**Core Concept**
Pemphigus is a group of autoimmune skin diseases characterized by the formation of blisters on the skin and mucous membranes. It is caused by the production of autoantibodies against desmoglein proteins, which are essential components of desmosomes that hold skin cells together.

**Why the Correct Answer is Right**
Pemphigus vulgaris is the most common form of pemphigus, accounting for approximately 70-80% of all cases. It primarily affects the skin and mucous membranes, with a predilection for the oral cavity. The autoantibodies in pemphigus vulgaris target desmoglein 3, leading to the disruption of skin cell adhesion and the formation of blisters.

**Why Each Wrong Option is Incorrect**
**Option A:** Pemphigus foliaceus is a rare form of pemphigus that primarily affects the skin, with a predilection for the face, scalp, and upper trunk. It is caused by autoantibodies against desmoglein 1.
**Option B:** Pemphigus erythematosus is a rare form of pemphigus that combines features of pemphigus foliaceus and lupus erythematosus. It is not the rarest variety of pemphigus.
**Option C:** Pemphigus vegetans is a rare variant of pemphigus vulgaris that is characterized by the formation of vegetating lesions on the skin. It is not the rarest variety of pemphigus.

**Clinical Pearl / High-Yield Fact**
Pemphigus is a classic example of an autoimmune disease that can be triggered by various factors, including infections, medications, and genetic predisposition. The presence of autoantibodies against desmoglein proteins is a hallmark of the disease.

**Correct Answer:** A. Pemphigus foliaceus is the rarest variety of pemphigus.