A 15-year-old boy has not yet begun puberty. His height is 145 cm, weight appropriate for height, smell is absent (anosmia). LH and FSH are very low. Testosterone is prepubertal. What is the MOST likely diagnosis?
- A Constitutional delay of puberty
- B Kallmann syndrome ✓
- C Primary hypogonadism (Klinefelter syndrome)
- D Isolated FSH deficiency
Explanation
Kallmann syndrome is hypogonadotropic hypogonadism (low LH and FSH) with anosmia/hyposmia, caused by defective migration of GnRH neurons and olfactory neurons from the olfactory placode during embryogenesis (mutations in KAL1, FGFR1, PROKR2, etc.). Anosmia in the setting of delayed puberty and low gonadotropins is the diagnostic clue distinguishing it from constitutional delay of puberty (which would have normal olfaction and a family history of delayed puberty). Constitutional delay has normal GnRH pulse amplitude once triggered. Primary hypogonadism (Klinefelter) would show ELEVATED LH and FSH with low testosterone. Isolated FSH deficiency is extremely rare and would spare LH.
Reference: Ghai Essential Pediatrics, 10th ed.
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