A patient with fat malabsorption has chronically low serum 25-hydroxyvitamin D. Which cascade of biochemical and skeletal events is most consistent with this condition?

• A. Decreased vitamin D → immediate loss of systemic phosphate with no change in calcium or PTH, causing isolated cortical thinning while trabecular bone remains preserved.
• B. Decreased vitamin D → increased intestinal absorption of calcium through compensatory upregulation of paracrine factors in the gut resulting in no net change in bone remodeling rates.
• C. Decreased vitamin D → decreased urinary calcium excretion leading to hypercalcemia → suppression of PTH and paradoxical increase in bone deposition and decreased fracture risk.
• D. Decreased vitamin D → reduced intestinal calcium absorption → mild hypocalcemia → secondary hyperparathyroidism → increased osteoclast-mediated bone resorption → progressive bone loss.

Answer: (D)