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 practice of purdah. On examination, she has proximal muscle weakness (grade 4/5), bone tenderness over the tibia and femur, and a waddling gait. Serum calcium is 7.2 mg/dL, phosphate 2.8 mg/dL, alkaline phosphatase 120 IU/L (elevated), and 25-hydroxyvitamin D is 12 ng/mL. X-ray shows loss of sharp metaphyseal margins and Looser's zones in the femur. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Nutritional Osteomalacia ### Key Features in This Case **Key Point:** The patient is an adult (38 years old) with vitamin D deficiency presenting with bone pain, proximal muscle weakness, and biochemical evidence of defective mineralization—this is osteomalacia, not rickets. **High-Yield:** Rickets occurs in children (open growth plates); osteomalacia occurs in adults (closed growth plates). Both result from vitamin D deficiency but manifest differently. ### Biochemical Pattern | Parameter | Finding | Interpretation | |-----------|---------|----------------| | 25-OH vitamin D | 12 ng/mL | **Severely deficient** (normal >30 ng/mL) | | Serum calcium | 7.2 mg/dL | Low (secondary hyperparathyroidism) | | Phosphate | 2.8 mg/dL | Low | | ALP | 120 IU/L | Elevated (bone turnover) | | PTH | Implied elevated | Response to hypocalcemia | ### Radiological Hallmarks of Osteomalacia 1. **Looser's zones** (pseudofractures)—pathognomonic, seen in femur here 2. Loss of sharp metaphyseal margins 3. Generalized osteopenia 4. Coxa vara deformity (in severe cases) ### Clinical Pearl **Proximal muscle weakness** is a cardinal feature of osteomalacia that distinguishes it from simple hypocalcemia. The weakness is due to direct vitamin D receptor effects on muscle, not just calcium depletion. ### Pathophysiology ```mermaid flowchart TD A[Vitamin D Deficiency<br/>Low sun exposure + poor diet]:::outcome A --> B[↓ 1,25-OH vitamin D production]:::outcome B --> C[↓ Intestinal Ca²⁺ and PO₄³⁻ absorption]:::outcome C --> D[Hypocalcemia + Hypophosphatemia]:::outcome D --> E[Secondary Hyperparathyroidism]:::outcome E --> F[Defective bone mineralization]:::outcome F --> G[Osteomalacia in adults<br/>Rickets in children]:::outcome G --> H[Looser's zones + muscle weakness]:::outcome ``` **Key Point:** The presence of **Looser's zones** is virtually diagnostic of osteomalacia and is rarely seen in other metabolic bone diseases. ### Why This Is Osteomalacia, Not Rickets - **Age:** 38 years (adult) → osteomalacia - **Growth plates:** Closed (no metaphyseal widening expected) - **Clinical presentation:** Bone pain + muscle weakness in adult - **Radiological finding:** Looser's zones are characteristic of osteomalacia **Mnemonic: OSTEO-MALACIA** = **O**lder adults, **S**oft bones, **T**enderness, **E**levated ALP, **O**steoid accumulation; **M**uscle weakness, **A**lkaline phosphatase ↑, **L**ooser's zones, **A**dult presentation, **C**alcium ↓, **I**ntestinal absorption ↓, **A**lbumin normal [cite:Robbins 10e Ch 26]