechanism behind lichenoid drug eruption.

**Clinical Pearl:** Lichenoid drug eruption is a clinically significant reaction as it may indicate a more severe systemic reaction to the drug, such as Stevens-Johnson syndrome or toxic epidermal necrolysis. Prompt recognition and discontinuation of the offending drug are essential in preventing further skin damage and systemic complications.