**Core Concept**
The question is testing the understanding of vertical transmission of HIV, which refers to the transmission of HIV from an infected mother to her child during pregnancy, childbirth, or breastfeeding. This concept is crucial in the field of HIV/AIDS, as it highlights the importance of prenatal care, antiretroviral therapy, and safe feeding practices to prevent mother-to-child transmission.

**Why the Correct Answer is Right**
The marker test for vertical transmission of HIV is the **HIV DNA PCR (Polymerase Chain Reaction)**. This test detects the presence of HIV DNA in the blood of the newborn, typically within the first few weeks of life. The HIV DNA PCR test is considered the gold standard for diagnosing HIV infection in infants and is used to confirm the diagnosis and monitor the effectiveness of antiretroviral therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a test for HIV. While HIV antibody tests are commonly used to diagnose HIV infection in adults, they may not detect the infection in infants who have been infected in utero.

**Option B:** This option is incorrect because it refers to a test for syphilis, not HIV.

**Option C:** This option is incorrect because it refers to a test for hepatitis B, not HIV.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the HIV DNA PCR test is not 100% sensitive, and a negative result does not rule out HIV infection. Therefore, healthcare providers should consider a combination of clinical assessment, laboratory tests, and follow-up evaluations to confirm the diagnosis of HIV infection in infants.

**Correct Answer:** A. HIV DNA PCR.