A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 14-year-old boy is brought to the physician by his parents for a well-child visit. The patient was born at 38 weeks' gestation via vaginal delivery and has been healthy. He attends a junior high school and is having difficulties keeping up with his classmates in many classes. He is at the 97th percentile for height and 50th percentile for weight. Vital signs are within normal limits. Cardiac examination shows a high-frequency midsystolic click that is best heard at the left fifth intercostal space. The patient has long extremities along with excess breast tissue bilaterally. He has no axillary hair. Genital examination shows reduced scrotal size and a normal sized penis. Which of the following tests is the most likely to diagnose the patient's underlying disorder?
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A
Serum IGF-1 measurementIncorrect. IGF-1 evaluates the GH axis (acromegaly, GH deficiency) — not chromosomal aneuploidy.
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B
UrinalysisIncorrect. Urinalysis would identify renal disease, not Klinefelter.
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C
Slit-lamp examinationIncorrect. Slit-lamp exam evaluates lens dislocation in Marfan or Wilson disease — Marfan has tall stature but typically normal sexual development, not gynecomastia or small testes.
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D
KaryotypingCorrect. Karyotyping definitively diagnoses Klinefelter syndrome (47,XXY) — the cause of this boy's tall stature, gynecomastia, small testes, and learning difficulties.
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E
EchocardiogramIncorrect. Echocardiogram would evaluate mitral valve prolapse or aortic root dilation but does not diagnose the underlying chromosomal disorder responsible for tall stature, gynecomastia, and small testes — karyotyping does.
↑ Tap an answer to reveal the reasoning
Answer: D. A 14-year-old boy with tall stature (97th percentile height), long extremities (eunuchoid proportions), gynecomastia (excess breast tissue), absent secondary sexual characteristics (no axillary hair), small testes, normal-sized penis, learning difficulties, and a high-frequency midsystolic click (mitral valve prolapse) is the classic phenotype for Klinefelter syndrome (47,XXY).
Klinefelter results from meiotic nondisjunction producing an extra X chromosome. Seminiferous tubule dysgenesis causes small, firm testes with hyalinization and azoospermia; Leydig cell dysfunction lowers testosterone, producing eunuchoid proportions and incomplete virilization. Elevated FSH and LH (primary hypogonadism) are diagnostic but the definitive test is KARYOTYPING, which shows 47,XXY (or rarely mosaic). Mitral valve prolapse and learning disabilities are recognized associations.
IGF-1 measurement evaluates growth hormone axis — relevant for acromegaly or GH deficiency, not Klinefelter. Urinalysis is irrelevant. Slit-lamp examination would be appropriate for suspected Marfan syndrome (lens dislocation) or Wilson disease (Kayser-Fleischer rings), but the gynecomastia, small testes, and learning difficulties fit Klinefelter, not Marfan.
**Why each option:**
**A.** IGF-1 evaluates the GH axis (acromegaly, GH deficiency) — not chromosomal aneuploidy.
**B.** Urinalysis would identify renal disease, not Klinefelter.
**C.** Slit-lamp exam evaluates lens dislocation in Marfan or Wilson disease — Marfan has tall stature but typically normal sexual development, not gynecomastia or small testes.
**D.** Karyotyping definitively diagnoses Klinefelter syndrome (47,XXY) — the cause of this boy's tall stature, gynecomastia, small testes, and learning difficulties.