A 34 years old patient came for antenatal ultrasound in her 2nd trimester. Her beta HCG level is raised, however AFP level is decreased. Patient has one 5 years old child who does not have any chromosomal abnormalities. Most likely diagnosis is?

**Core Concept**
The underlying principle being tested involves the interpretation of maternal serum markers during pregnancy, specifically the levels of **beta human chorionic gonadotropin (beta HCG)** and **alpha-fetoprotein (AFP)**. These markers are crucial in screening for chromosomal abnormalities and fetal defects.

**Why the Correct Answer is Right**
A raised **beta HCG** level in conjunction with a decreased **AFP** level is suggestive of a chromosomal abnormality, particularly **Down syndrome (Trisomy 21)**. This is because elevated beta HCG is associated with an increased risk of Down syndrome, while decreased AFP levels are also commonly seen in such cases.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not correlate with the specific pattern of raised beta HCG and decreased AFP.
**Option B:** Incorrect as it is not directly related to the described serum marker pattern.
**Option C:** Incorrect because, although it involves a chromosomal issue, it does not match the specific markers' pattern given.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that while these markers can suggest an increased risk, they are screening tools and not diagnostic. An elevated beta HCG with a decreased AFP should prompt further diagnostic testing, such as amniocentesis or chorionic villus sampling, to confirm the presence of a chromosomal abnormality.

**Correct Answer:** D. Down syndrome (Trisomy 21)

✓ Correct Answer: A. Down syndrome