**Core Concept**
The triple test in obstetrics evaluates maternal serum levels of human chorionic gonadotropin (hCG), alpha-fetoprotein (AFP), and unconjugated estriol (uE3) to assess the risk of fetal abnormalities, particularly neural tube defects and chromosomal disorders like trisomy 21.

**Why the Correct Answer is Right**
The triple test measures hCG (A), AFP (B), and unconjugated estriol (C), all of which are produced during pregnancy and reflect placental and fetal function. Inhibin A (D) is not included in the standard triple test; it is part of the **quadruple test** (along with hCG, AFP, and estriol) and is used to screen for trisomy 21, especially in the context of Down syndrome. Inhibin A is produced by the ovaries and placenta but is not a component of the classic triple test.

**Why Each Wrong Option is Incorrect**
Option A: hCG is a key component of the triple test and is measured to assess fetal development and placental function.
Option B: AFP is measured to evaluate neural tube defect risk and is a standard part of the triple test.
Option C: Unconjugated estriol (uE3) reflects placental function and is included in the triple test.
Option D: Inhibin A is not part of the triple test; it is part of the quadruple test and is not measured in the standard triple test.

**Clinical Pearl / High-Yield Fact**
The triple test includes hCG, AFP, and unconjugated estriol — remember the mnemonic **"H-A-F-E"** (HCG, AFP, Estriol). Inhibin A is only relevant in the **quadruple test**, not the triple test.

✓ Correct Answer: D. Inhibin A