**Core Concept**
Nikolsky's sign is a clinical indicator of intraepithelial blistering diseases, where gentle pressure or rubbing of the skin results in the exfoliation of the outermost layer of the skin. This phenomenon occurs due to the separation of the epidermis from the dermis, which is compromised in diseases characterized by autoantibodies against adhesion molecules.

**Why the Correct Answer is Right**
In pemphigus, autoantibodies target desmoglein 3 and 1 proteins, which are crucial for maintaining the integrity of epidermal cell-to-cell adhesion. The loss of these adhesion molecules leads to the formation of intraepithelial blisters. When Nikolsky's sign is applied, the gentle pressure causes the already weakened epidermal-dermal junction to separate, resulting in the exfoliation of the outermost layer of the skin. This characteristic clinical finding is a hallmark of pemphigus.

**Why Each Wrong Option is Incorrect**
**Option B:** Dermatitis herpetiformis is a skin condition characterized by intense pruritic vesicles and is associated with celiac disease. It is not associated with intraepidermal blistering and does not exhibit Nikolsky's sign.

**Option C:** Pemphigoid refers to a group of autoimmune diseases that result in subepidermal blistering, not intraepidermal blistering. Therefore, Nikolsky's sign is typically negative in pemphigoid.

**Option D:** Rubella is a viral infection that causes a self-limiting rash, but it is not associated with intraepidermal blistering or Nikolsky's sign.

**Clinical Pearl / High-Yield Fact**
Nikolsky's sign is a critical clinical finding that helps differentiate pemphigus from other blistering diseases. Remember that pemphigus is the only condition among the options that exhibits intraepidermal blistering and a positive Nikolsky's sign.

**Correct Answer:**
✓ Correct Answer: A. Pemphigus.