A 35-year-old woman from rural India presents with bilateral angular cheilitis, glossitis, and a photosensitive dermatitis on sun-exposed areas. She works as a manual laborer with a diet predominantly of polished rice and lentils. Which vitamin deficiency is the most common cause of this clinical presentation?

Explanation

## Clinical Presentation Analysis **Key Point:** The triad of **photosensitive dermatitis on sun-exposed areas**, glossitis, and angular cheilitis in a patient with a diet predominantly of polished rice and lentils is the classic presentation of **pellagra**, caused by **Vitamin B3 (niacin) deficiency**. ### Why Niacin (B3) Deficiency — Not Riboflavin (B2)? | Feature | Niacin (B3) Deficiency (Pellagra) | Riboflavin (B2) Deficiency | Thiamine (B1) Deficiency | |---------|-----------------------------------|---------------------------|--------------------------| | **Photosensitive dermatitis (sun-exposed)** | ✓ **(hallmark — Casal's necklace)** | ✗ (seborrheic, not photosensitive) | ✗ | | **Glossitis** | ✓ (beefy red tongue) | ✓ (magenta tongue) | ✗ | | **Angular cheilitis** | ✓ (can occur) | ✓ (hallmark) | ✗ | | **Diarrhea** | ✓ | ✗ | ✗ | | **Dementia / neuropsychiatric** | ✓ (3 Ds: Dermatitis, Diarrhea, Dementia) | Minimal | Wernicke-Korsakoff | | **Corneal vascularization** | ✗ | ✓ (unique sign) | ✗ | **High-Yield:** The **photosensitive dermatitis on sun-exposed areas** is the single most discriminating feature here. This is the hallmark of pellagra (niacin deficiency), not riboflavin deficiency. Riboflavin deficiency causes seborrheic dermatitis in non-photosensitive distributions. The classic "Casal's necklace" — a hyperpigmented, scaly rash around the neck — is pathognomonic for pellagra. ### Dietary Basis **Clinical Pearl:** Pellagra is classically associated with: - Diets predominantly of **maize (corn) or polished rice** — both poor in bioavailable niacin - Low intake of **meat, fish, eggs, and dairy** (rich niacin sources) - Lentils provide some niacin but are insufficient when polished rice dominates - Niacin can also be synthesized from tryptophan (60 mg tryptophan → 1 mg niacin), but this pathway is inadequate when dietary protein is poor ### The 4 Ds of Pellagra (Park's Textbook of Preventive and Social Medicine) 1. **Dermatitis** — photosensitive, symmetric, on sun-exposed skin 2. **Diarrhea** — due to mucosal inflammation of the GI tract 3. **Dementia** — neuropsychiatric manifestations 4. **Death** — if untreated ### Biochemical Basis Niacin (as NAD⁺/NADP⁺) is essential for over 400 oxidation-reduction reactions. Deficiency impairs energy metabolism in rapidly dividing epithelial cells (skin, GI mucosa), causing the characteristic mucocutaneous and neurological manifestations. **Reference:** Park's Textbook of Preventive and Social Medicine (25th ed.); Harrison's Principles of Internal Medicine (21st ed.), Chapter on Vitamin Deficiencies.