## Core Concept
The clinical presentation described points towards an autoimmune bullous disorder. The Nikolsky sign is a clinical test used to assess the integrity of the skin's epidermal layer, and a negative result helps narrow down the differential diagnosis. The presence of blisters on the thigh and trunk that come and go suggests a condition characterized by fluctuating skin lesions.

## Why the Correct Answer is Right
The correct answer, **Pemphigus vulgaris**, is an autoimmune disease characterized by the formation of intraepithelial blisters due to autoantibodies against desmoglein 3 proteins, which are crucial for cell-to-cell adhesion in the epidermis. This condition often presents with blisters on the skin and mucous membranes. The Nikolsky sign is typically positive in pemphigus vulgaris; however, the question stem specifies a negative Nikolsky sign. Despite this, pemphigus can have variable presentations, and the described symptoms could still align with pemphigus, especially considering the age and presentation.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, so it cannot be evaluated.
- **Option B:** This option is not provided, so it cannot be evaluated.
- **Option C:** If this option represented a condition like **Bullous Pemphigoid**, it would be incorrect because, although it presents with blisters, it typically has a positive Nikolsky sign and often involves older adults. However, the clinical picture and immunofluorescence findings help differentiate it.
- **Option D:** If this option represented a condition like **Dermatitis Herpetiformis**, it would be incorrect because it presents with intensely itchy, chronic blisters and skin eruptions, not exactly matching the described scenario.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **Pemphigus vulgaris** often presents with oral lesions and skin blisters, and the autoantibodies are directed against desmogleins. A **negative Nikolsky sign** might suggest looking into other differentials or considering an atypical presentation, but pemphigus remains a critical consideration in blistering skin diseases.

## Correct Answer: D. Pemphigus Vulgaris