**Core Concept**
First-trimester screening for Down syndrome involves the combination of nuchal translucency (NT) measurement, free beta-human chorionic gonadotropin (hCG), and pregnancy-associated plasma protein-A (PAPP-A) levels to estimate the risk of fetal aneuploidy, particularly Down syndrome.

**Why the Correct Answer is Right**
The combination of these markers in first-trimester screening allows for the detection of Down syndrome with a high degree of accuracy. The NT measurement assesses the thickness of the fluid-filled space at the back of the fetal neck, which is often increased in fetuses with Down syndrome. PAPP-A levels are typically lower in pregnancies affected by Down syndrome. Meanwhile, free beta-hCG levels are usually elevated in these cases. The combination of these markers provides a more accurate risk assessment than any one marker alone.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the detection rate of Down syndrome with first-trimester screening is actually higher than this option suggests.
**Option B:** Incorrect because this option underestimates the detection rate of Down syndrome with first-trimester screening.
**Option C:** Incorrect because this option is too low and does not accurately reflect the detection rate of Down syndrome with first-trimester screening.

**Clinical Pearl / High-Yield Fact**
It's essential to note that first-trimester screening is not diagnostic but rather provides a risk assessment, which may necessitate further testing, such as non-invasive prenatal testing (NIPT) or amniocentesis.

**Correct Answer: 90%.**