Human chromosomal aneuploidy: Klinefelter syndrome results from which sex chromosome complement?

Introduction / Context:
Aneuploidies arise from meiotic nondisjunction and lead to characteristic clinical syndromes. Klinefelter syndrome is one of the most common sex chromosome aneuploidies in males, with important reproductive and endocrine consequences.

Given Data / Assumptions:

• Phenotype: typically tall stature, small testes, gynecomastia, infertility, and variable learning differences.
• Karyotype involves an extra X chromosome in a phenotypic male.

Concept / Approach:
Klinefelter syndrome corresponds to the presence of two X chromosomes and one Y chromosome (47,XXY). Lyonization (X inactivation) occurs, but the extra X still affects gene dosage and development. Mosaic forms (e.g., 46,XY/47,XXY) also exist but the canonical form is XXY.

Step-by-Step Solution:

Verification / Alternative check:
Karyotyping and chromosomal microarray confirm the extra X; elevated FSH/LH with low testosterone supports primary testicular failure consistent with XXY.

Why Other Options Are Wrong:

• XXX: Triple X syndrome (female), often subtle phenotype.
• XYY: Jacobs syndrome (male), different features.
• XO: Turner syndrome (female monosomy X).
• XXXX: rare tetrasomy with distinct female phenotype.

Common Pitfalls:
Confusing XXY with XYY due to both involving an extra sex chromosome; overlooking that Klinefelter refers to males with an extra X.

Final Answer:
XXY