**Core Concept**
The question revolves around the concept of hypersensitivity reactions to anti-HIV medications, specifically the management of allergic reactions and the decision to rechallenge with the same drug. This is a critical aspect of pharmacovigilance and treatment of HIV infection.

**Why the Correct Answer is Right**
The correct answer is **A. Abacavir**. Abacavir is a nucleoside reverse transcriptase inhibitor (NRTI) known to cause a severe hypersensitivity reaction in some patients. This reaction is often associated with fever, rash, and other systemic symptoms. The mechanism behind this reaction is not fully understood, but it is thought to involve an immune-mediated response to the drug. Rechallenge with Abacavir after a history of hypersensitivity reaction is strongly contraindicated, as it can lead to life-threatening consequences.

**Why Each Wrong Option is Incorrect**
* **Option B:** Nelfinavir, another protease inhibitor, can cause a range of side effects, but it is not typically associated with severe hypersensitivity reactions.
* **Option C:** Efavirenz, a non-nucleoside reverse transcriptase inhibitor (NNRTI), can cause neuropsychiatric side effects and skin reactions, but rechallenge is not strictly contraindicated in case of a hypersensitivity reaction.
* **Option D:** Stavudine, a nucleoside reverse transcriptase inhibitor, can cause mitochondrial toxicity and peripheral neuropathy, but it is not typically associated with severe hypersensitivity reactions.

**Clinical Pearl / High-Yield Fact**
When managing HIV infection, it's essential to recognize the risk of hypersensitivity reactions to certain medications, such as Abacavir, and to carefully weigh the benefits and risks of rechallenge.

**Correct Answer:** Abacavir.