Nikolsky sign not present in -a) Pemphigusb) Pemphigoidc) Vitiligod) Staphylococcal scalded syndrome

**Core Concept**
The Nikolsky sign is a clinical indicator of intraepidermal cleavage, where gentle pressure on a blister results in the top layer of the skin shearing off. This sign is typically present in conditions characterized by acantholysis, a process where keratinocytes separate from each other due to loss of intercellular adhesion.

**Why the Correct Answer is Right**
Pemphigus is a group of autoimmune diseases that primarily affect the skin and mucous membranes. The pathophysiology of pemphigus involves autoantibodies against desmogleins, a family of adhesion molecules responsible for holding keratinocytes together. In pemphigus, the autoantibodies cause acantholysis, resulting in the formation of intraepidermal blisters and a positive Nikolsky sign. In contrast to pemphigus, pemphigoid diseases (such as bullous pemphigoid) primarily affect the basement membrane zone, resulting in subepidermal blisters and a negative Nikolsky sign.

**Why Each Wrong Option is Incorrect**
**Option A:** Pemphigus is characterized by a positive Nikolsky sign due to intraepidermal acantholysis.
**Option B:** Pemphigoid diseases, like bullous pemphigoid, primarily affect the basement membrane zone and result in subepidermal blisters, making the Nikolsky sign negative.
**Option C:** Vitiligo is an autoimmune disease characterized by the loss of melanocytes, resulting in areas of skin depigmentation. While vitiligo can be associated with other autoimmune diseases, it is not typically associated with a positive Nikolsky sign.
**Option D:** Staphylococcal scalded skin syndrome (SSSS) is caused by exfoliative toxins produced by Staphylococcus aureus. These toxins cleave desmoglein 1, leading to intraepidermal blisters and a positive Nikolsky sign.

**Clinical Pearl / High-Yield Fact**
The Nikolsky sign is a useful clinical indicator of intraepidermal cleavage, but it is not specific for any particular disease. It is essential to consider the clinical context, including the distribution and characteristics of the blisters, to arrive at an accurate diagnosis.

**Correct Answer:** B. Pemphigoid