**Question:** A 70-year-old man develops multiple pruritic skin lesions and bullae mostly in the axillae and around the medial aspects of his groin and thighs. There are some lesions on his forearms and on his lower legs (first appeared in this location), and moderately painful oral lesions. Nikolsky sign is negative. There is no eye involvement. Which of the following is the most likely diagnosis?

A. Bullous pemphigoid
B. Erythema multiforme
C. Lichen planus
D. Systemic lupus erythematosus

**Core Concept:**

The question describes a patient presenting with blisters, pruritus, and oral lesions. These symptoms are consistent with a blistering skin disorder, among which we can differentiate between four options: bullous pemphigoid, erythema multiforme, lichen planus, and systemic lupus erythematosus.

**Why the Correct Answer is Right:**

The correct diagnosis is **A. Bullous pemphigoid**. This is a subepidermal blistering autoimmune disorder, characterized by autoantibodies targeting the hemidesmosome proteins, resulting in blister formation. The patient's symptoms and clinical findings are consistent with this diagnosis, including the location of lesions and the absence of Nikolsky sign (negative sign).

**Why Other Options are Incorrect:**

- **B. Erythema multiforme:** This condition primarily involves mucous membranes and is caused by infections or medications, whereas the patient's oral lesions are not a key feature in this case.

- **C. Lichen planus:** Lichen planus typically presents with symmetrical, targetoid lesions, and is more commonly associated with oral lesions; the patient's clinical features do not match these.

- **D. Systemic lupus erythematosus:** Lupus is a systemic autoimmune disease with a diverse set of symptoms, and while it may cause skin lesions, it does not present with the described clinical symptoms and signs.

**Clinical Pearl:**

Bullous pemphigoid is a distinct entity, frequently presenting in elderly populations, characterized by subepidermal blisters, typically in flexural areas, and associated with pruritus and oral lesions. The negative Nikolsky sign is a significant clue to this diagnosis.

Please note that the provided question is a clinical challenge trying to mimic other conditions like bullous pemphigus, pemphigus herpeticum, or pemphigus foliaceus. However, the correct answer is Bullous Pemphigoid due to the described clinical features, including negative Nikolsky sign and the presence of oral lesions.