## Core Concept
Karyotyping of a fetus is a crucial prenatal test used to detect chromosomal abnormalities. This procedure involves analyzing the number and structure of a fetus's chromosomes. It is commonly performed through amniocentesis or chorionic villus sampling (CVS).

## Why the Correct Answer is Right
The correct answer, , involves understanding the common methods for fetal karyotyping. Amniocentesis (A) and CVS (C) are well-established techniques for obtaining fetal cells for karyotyping. Another method, percutaneous umbilical blood sampling (PUBS) or cordocentesis (D), can also be used, especially when a rapid diagnosis is needed or when previous tests were inconclusive. However, the option that stands out as not routinely used for karyotyping is not explicitly listed, implying we need to assess each option's relevance.

## Why Each Wrong Option is Incorrect
- **Option A:** Amniocentesis is a standard method for fetal karyotyping, involving the withdrawal of amniotic fluid to analyze fetal cells.
- **Option C:** CVS is another common technique, where a sample of chorionic villi is taken for chromosomal analysis.
- **Option D:** PUBS or cordocentesis, though more invasive, can be used for karyotyping, especially in the second or third trimester.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **amniocentesis** and **CVS** are the primary methods for fetal karyotyping. **Percutaneous umbilical blood sampling (PUBS)** is typically reserved for specific situations due to its invasive nature.

## Correct Answer Line
**Correct Answer: B.**