**Core Concept**
Pemphigus is a group of autoimmune blistering diseases characterized by the production of autoantibodies against desmoglein proteins, leading to the loss of cell-to-cell adhesion and subsequent blister formation.

**Why the Correct Answer is Right**
The correct answer is based on the fact that the Nikolsky's sign is a clinical manifestation of pemphigus, where gentle pressure on the skin causes it to blister or separate. This is due to the autoantibodies against desmoglein 1 and 3 proteins, which are crucial for maintaining the integrity of the epidermal layer. The presence of Nikolsky's sign is a key diagnostic feature of pemphigus, particularly pemphigus vulgaris.

**Why Each Wrong Option is Incorrect**
* **Option A:** A biopsy from the affected area may show acantholysis, but it is not the most important bedside investigation.
* **Option B:** Blood tests may show elevated levels of autoantibodies, but they are not as immediately helpful as a bedside sign like Nikolsky's sign.
* **Option C:** A skin scraping may show the presence of acantholytic cells, but it is not as sensitive as Nikolsky's sign.
* **Option D:** A Tzanck smear may show the presence of multinucleated giant cells, but it is not specific for pemphigus.

**Clinical Pearl / High-Yield Fact**
Remember the "N" of Nikolsky's sign: **N**ormal skin pressure can cause the skin to **N**ot **I**ntact, leading to blister formation.

**Correct Answer:** .