Correct Answer: D. Scurvy
Scurvy (vitamin C deficiency) presents with bilateral knee pain and characteristic radiological findings that distinguish it from other metabolic bone diseases. The key discriminating feature is the Wimberger ring sign (sclerotic ring around epiphysis) and Scorbutic rosary (metaphyseal spurs/irregularities) visible on X-ray, combined with normal bone mineral density. Unlike rickets, which shows decreased mineralization and widened growth plates, scurvy maintains normal overall density but exhibits metaphyseal lucency with a dense metaphyseal line (white line of Frankel). The clinical presentation—bilateral joint pain, swelling, and potential bleeding manifestations—reflects defective collagen synthesis due to absent vitamin C-dependent hydroxylation of proline and lysine. In Indian pediatric populations, scurvy occurs in children with restricted diets (exclusive boiled milk, no fresh fruits/vegetables), particularly in urban settings with poor nutritional awareness. The normal bone mineral density is the critical clue that excludes rickets, which would show hypomineralization. Treatment with vitamin C supplementation (100–200 mg daily) rapidly resolves symptoms within 2–4 weeks, confirming the diagnosis retrospectively.
Why the other options are wrong
A. Metaphyseal dysplasia — Metaphyseal dysplasias are primary skeletal dysplasias (genetic/developmental) presenting with permanent structural abnormalities of the metaphysis, not reversible metabolic changes. They do not present acutely with bilateral joint pain and are not associated with nutritional deficiency. The clinical history of dietary insufficiency and rapid response to vitamin C supplementation rules out this congenital disorder. B. Rickets — While rickets also causes metaphyseal changes and joint pain, it characteristically shows decreased bone mineral density with widened growth plates and loss of sharp metaphyseal margins. Scurvy maintains normal overall bone density with a pathognomonic dense metaphyseal line. The normal BMD in this case is the discriminating feature that excludes rickets, which would require calcium/phosphate/vitamin D supplementation rather than vitamin C. C. Pycnodysostosis — Pycnodysostosis is a rare autosomal recessive osteoclast dysfunction disorder (CTSK gene mutation) causing increased bone density (osteosclerosis), short stature, and dental abnormalities. It is a permanent genetic condition, not reversible with nutritional supplementation. The normal BMD and acute presentation with dietary history make this unlikely; pycnodysostosis would show hyperdensity, not the characteristic scurvy findings.
High-Yield Facts
- Wimberger ring sign (sclerotic ring around epiphysis) is pathognomonic for scurvy on X-ray.
- White line of Frankel (dense metaphyseal line) distinguishes scurvy from rickets, which shows loss of sharp margins.
- Normal bone mineral density in scurvy vs. decreased BMD in rickets is the key radiological discriminator.
- Vitamin C deficiency impairs collagen synthesis, causing defective osteoid matrix and metaphyseal hemorrhages.
- Scorbutic rosary (metaphyseal spurs/irregularities) and Pelkan spurs (lateral metaphyseal projections) are characteristic findings.
- Treatment: 100–200 mg vitamin C daily produces clinical improvement in 2–4 weeks and radiological healing in 6–8 weeks.
Mnemonics
SCURVY Radiological Signs Sclerotic ring (Wimberger) | Corbutic rosary | Undermine zone (Pelkan spurs) | Radiolucent metaphysis | Vitamin C deficiency | Yellow-white line (Frankel) Scurvy vs Rickets (Quick Discriminator) Scurvy = Normal density + Dense metaphyseal line | Rickets = Low density + Widened growth plate — Use when X-ray shows normal BMD with metaphyseal changes.
NBE Trap
NBE may pair rickets with metaphyseal changes to trap students who recognize abnormal metaphyseal findings but forget that rickets causes decreased bone density while scurvy maintains normal density. The normal BMD statement is the key discriminator.
Clinical Pearl
In Indian pediatric practice, scurvy is increasingly seen in urban children fed exclusively on boiled milk and refined foods without fresh fruits/vegetables. The rapid clinical response to vitamin C supplementation (bleeding gums resolve, joint swelling subsides within days) makes this a high-yield diagnosis for bedside recognition and confirms the diagnosis without waiting for confirmatory tests.
_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 8 (Bone and Joint Diseases); OP Ghai Pediatric Nursing, Ch. 12 (Nutritional Disorders)_