**Core Concept**
The question tests the student's knowledge of a rare but important condition known as fixed drug eruption (FDE), which is a type of adverse drug reaction that occurs in response to repeated exposure to a specific medication. In FDE, the lesion typically heals with post-inflammatory hyperpigmentation, and the reaction is often accompanied by a temporal relationship between the onset of symptoms and the ingestion of the offending drug.

**Why the Correct Answer is Right**
The correct answer, fixed drug eruption, is the most likely diagnosis in this scenario because the patient developed a vesiculobullous lesion on the glans soon after taking diclofenac, a nonsteroidal anti-inflammatory drug (NSAID). FDE is a type of drug reaction that is characterized by the formation of a skin lesion at the site of drug exposure, which in this case is the glans penis. The temporal relationship between the ingestion of diclofenac and the onset of symptoms is consistent with FDE, and the healing of the lesion with hyperpigmentation is also a characteristic feature of this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Behcet's syndrome is a form of vasculitis that can affect the skin, mucous membranes, and other organs, but it does not typically present with vesiculobullous lesions or a temporal relationship between symptoms and drug ingestion.

**Option B:** Herpes genitalis is a viral infection caused by the herpes simplex virus, which can cause vesiculobullous lesions on the genitalia. However, the timing of the lesion in relation to the ingestion of diclofenac does not support this diagnosis.

**Option D:** Pemphigus vulgaris is an autoimmune disease that affects the skin and mucous membranes, causing blistering lesions. While it can present with vesiculobullous lesions, it is not typically associated with a temporal relationship between symptoms and drug ingestion.

**Clinical Pearl / High-Yield Fact**
A key feature of fixed drug eruption is the recurrence of the lesion at the same site with subsequent exposure to the offending drug, which can be useful in making the diagnosis. This is often referred to as the "Boil and Bubble" sign, where the lesion "boils" or erupts with each subsequent exposure to the drug.

✓ Correct Answer: C. fixed drug erruption