**Core Concept**
Pemphigus vulgaris and bullous pemphigoid are two autoimmune bullous diseases that present with blistering skin lesions. The key difference lies in the presence or absence of Nikolsky's sign, which is a positive sign in pemphigus vulgaris and a negative sign in bullous pemphigoid.

**Why the Correct Answer is Right**
The patient's presentation of multiple blisters on the trunk and thighs, with a negative Nikolsky's sign, makes bullous pemphigoid a more likely diagnosis. Bullous pemphigoid is characterized by the deposition of autoantibodies against the basement membrane zone, leading to the formation of blisters. The absence of Nikolsky's sign, which is a positive sign in pemphigus vulgaris where gentle pressure on the skin causes the blister to rupture, is a distinguishing feature of bullous pemphigoid.

**Why Each Wrong Option is Incorrect**
**Option A:** Pemphigus vulgaris is unlikely because the patient has a negative Nikolsky's sign, which is typically positive in pemphigus vulgaris.
**Option B:** Dermatitis herpetiformis is a chronic autoimmune skin disease characterized by intensely itchy blisters, but it usually involves the elbows, knees, and buttocks, not the trunk and thighs.
**Option C:** Erythema multiforme is a skin condition characterized by target lesions, but it does not typically present with multiple blisters.
**Option D:** Stevens-Johnson syndrome is a severe skin condition characterized by blistering and peeling of the skin, but it usually involves the mucous membranes and is often a reaction to medication.

**Clinical Pearl / High-Yield Fact**
Bullous pemphigoid is a common autoimmune bullous disease in the elderly population, and it is essential to distinguish it from pemphigus vulgaris, as the treatment and prognosis differ between the two conditions.

**Correct Answer: D. Stevens-Johnson syndrome is a severe skin condition characterized by blistering and peeling of the skin, but it usually involves the mucous membranes and is often a reaction to medication.**