**Core Concept**
The peripartum period is a critical time for preventing mother-to-child transmission (MTCT) of HIV. Antiretroviral therapy (ART) is a cornerstone in reducing the risk of HIV transmission from mother to child.

**Why the Correct Answer is Right**
The provisional short-course antiretroviral regimen, typically involving a combination of zidovudine (AZT) and lamivudine (3TC), is administered to the mother during the peripartum period. This regimen works by reducing the viral load in the mother's blood, thereby decreasing the risk of transmission to the baby during delivery. The mechanism involves the inhibition of reverse transcriptase, an enzyme essential for HIV replication.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the question specifically asks about the peripartum period, and the regimen mentioned is typically administered during this time, not before or after.
**Option B:** This option is incorrect because, while zidovudine (AZT) is a crucial component of the regimen, it is not the sole reason for the reduction in vertical transmission risk.
**Option C:** This option is incorrect because the regimen in question is a combination of antiretroviral drugs, not a single agent.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the peripartum antiretroviral regimen should be initiated as soon as possible after diagnosis of HIV infection in pregnancy, ideally within 14 days of delivery.

**Correct Answer: D. 50-60% reduction in the risk of vertical transmission**