A 38-year-old woman from rural Bihar presents with progressive bone pain, muscle weakness, and difficulty climbing stairs for the past 8 months. She reports minimal sun exposure due to cultural practices. On examination, she has proximal muscle weakness (grade 4/5 in hip flexors), bowing of legs, and tenderness over the tibiae. Serum calcium is 7.8 mg/dL (normal 8.5–10.5), phosphate is 2.1 mg/dL (normal 2.5–4.5), alkaline phosphatase is 120 IU/L (normal 30–120), and 25-hydroxyvitamin D is 12 ng/mL (normal >30). X-ray shows loss of sharp corticomedullary differentiation and Looser zones in the femur. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Osteomalacia ### Key Clinical Features **Key Point:** Osteomalacia is the adult equivalent of rickets, characterized by defective mineralization of osteoid matrix due to vitamin D deficiency. This patient presents with the classic triad: 1. **Low 25-hydroxyvitamin D** (12 ng/mL; normal >30) — the diagnostic hallmark 2. **Biochemical pattern**: Low calcium, low phosphate, elevated alkaline phosphatase 3. **Clinical manifestations**: Proximal muscle weakness, bone pain, bowing deformity, Looser zones on X-ray ### Pathophysiology ```mermaid flowchart TD A[Vitamin D Deficiency]:::urgent --> B[↓ Intestinal Ca²⁺ Absorption] A --> C[↓ Renal 1,25-OH Vitamin D] B --> D[Hypocalcemia] C --> D D --> E[Secondary Hyperparathyroidism] E --> F[↑ ALP, ↑ PTH] F --> G[Defective Osteoid Mineralization] G --> H[Osteomalacia]:::outcome H --> I[Bone Pain, Muscle Weakness, Deformity] I --> J[Looser Zones on X-ray]:::outcome ``` ### Biochemical Markers | Parameter | Osteomalacia | Osteoporosis | Hyperparathyroidism | |-----------|--------------|--------------|---------------------| | Calcium | ↓ or Normal | Normal | ↑ | | Phosphate | ↓ | Normal | ↓ | | ALP | ↑↑ | Normal | ↑ | | 25-OH Vitamin D | ↓↓ (<20) | Normal | Normal | | PTH | ↑ | Normal | ↑ | | Looser Zones | Present | Absent | Absent | **High-Yield:** Looser zones (pseudofractures) are pathognomonic for osteomalacia — they are bands of unmineralized osteoid perpendicular to the bone surface, best seen in femur, tibia, and pelvis. ### Clinical Pearl **Clinical Pearl:** Proximal muscle weakness in osteomalacia is due to direct vitamin D deficiency affecting muscle cells (vitamin D receptors are present in skeletal muscle), not just from bone pain. This improves rapidly with vitamin D replacement. ### Risk Factors in This Patient - **Minimal sun exposure** (cultural/geographic) → ↓ skin synthesis of vitamin D - **Rural Bihar** (high latitude, seasonal variation, air pollution) → reduced UVB penetration - **Female** (higher prevalence in women due to clothing practices in South Asia) ### Diagnostic Criteria **Key Point:** 25-hydroxyvitamin D <20 ng/mL is diagnostic of vitamin D deficiency; <12 ng/mL indicates severe deficiency. [cite:Harrison 21e Ch 402]