**Core Concept**
Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Diagnostic serum markers are used to identify fetal aneuploidies, particularly Down's syndrome, during pregnancy. These markers are typically measured in maternal serum between 15 and 20 weeks of gestation.

**Why the Correct Answer is Right**
The most commonly used serum markers for Down's syndrome include:
* Alpha-fetoprotein (AFP): a protein produced by the fetus that can be detected in maternal serum. Elevated levels are associated with an increased risk of Down's syndrome.
* Human chorionic gonadotropin (hCG): a hormone produced by the placenta that can also be detected in maternal serum. Elevated levels are associated with an increased risk of Down's syndrome.
* Unconjugated estriol (uE3): an estrogen produced by the fetus that can be detected in maternal serum. Low levels are associated with an increased risk of Down's syndrome.
* Inhibin A: a protein produced by the placenta that can be detected in maternal serum. Elevated levels are associated with an increased risk of Down's syndrome.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not listed as a diagnostic serum marker for Down's syndrome.
* **Option B:** Not listed as a diagnostic serum marker for Down's syndrome.
* **Option C:** Not listed as a diagnostic serum marker for Down's syndrome.

**Clinical Pearl / High-Yield Fact**
The quad screen, which measures AFP, hCG, uE3, and inhibin A, is a commonly used screening test for Down's syndrome during pregnancy. However, it is essential to note that a positive result does not confirm Down's syndrome, and further diagnostic testing is required to confirm the diagnosis.

**Correct Answer:**
(Unfortunately, the options were not provided, but I would assume the question is asking which of the following is NOT a diagnostic serum marker for Down's syndrome.)