A 22-year-old man complains about his inability to conceive a child. On physical examination, the patient is noted to be tall (6 ft, 5 in) and exhibits gynecomastia and testicular atrophy. Laboratory studies demonstrate increased serum levels of follicle-stimulating hormone. Cytogenetic studies reveal a chromosomal abnormality. What is the most common cause of this patient’s chromosomal abnormality?

**Core Concept**
Klinefelter syndrome (KS) is a chromosomal disorder characterized by an extra X chromosome in males, leading to hypogonadism, gynecomastia, and infertility. This condition is often associated with tall stature and increased levels of follicle-stimulating hormone (FSH).

**Why the Correct Answer is Right**
The most common cause of Klinefelter syndrome is a non-disjunction event during meiosis, resulting in a 47,XXY karyotype. This error occurs when a pair of X chromosomes fails to separate properly during gametogenesis, leading to an extra X chromosome in the sperm or egg. The resulting zygote inherits the extra X chromosome, resulting in Klinefelter syndrome.

**Why Each Wrong Option is Incorrect**

* **Option A:** Klinefelter syndrome is not caused by a deletion of the Y chromosome, which would result in a different condition known as 45,X or Turner syndrome.
* **Option B:** Mosaicism, where a mixture of normal and abnormal cells is present, can occur in Klinefelter syndrome, but it is not the most common cause of the chromosomal abnormality.
* **Option D:** Chromosomal translocations, where a segment of a chromosome breaks off and attaches to another chromosome, are not a common cause of Klinefelter syndrome.

**Clinical Pearl / High-Yield Fact**
Klinefelter syndrome is the most common sex chromosome disorder in males, affecting approximately 1 in every 650 males. It is essential to recognize the characteristic features of KS, including tall stature, gynecomastia, and infertility, and to investigate cytogenetic studies to confirm the diagnosis.

**Correct Answer:** C. Non-disjunction during meiosis.

✓ Correct Answer: C. Meiotic nondisjunction