**Core Concept**
Pemphigus vulgaris is an autoimmune disease characterized by the formation of blisters on the skin and mucous membranes due to the presence of autoantibodies against desmoglein 3 proteins, which are crucial for cell-to-cell adhesion.

**Why the Correct Answer is Right**
The patient's presentation of multiple blisters on the trunk and thighs, which come and go, is consistent with pemphigus vulgaris. The absence of Nikolsky's sign, which is a positive sign in bullous pemphigoid, further supports this diagnosis. The autoantibodies in pemphigus vulgaris target the desmoglein 3 proteins, leading to the breakdown of skin cell adhesion and the formation of blisters.

**Why Each Wrong Option is Incorrect**
**Option A:** Bullous pemphigoid is a different autoimmune disease characterized by the presence of autoantibodies against bullous pemphigoid antigens 1 and 2, which are located in the basement membrane zone. Bullous pemphigoid often presents with a positive Nikolsky's sign.
**Option B:** Pemphigus foliaceus is another autoimmune disease that targets desmoglein 1 proteins, leading to the formation of blisters on the skin. However, it typically presents with a more widespread and superficial distribution of blisters, often involving the face and upper torso.
**Option C:** Dermatitis herpetiformis is an autoimmune disease characterized by the presence of autoantibodies against tissue transglutaminase, leading to the formation of blisters that are intensely itchy. It often presents with a more widespread distribution of lesions and is associated with celiac disease.
**Option D:** Stevens-Johnson syndrome is a severe skin and mucous membrane disorder caused by an adverse reaction to medication or infection. It presents with widespread blistering and often involves the eyes, genitalia, and mucous membranes.

**Clinical Pearl / High-Yield Fact**
Pemphigus vulgaris is a classic exam question and is often associated with a positive direct immunofluorescence test showing intercellular IgG deposits on the skin.

**Correct Answer:** C. Pemphigus vulgaris.