A 22-year-old man is evaluated for infertility. Physical examination reveals tall stature, small firm testes (volume 4 mL bilaterally), and mild gynecomastia. Serum FSH and LH are elevated, with low testosterone. Which karyotype is most likely?
- A 45,X
- B 47,XXY ✓
- C 47,XYY
- D 47,XY,+21
Correct answer: B. 47,XXY
Explanation
Klinefelter syndrome (47,XXY) is the most common cause of primary testicular failure and hypogonadism in men. The classic triad is small firm testes, hypergonadotropic hypogonadism (elevated FSH/LH with low testosterone), and infertility. Tall stature results from delayed epiphyseal closure due to androgen deficiency. 45,X is Turner syndrome in females. 47,XYY is usually phenotypically normal with preserved fertility. 47,XY,+21 is Down syndrome.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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