**Question:** A 41-year-old had a baby with Down syndrome 10 years ago. She is anxious to know the chromosome status of her fetus in a current pregnancy. The test that has the fastest lab processing time for karyotype is:

A. Amniocentesis
B. Chorionic villus sampling (CVS)
C. Gastroschisis ultrasound
D. Placenta biopsy

**Correct Answer:** B. Chorionic villus sampling (CVS)

**Core Concept:**
Down syndrome, also known as trisomy 21, is a genetic disorder caused by an extra copy of chromosome 21. In this case, the patient has a history of delivering a baby with Down syndrome 10 years ago, which indicates that she is a carrier of the trisomy 21 chromosomal abnormality.

**Why the Correct Answer is Right:**
Chorionic villus sampling (CVS) is a prenatal diagnostic test used to evaluate the chromosome status of the fetus. It involves obtaining a small sample of placental tissue (chorionic villi) from the first trimester (11-14 weeks) to analyze the fetal chromosomes. CVS has a faster turnaround time compared to amniocentesis, which involves the collection of amniotic fluid cells from the uterus. While both CVS and amniocentesis can detect trisomy 21, CVS has a faster processing time, making it the preferred option for the patient's situation.

**Why Each Wrong Option is Incorrect:**
A. Amniocentesis (A) is another prenatal diagnostic test used for fetal karyotyping, but it has a longer processing time compared to CVS.
C. Gastroschisis ultrasound (C) is a diagnostic test for gastroschisis, a congenital abdominal wall defect, not chromosome analysis.
D. Placenta biopsy (D) is a test used to evaluate the placenta, not the fetus.

**Core Concept:**
In medical diagnosis, understanding the correct test procedures and their specific applications is essential for accurate medical decisions. In this case, the patient is concerned about the genetic status of her fetus and wants an efficient procedure.

**Why the Correct Answer is Right:**
Chorionic villi sampling (CVS) is the fastest method for analyzing fetal chromosomes, making it useful for this patient's situation. It offers faster results to help the patient make informed decisions about her pregnancy.

**Why Each Wrong Option is Incorrect:**
Amniocentesis (A) is a slower procedure compared to CVS, which might not meet the patient's requirements.
Gastroschisis ultrasound (C) is a diagnostic test for gastroschisis, a congenital abdominal wall defect, not chromosome analysis.
Placenta biopsy (D) is a test for evaluating the placenta, not the fetus.