**Question:** An HIV positive, 36 years old female on AZT (zidovudine) therapy. Which of the following 1st trimester markers of Down Syndrome would be affected by AZT?

**Core Concept:** Azidothymidine (AZT), a nucleoside reverse transcriptase inhibitor (NRTI), is used to treat HIV infection. As a first-line therapy, it can cross the placenta and affect the development of the fetus.

**Why the Correct Answer is Right:** Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. In the first trimester, certain markers can be used to screen for Down syndrome, including:

1. Nuchal translucency (NT) measurement: The thickness of the fluid-filled area at the back of the neck in the fetus.
2. Pregnancy-specific beta-human chorionic gonadotropin (β-hCG) and alpha-fetoprotein (AFP) levels: These are proteins present in the blood during pregnancy.
3. Free beta-hCG and free β-hGFR: These are specific forms of β-hCG and β-hCG/AFP ratios.

**Why Each Wrong Option is Incorrect:**

A. Nuchal translucency (NT) measurement is a marker affected by AZT. AZT crosses the placenta and can increase the NT, making Down syndrome screening more likely to be positive.

B. Pregnancy-specific beta-human chorionic gonadotropin (β-hCG) and alpha-fetoprotein (AFP) levels are not affected by AZT. AZT does not directly impact these markers, but the increased NT could lead to higher β-hCG and AFP levels, contributing to false positive results.

C. Free beta-hCG and free β-hCG/AFP ratios are affected by AZT. As mentioned above, AZT can increase nuchal translucency, which could lead to false positive results in Down syndrome screening.

D. None of the above markers is directly affected by AZT. However, increased NT caused by AZT can lead to false positive results in Down syndrome screening.

**Correct Answer:** C. Free beta-hCG and free β-hCG/AFP ratios are affected by AZT. As mentioned above, AZT can increase nuchal translucency, which could lead to false positive results in Down syndrome screening.

**Why Each Wrong Option is Incorrect:**

A. Nuchal translucency (NT) measurement is not directly affected by AZT. AZT does not directly impact this marker, but the increased NT could lead to false positive results in Down syndrome screening.

B. Pregnancy-specific beta-hCG and alpha-fetoprotein (AFP) levels are not affected by AZT. AZT does not directly impact these markers but the increased NT caused by AZT can lead to false positive results in Down syndrome screening.

D. None of the above markers is directly affected by AZT. However, increased NT caused