A 16-year-old boy is evaluated for gynaecomastia. He is tall with an arm span exceeding his height, has small firm testes (about 4 mL bilaterally), sparse facial hair, and a history of scholastic difficulty. Serum testosterone is low with markedly elevated FSH and LH. What is the most likely karyotype?

• A 46,XX male
• B 47,XYY
• C 45,X/46,XY mosaicism
• D 47,XXY ✓

Explanation

Hypergonadotropic hypogonadism with small firm testes, gynaecomastia, eunuchoid proportions, and borderline intellect defines Klinefelter syndrome, caused by 47,XXY in about 90 percent of cases. 47,XYY males are tall but have normal fertility and normal testosterone with no gynaecomastia. Mixed gonadal dysgenesis (45,X/46,XY) presents with ambiguous genitalia or female external genitalia, and SRY-translocated 46,XX males usually lack the typical small firm testes and learning disability pattern.

Reference: Nelson Textbook of Pediatrics, 22nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.