A 7-month-old exclusively breastfed infant presents with irritability, refused to bear weight on one limb, and periosteal reaction of the femur and tibia on X-ray. Alkaline phosphatase is elevated and serum phosphorus is low. Mother is a dark-skinned immigrant who wears full-body covering clothing. The pathophysiology of the bone disease involves:

• A Defective osteoclast function due to vitamin D deficiency causing failure of bone remodelling
• B Inadequate calcium absorption due to 1,25-dihydroxyvitamin D deficiency, leading to secondary hyperparathyroidism and hypophosphataemia ✓
• C Primary phosphate wasting through the kidney due to FGF-23 excess
• D Collagen cross-linking defect from vitamin C deficiency causing subperiosteal haematomas

Explanation

This is nutritional rickets in an exclusively breastfed infant of a vitamin D-deficient mother. Vitamin D deficiency → reduced 1,25-dihydroxyvitamin D → impaired intestinal calcium absorption → hypocalcaemia → secondary hyperparathyroidism → PTH-mediated phosphate wasting via FGF-23 pathway → hypophosphataemia → defective bone mineralisation (osteomalacia/rickets). Periosteal reactions, metaphyseal fraying, cupping, and ALP elevation are the hallmarks. Scurvy (option D) causes periosteal haematomas but occurs at 6–24 months and features a different clinical picture.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.