Okay, let's tackle this question. The user provided a scenario where a woman has chorionic villous sampling (CVS) at 11 weeks gestation, and the result shows two cell lines: 46 XY and 47 XY +21. The options are not given, but the correct answer is missing. My job is to create a detailed explanation based on the core concept.

First, I need to recall what CVS is. It's a prenatal test done in the first trimester to detect chromosomal abnormalities. The result here shows two different cell lines. A 46 XY is a normal male karyotype, but the 47 XY +21 is trisomy 21, which is Down syndrome. So, the presence of two cell lines suggests mosaicism. Mosaicism in CVS can be a challenge because it might be due to a true fetal mosaicism or a technical artifact from the sample.

Next, the next step would be to confirm the diagnosis. Since CVS can sometimes have confined placental mosaicism, where the abnormal cells are in the placenta and not the fetus, the correct approach is to perform amniocentesis to check the amniotic fluid cells, which are fetal-derived. This would help determine if the trisomy 21 is present in the fetus or just in the placenta.

Now, looking at the possible incorrect options. If an option suggests terminating the pregnancy without confirmation, that's wrong. Another might be to do another CVS, but that's redundant and doesn't address the placental vs fetal issue. Ultrasound alone can't confirm mosaicism. Non-invasive prenatal testing (NIPT) might be a distractor, but it's not as definitive as amniocentesis for mosaicism.

The clinical pearl here is that confined placental mosaicism is possible with CVS, so confirmation with amniocentesis is necessary to avoid misdiagnosis. Students should remember that CVS can sometimes show abnormalities not present in the fetus, so further testing is crucial.

Putting it all together, the correct answer is to perform amniocentesis for confirmation. The explanation should cover mosaicism, the limitations of CVS, and the need for fetal karyotype confirmation.

**Core Concept**
Chorionic villus sampling (CVS) at 11 weeks may reveal **fetal chromosomal mosaicism** (e.g., 46 XY and 47 XY +21), but **confined placental mosaicism (CPM)** is a critical differential. CPM occurs when abnormal cells are restricted to placental tissue, not the fetus. Confirming fetal karyotype is essential to distinguish true fetal aneuploidy from placental mosaicism.

**Why the Correct Answer is Right**
The presence of two cell lines (46 XY and 47 XY +21) in CVS suggests **mosaicism**. However, **confined placental mosaicism** is common in early CVS (1–5% of cases), where trisomy 21 cells are confined to the placenta. The next step is **amniocentesis** to obtain fetal-derived cells for karyotyping. This confirms whether the trisomy 21 is present in the fetus (true mosaicism) or restricted to the plac

✓ Correct Answer: C. Do amniocentesis for further confirmation