**Core Concept**
Carbamazepine-induced Stevens-Johnson syndrome (SJS) is a severe cutaneous adverse reaction (SCAR) that is influenced by genetic predisposition. The HLA-B gene plays a crucial role in the immune system and has been associated with various drug reactions. Specifically, certain HLA-B alleles increase the risk of developing SJS when exposed to specific medications.

**Why the Correct Answer is Right**
The correct answer is HLA-B*1502, which is a specific allele of the HLA-B gene. Studies have shown that individuals with HLA-B*1502 are at an increased risk of developing SJS when taking carbamazepine. This is because the HLA-B*1502 allele is associated with an abnormal immune response to carbamazepine, leading to the activation of cytotoxic T cells and the subsequent release of pro-inflammatory cytokines. This results in the characteristic skin and mucosal lesions seen in SJS. The exact mechanism is not fully understood, but it is believed to involve the presentation of carbamazepine metabolites to T cells via the HLA-B*1502 molecule.

**Why Each Wrong Option is Incorrect**
* **Option A:** HLA-B*5801 is associated with allopurinol-induced SJS, not carbamazepine-induced SJS.
* **Option C:** HLA-B*5701 is associated with abacavir-induced hypersensitivity, not carbamazepine-induced SJS.
* **Option D:** HLA-B27 is associated with ankylosing spondylitis and other spondyloarthropathies, not carbamazepine-induced SJS.

**Clinical Pearl / High-Yield Fact**
When prescribing carbamazepine, it is essential to screen patients for the presence of HLA-B*1502, particularly in individuals of Asian ancestry, as they are at a higher risk of developing SJS.

**✓ Correct Answer: B. HLA-B* 1502**