**Core Concept**
The question is testing the knowledge of hypersensitivity reactions to antiretroviral therapy in HIV management. **Hypersensitivity reactions** are a type of adverse drug reaction that can be severe and life-threatening, requiring careful management and avoidance of the causative agent.

**Why the Correct Answer is Right**
The correct answer is **Lamivudine** (3TC). Lamivudine is a nucleoside reverse transcriptase inhibitor (NRTI) that is commonly used in combination with other antiretroviral agents to treat HIV infection. However, it is known to cause severe hypersensitivity reactions, including Stevens-Johnson syndrome and toxic epidermal necrolysis, in some patients. Once a patient has experienced a hypersensitivity reaction to lamivudine, it is generally contraindicated to re-challenge with the same medication due to the risk of severe and potentially life-threatening reactions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Zidovudine (AZT) is another NRTI that is used to treat HIV infection. While it can cause adverse reactions, it is not typically associated with severe hypersensitivity reactions like lamivudine.
* **Option B:** Efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI) that is used to treat HIV infection. It can cause central nervous system side effects, but it is not typically associated with severe hypersensitivity reactions.
* **Option C:** Abacavir is an NRTI that is used to treat HIV infection. It can cause hypersensitivity reactions, but it is not as severe as those associated with lamivudine.

**Clinical Pearl / High-Yield Fact**
When managing patients with HIV, it is essential to carefully monitor for signs of hypersensitivity reactions, including fever, rash, and mucocutaneous lesions. If a patient experiences a hypersensitivity reaction to an antiretroviral medication, it is crucial to discontinue the medication and switch to an alternative regimen to avoid severe and potentially life-threatening reactions.

**Correct Answer:** C.