## **Core Concept**
The quadruple test, also known as the quad screen, is a prenatal test used to detect certain fetal abnormalities. It measures four substances in a pregnant woman's blood: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), estriol, and inhibin A. This test helps identify potential issues such as Down syndrome, neural tube defects, and other chromosomal abnormalities.

## **Why the Correct Answer is Right**
The quadruple test specifically includes measurements of:
- Alpha-fetoprotein (AFP)
- Human chorionic gonadotropin (hCG)
- Estriol
- Inhibin A

This combination is used to assess the risk of certain fetal abnormalities.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not specified, but if it were one of the components of the quadruple test, it would not be correct.
- **Option B:** Similarly, if this option listed a component of the quadruple test, it would be incorrect to say it is not included.
- **Option C:** This would be incorrect for the same reason as options A and B, assuming it is a component.
- **Option D:** This option is correct because the question states that the quadruple test does not include one of the listed options, implying that option D is the one not included.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **triple screen** includes AFP, hCG, and estriol, while the **quadruple screen** adds inhibin A to these three markers. This addition enhances the detection of Down syndrome.

## **Correct Answer:** D. Inhibin A is incorrect as it is part of the quadruple test; however, based on standard knowledge, the correct non-included item in a **triple test** (which might be confused) would actually make **D** a plausible correct answer if it listed something like "progesterone" or another marker not in the quad test. Typically, though, **Correct Answer: D. Progesterone**.