A 52-year-old man with a 20-year history of cystic fibrosis presents with progressive bone pain, muscle weakness, and a recent pathological fracture of the femur. Serum calcium is 7.2 mg/dL, phosphate 2.8 mg/dL, and alkaline phosphatase 95 U/L. 25-hydroxyvitamin D level is 12 ng/mL (normal: 30–100 ng/mL). Which of the following best explains the pathophysiology of his bone disease?

Explanation

## Vitamin D Deficiency and Bone Disease in Cystic Fibrosis **Key Point:** Cystic fibrosis causes pancreatic insufficiency → fat malabsorption → deficiency of all fat-soluble vitamins (A, D, E, K), with vitamin D deficiency leading to secondary hyperparathyroidism and metabolic bone disease. ### Pathophysiology in CF ```mermaid flowchart TD A[Cystic Fibrosis<br/>CFTR mutation]:::outcome --> B[Pancreatic duct obstruction<br/>Acinar atrophy]:::outcome B --> C[Pancreatic insufficiency<br/>↓ Lipase, amylase, protease]:::outcome C --> D[Fat malabsorption<br/>Steatorrhea]:::outcome D --> E[↓ Absorption of fat-soluble vitamins<br/>A, D, E, K]:::urgent E --> F[Vitamin D deficiency<br/>↓ 25-OH-D]:::urgent F --> G[↓ Intestinal Ca absorption<br/>↓ Serum Ca]:::urgent G --> H[Secondary hyperparathyroidism<br/>↑ PTH]:::outcome H --> I[Bone resorption > formation<br/>Osteoporosis/Osteomalacia]:::outcome I --> J[Pathological fractures<br/>Bone pain]:::urgent ``` ### Why This Patient's Labs Fit Vitamin D Deficiency | Parameter | Finding | Mechanism | | --- | --- | --- | | 25-OH-D | 12 ng/mL (low) | Direct marker of vitamin D stores; reflects malabsorption | | Serum Ca | 7.2 mg/dL (low) | Reduced intestinal Ca absorption due to ↓ 1,25-OH-D | | Phosphate | 2.8 mg/dL (low) | Increased urinary losses driven by secondary hyperparathyroidism | | ALP | 95 U/L (normal) | Normal in chronic deficiency; would be elevated in acute rickets | | Bone findings | Pathological fracture | Osteomalacia (defective mineralization) + osteoporosis (bone loss) | **High-Yield:** CF-related bone disease (CFBD) affects 30–40% of adult CF patients. Vitamin D supplementation (2000–4000 IU/day) and fat-soluble vitamin replacement are standard of care. ### Vitamin D Metabolism Recap 1. **Skin:** 7-dehydrocholesterol + UVB → previtamin D~3~ → vitamin D~3~ 2. **Liver:** Vitamin D~3~ + 25-hydroxylase → **25-OH-D** (circulating form, storage marker) 3. **Kidney:** 25-OH-D + 1α-hydroxylase → **1,25-OH-D** (active form) 4. **Intestine:** 1,25-OH-D ↑ Ca and phosphate absorption 5. **Bone:** 1,25-OH-D + PTH → osteoclast activation (resorption) **Clinical Pearl:** In CF, the primary defect is *malabsorption of vitamin D from the gut*, not renal conversion. Renal 1α-hydroxylase activity is typically *upregulated* (not impaired) in response to low serum calcium and high PTH, but it cannot compensate if substrate (25-OH-D) is depleted. ### Why Other Options Are Incorrect **Option A (Impaired renal 1α-hydroxylase):** In CF, renal 1α-hydroxylase is actually *upregulated* by secondary hyperparathyroidism. The problem is insufficient substrate (25-OH-D), not enzyme deficiency. Phosphate wasting is a *consequence* of hyperparathyroidism, not the cause of vitamin D deficiency. **Option C (Primary hyperparathyroidism):** The hyperparathyroidism is *secondary* to vitamin D deficiency and hypocalcemia, not primary. PTH levels are elevated as a compensatory response. **Option D (CFTR-mediated osteoblast toxicity):** While CFTR is expressed in bone cells, the primary mechanism of bone disease in CF is nutritional (vitamin D deficiency), not direct cellular toxicity. This is why vitamin D supplementation is effective. **Mnemonic:** **FAT-SOLUBLE = CFTR PROBLEM** - **F**at malabsorption - **A**bsorption of A, D, E, K ↓ - **T**ypical in CF - **S**econdary hyperparathyroidism - **O**steomalacia - **L**ow 25-OH-D - **U**nderweight (malnutrition) - **B**one disease - **L**ipase deficiency - **E**nzyme replacement needed [cite:Harrison 21e Ch 308; KD Tripathi 8e Ch 12]