A 6-month-old exclusively breastfed infant presents with generalized seizures. Serum calcium is 6.8 mg/dL with markedly elevated alkaline phosphatase. Serum 25-OH Vitamin D is undetectable. The most likely clinical syndrome and its mechanism is:
- A Neonatal hypomagnesaemia; magnesium deficiency inhibits PTH secretion
- B DiGeorge syndrome; absent parathyroid glands
- C Classic rickets; deficient 1,25-OH vitamin D impairs intestinal calcium absorption ✓
- D Pseudohypoparathyroidism; PTH resistance
Correct answer: C. Classic rickets; deficient 1,25-OH vitamin D impairs intestinal calcium absorption
Explanation
Exclusively breastfed infants (particularly those with dark skin and limited sun exposure) are at high risk of vitamin D deficiency rickets. Undetectable 25-OH vitamin D causes deficient 1,25-OH vitamin D synthesis, impairing intestinal calcium and phosphate absorption. The resulting hypocalcaemia can trigger tetanic seizures, especially around 6 months when growth demands increase. Elevated ALP reflects bone turnover from secondary hyperparathyroidism. DiGeorge and pseudohypoparathyroidism would have detectable vitamin D levels.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.