A 35-year-old woman from Punjab presents with progressive bone pain, muscle weakness, and difficulty climbing stairs. Investigations show serum calcium 7.8 mg/dL, phosphate 2.9 mg/dL, alkaline phosphatase 120 U/L, and 25-hydroxyvitamin D level of 15 ng/mL. What is the most common cause of osteomalacia in this adult patient?

Explanation

## Most Common Cause of Osteomalacia in Adults **Key Point:** Nutritional vitamin D deficiency due to inadequate dietary intake and/or insufficient sunlight exposure is the most common cause of osteomalacia in adults, particularly in regions with limited sun exposure or dietary restrictions. ### Epidemiology of Osteomalacia - Osteomalacia is the adult equivalent of rickets (defective mineralization of osteoid) - Nutritional deficiency accounts for >70% of osteomalacia cases in developed countries - Prevalence is even higher in developing countries and high-latitude regions - Women and elderly populations are at higher risk ### Clinical Presentation in This Case The patient presents with classic osteomalacia features: - **Musculoskeletal:** Diffuse bone pain, muscle weakness, difficulty with stairs (proximal myopathy) - **Age group:** 35 years (adult osteomalacia) - **Geography:** Punjab (high latitude, variable sun exposure) - **Biochemical pattern:** Low calcium, low phosphate, mildly elevated ALP, **low 25-OH D (15 ng/mL)** ### Diagnostic Threshold **High-Yield:** 25-hydroxyvitamin D levels: - **<10 ng/mL:** Severe deficiency (osteomalacia likely) - **10–20 ng/mL:** Deficiency (osteomalacia present) - **20–30 ng/mL:** Insufficiency (subclinical) - **>30 ng/mL:** Sufficient This patient's level of 15 ng/mL is diagnostic of vitamin D deficiency osteomalacia. ### Differential Diagnosis of Osteomalacia Causes | Cause | 25-OH D | 1,25-OH₂ D | Ca | PO₄ | PTH | Prevalence | |-------|---------|-----------|----|----|-----|------------| | **Nutritional deficiency** | **Very low** | Low | Low | Low | ↑ | **Most common** | | Malabsorption | Very low | Low | Low | Low | ↑ | Common (secondary) | | Renal disease | Normal/Low | Low | Low | High | ↑ | Secondary to CKD | | Hypophosphatemia | Normal | High | Normal | Very low | Normal | Rare genetic | **Clinical Pearl:** The 25-hydroxyvitamin D level is the **gold standard** for diagnosing nutritional vitamin D deficiency. A level <20 ng/mL in the context of bone pain and biochemical abnormalities is diagnostic of osteomalacia. ### Risk Factors for Nutritional Vitamin D Deficiency in Adults 1. **Dietary insufficiency:** Low intake of fortified milk, fatty fish, egg yolks 2. **Limited sun exposure:** Indoor work, cultural clothing (hijab, burqa), high latitude, winter months 3. **Aging:** Reduced skin synthesis of vitamin D with age 4. **Darker skin pigmentation:** Reduced UVB penetration in high-latitude regions 5. **Vegetarian/vegan diet:** Limited dietary sources of vitamin D **Mnemonic — Causes of Osteomalacia (DRAM):** **D**ietary deficiency (most common) → **R**enal disease → **A**bsorption defects → **M**etabolic (genetic hypophosphatemia)