Nikolsky sign is positive in all of the following except:
**Core Concept**
The Nikolsky sign is a clinical indicator of epidermal cleavage, which occurs when the outer layer of the skin (epidermis) separates from the inner layer (dermis) due to various pathological processes. This sign is particularly useful in diagnosing conditions that affect the skin's integrity, such as pemphigus.
**Why the Correct Answer is Right**
The Nikolsky sign is positive when gentle pressure applied to the skin results in the separation of the epidermis from the dermis, often accompanied by the formation of a blister or erosion. This phenomenon is observed in conditions like pemphigus, where autoantibodies target desmoglein 3 proteins, disrupting intercellular adhesion. The resulting epidermal cleavage is a hallmark of this disease. In contrast, conditions that primarily affect the dermis, such as scleroderma, do not exhibit a positive Nikolsky sign.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely a condition that affects the epidermis, such as pemphigus, where the Nikolsky sign is indeed positive. Therefore, it is an incorrect choice.
**Option B:** This option is also a condition that affects the epidermis, such as staphylococcal scalded skin syndrome (SSSS), where the Nikolsky sign is positive due to the exfoliative toxins produced by the bacteria. Thus, it is an incorrect choice.
**Option C:** This option is likely a condition that primarily affects the dermis, such as scleroderma, where the epidermis is not directly involved, and the Nikolsky sign is negative. Therefore, it is the correct answer.
**Clinical Pearl / High-Yield Fact**
To remember the Nikolsky sign, recall the phrase "Nikolsky's nodules nicely nip off" to associate the clinical sign with conditions that cause epidermal cleavage.
**Correct Answer: C. Scleroderma.**