**Core Concept**
Down syndrome is a genetic disorder caused by an extra copy of chromosome 21, leading to various physiological and biochemical changes. One of the critical markers in prenatal screening for Down syndrome is the level of alpha-fetoprotein (AFP), a protein produced by the fetus.

**Why the Correct Answer is Right**
Decreased AFP levels are associated with Down syndrome because the extra genetic material in the fetus results in abnormal fetal development, including impaired liver function. The liver is responsible for producing AFP, and decreased levels are indicative of abnormal fetal development. In Down syndrome, the AFP levels are typically lower than the expected range, making it a useful marker for screening. The decreased AFP levels can be attributed to the impaired liver function and abnormal development of the fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** Raised PAPP-A level is associated with an increased risk of Down syndrome. PAPP-A (pregnancy-associated plasma protein-A) is a protein produced by the placenta, and elevated levels are often seen in pregnancies affected by Down syndrome.

**Option B:** Decreased beta-HCG level is not typically associated with Down syndrome. Beta-HCG (human chorionic gonadotropin) is a hormone produced by the placenta, and its levels are usually elevated in pregnancies affected by Down syndrome.

**Option C:** Raised estriol level is associated with an increased risk of Down syndrome. Estriol is an estrogen produced by the placenta, and elevated levels are often seen in pregnancies affected by Down syndrome.

**Option D:** (This is the correct answer, so it's not included here)

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Down syndrome can be screened for using a combination of markers, including AFP, PAPP-A, beta-HCG, and estriol levels. Each of these markers has a specific role in the screening process, and abnormal levels can indicate an increased risk of Down syndrome.

**✓ Correct Answer: D. Decreased AFP level**