**Question:** A 50 year old male known case of myasthenia gravis presents with erythematous, shallow erosions with few blisters and scales. Oral mucosa is not involved. Immunopathology demonstrates IgG deposition on keratinocytes and auto antibodies against Dsg -1. The diagnosis is

A. Eczema
B. Pemphigus vulgaris
C. Bullous pemphigoid
D. Dermatomyositis

**Correct Answer:** **B. Pemphigus vulgaris**

**Core Concept:**
Pemphigus is a group of rare autoimmune blistering diseases affecting skin and mucous membranes. In pemphigus, autoantibodies target desmoglein proteins (Dsg) present on keratinocytes, leading to blister formation.

**Why the Correct Answer is Right:**
In this case, the patient has symptoms like erythematous, shallow erosions with few blisters and scales, and IgG deposition on keratinocytes. Additionally, autoantibodies are directed against Dsg-1. These findings are characteristic of Pemphigus vulgaris, an autoimmune blistering disease. The presence of IgG deposition and Dsg-1 autoantibodies point towards Pemphigus vulgaris as the correct diagnosis as opposed to other conditions.

**Why Each Wrong Option is Incorrect:**
A. Eczema (Atopic Dermatitis) is a different skin disorder characterized by itching and dry, scaly skin, not the described symptoms of the patient.
C. Bullous pemphigoid is an autoimmune blistering disease, yet it is characterized by tense bullae and subepidermal blisters, while the patient presents with shallow erosions and scales.
D. Dermatomyositis is a systemic autoimmune disease affecting muscles and skin, not just mucous membranes.

**Clinical Pearl:**
Pemphigus vulgaris is a diagnostic challenge due to its overlap with other autoimmune blistering diseases. However, the presence of IgG deposition and Dsg-1 autoantibodies helps differentiate Pemphigus vulgaris from other conditions like eczema, bullous pemphigoid, and dermatomyositis. This is crucial in guiding treatment decisions and avoiding misdiagnosis.