vitamin-d-deficiency — SAQ

Stem

A 4-year-old girl of South Asian origin is brought by her parents because her legs have become increasingly bowed over the past year and she is reluctant to walk. She was exclusively breastfed until 18 months, takes no supplements, and the family cook with a vegan diet. On examination she has genu varum, enlarged and tender wrists, a palpable costochondral beading (rachitic rosary), frontal bossing and short stature (below the 3rd centile). Her 25-hydroxyvitamin D is 11 ng/mL; calcium 1.95 mmol/L (low), phosphate 0.7 mmol/L (low), PTH 18 pmol/L (high), alkaline phosphatase 560 U/L (high). Renal and hepatic function are normal.^[1]

Questions

a) What is the diagnosis, and which four biochemical features in the stem support it? (2 marks)

Model answer: Nutritional rickets from severe vitamin D deficiency (1 mark).^[1] Any four supporting features from: 25-OH vitamin D of 11 ng/mL (deficient, under 20 ng/mL); low calcium; low phosphate from PTH-driven phosphaturia; high PTH (secondary hyperparathyroidism); high alkaline phosphatase from increased osteoblast activity (1 mark). The risk factors — South Asian pigmentation, exclusive breastfeeding (breast milk is vitamin D poor), no supplementation and a vegan diet — make a nutritional cause most likely.

b) What radiographic finding at the wrists would confirm the diagnosis, and what is the underlying bone abnormality? (2 marks)

Model answer: Wrist radiographs show WIDENED, CUPPED and FRAYED metaphyses with splaying of the distal radial and ulnar growth plates (1 mark). The underlying abnormality is defective mineralisation of the hypertrophic cartilage of the growth plate — unmineralised cartilage accumulates, the metaphysis widens and the softened bone bows under weight (1 mark).^[1]

c) Outline the treatment. (4 marks)

Model answer:^[3] Treat with cholecalciferol (or ergocalciferol) — validated loading: oral 50,000 IU weekly for 8 weeks (or 200,000 IU monthly, or IM 600,000 IU monthly), then maintenance 50,000 IU monthly or bimonthly (1 mark). Give ADEQUATE CALCIUM — 1000 to 1200 mg/day through diet or supplement, because vitamin D without calcium does not heal rickets (1 mark). Because she has hypocalcaemia, ensure calcium is corrected and watch for symptomatic hypocalcaemia (seizures) as mineralisation accelerates — the hungry-bone effect (1 mark). Address the underlying cause: dietary review, education on safe sun exposure, and lifelong maintenance supplementation; the whole family should be screened and supplemented (1 mark). With normal renal function, native cholecalciferol (not calcitriol) is correct.

d) She has an 8-week-old sibling who is exclusively breastfed. What preventive advice and supplementation do you give, and why? (2 marks)

Model answer:^[1] Breast milk is VITAMIN D POOR, so all infants — especially exclusively breastfed babies — should receive 400 IU/day of vitamin D from birth to prevent rickets (1 mark). The mother should also be supplemented to maintain maternal sufficiency (600 to 1000+ IU/day). This simple, universal measure prevents nutritional rickets entirely (1 mark).

References4ShowHide

1. [1]Munns CF, Shaw N, Kiely M, et al. Global Consensus Recommendations on Prevention and Management of Nutritional Rickets J Clin Endocrinol Metab, 2016.PMID 26745253
2. [2]Amrein K, Scherkl M, Hoffmann M, et al. Vitamin D deficiency 2.0: an update on the current status worldwide Eur J Clin Nutr, 2020.PMID 31959942
3. [3]Hassan AB, Hozayen RF, Alotaibi RA, et al. Therapeutic and maintenance regimens of vitamin D3 supplementation in healthy adults: A systematic review Cell Mol Biol (Noisy-le-grand), 2018.PMID 30511630
4. [4]Sanders KM, Stuart AL, Williamson EJ, et al. Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial JAMA, 2010.PMID 20460620