A 4-month-old exclusively breastfed infant (no sunlight exposure) presents with convulsions. Serum calcium is 6.5 mg/dL, phosphorus is 5.8 mg/dL, and serum 25(OH)D is undetectable. What is the MECHANISM of hypocalcemia in this case?

• A Primary hypoparathyroidism
• B Decreased intestinal calcium absorption due to vitamin D deficiency ✓
• C Hyperphosphatemia from formula feeds
• D Calcium chelation by high breast milk phytate

Explanation

In vitamin D deficiency rickets presenting with neonatal/infantile hypocalcemia, the mechanism is: vitamin D deficiency → decreased active vitamin D (1,25-dihydroxycholecalciferol) → impaired calcium absorption from the gut → hypocalcemia → secondary hyperparathyroidism (PTH rises) → phosphaturia → low serum phosphorus. In this case, serum phosphorus is paradoxically elevated, suggesting early vitamin D deficiency before PTH has had time to cause significant phosphaturia, OR hypoparathyroidism should be considered. Low 25(OH)D confirms nutritional vitamin D deficiency as the primary cause. Breast milk has low vitamin D content (~25 IU/L), insufficient for infants with no sunlight exposure.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.