**Question:** A patient presents with erythematous scaly lesions on extensor aspect of elbows and knees. The clinical diagnosis is got by –

A. Psoriasis
B. Eczema
C. Contact dermatitis
D. Lichen planus

**Correct Answer:** **D. Lichen Planus:**

**Core Concept:** Lichen Planus (LP) is a chronic immunemediated mucocutaneous disease characterized by pruritic, lacy, or reticulated, violaceous, or purpuric papules and plaques. LP commonly affects the mucous membranes and skin, particularly on extensor surfaces like elbows, knees, and shins.

**Why the Correct Answer is Right:** Lichen Planus is an autoimmune disorder, where the body's immune system mistakenly attacks the skin cells. The lesions present on extensor surfaces like elbows, knees, and shins are a characteristic feature of LP. In this case, the correct diagnosis is based on the clinical presentation of the patient.

**Why Each Wrong Option is Incorrect:**

A. Psoriasis: Psoriasis presents with well-demarcated, sharply defined, and polycyclic, erythematous, and silvery white scaly plaques. LP and psoriasis differ in presentation and distribution.

B. Eczema: Eczema is a chronic inflammatory skin disorder with pruritic, vesicles, and weeping vesicles or bullae. LP and eczema differ in presentation and distribution.

C. Contact Dermatitis: Contact dermatitis is an acute or subacute allergic or irritant reaction to an external stimulus. LP and contact dermatitis differ in presentation and distribution.

**Clinical Pearl:**

Lichen Planus is a commonly seen clinical entity in dermatology practice, particularly in patients with a history of stress or medications. Recognizing the clinical presentation can help in making an accurate diagnosis, which is crucial for appropriate management and prognosis.