A 52-year-old male construction worker from Delhi presents with a 3-month history of painful, burning feet and hands, with a sharply demarcated erythematous rash on sun-exposed areas of his face, neck, and forearms. He has diarrhea on most days and reports poor appetite. On examination, he is irritable and confused. His diet consists mainly of corn and legumes with minimal vegetables, eggs, or meat. What is the most likely diagnosis?

## Clinical Diagnosis: Niacin (Vitamin B3) Deficiency — Pellagra ### The "4 Ds" of Pellagra **Mnemonic: "4 Ds of Pellagra"** - **Dermatitis** (symmetric, sun-exposed areas — photosensitive rash) - **Diarrhea** (chronic, often bloody) - **Dementia** (irritability, confusion, progressing to psychosis) - **Death** (if untreated) **Key Point:** This patient has all four cardinal features of pellagra. The sharply demarcated erythematous rash on sun-exposed areas (face, neck, forearms) is pathognomonic — it follows the distribution of sun exposure and is called **Casal's collar** when it encircles the neck. ### Pathophysiology **High-Yield:** Niacin (vitamin B3) is a precursor for NAD^+^ and NADP^+^, essential cofactors in energy metabolism and DNA repair. Deficiency impairs: 1. Cellular energy production (ATP synthesis) 2. DNA repair mechanisms 3. Neurotransmitter synthesis (serotonin, dopamine) This explains the multi-system involvement: skin (high cell turnover), GI tract (mucosal damage), and CNS (neurotransmitter depletion). ### Why This Patient Is at Risk | Risk Factor | Present in This Patient | Explanation | |---|---|---| | **Corn-based diet** | ✓ | Niacin in corn is bound (unavailable) unless treated with alkali | | **Low animal protein** | ✓ | No eggs/meat = no dietary niacin or tryptophan (precursor) | | **Poor socioeconomic status** | ✓ | Construction worker, limited food diversity | | **Malabsorption** | Implied (diarrhea) | Chronic diarrhea reduces niacin absorption | | **Alcohol use** | Not stated | Would increase risk further | **Clinical Pearl:** In populations dependent on corn as the staple grain, pellagra is endemic unless the corn is treated with lime (nixtamalization), which releases bound niacin. This is why pellagra is rare in Mexico (where corn is nixtamalized) but was common in the American South and parts of India. ### Differential Diagnosis: Why Not the Others? | Feature | Niacin Deficiency | B6 Deficiency | B12 Deficiency | Zinc Deficiency | |---|---|---|---|---| | **Photosensitive dermatitis** | ✓ (Casal's collar) | ✗ | ✗ | ✓ (perioral/perianal) | | **Diarrhea** | ✓ (prominent) | ✗ | ✗ | ✓ | | **Dementia/CNS symptoms** | ✓ (irritability, confusion) | ✗ | ✓ (but with SCD signs) | ✗ | | **Dorsal column signs** | ✗ | ✗ | ✓ | ✗ | | **Corn-based diet risk** | ✓ (HIGH) | ✗ | ✗ | ✗ | | **Rash distribution** | Sun-exposed (sharp demarcation) | Generalized | None | Perioral/perianal | **Why not B6?** B6 deficiency causes seizures, peripheral neuropathy, and seborrheic dermatitis (not photosensitive), but NOT the classic 4 Ds or Casal's collar. **Why not B12?** B12 deficiency presents with subacute combined degeneration (dorsal column signs, hyperreflexia, Romberg sign), not the photosensitive dermatitis or diarrhea seen here. **Why not zinc?** Zinc deficiency causes perioral and perianal dermatitis (acral distribution), alopecia, and diarrhea, but NOT the photosensitive rash on face/neck/forearms or the CNS symptoms of pellagra. ### Diagnostic Confirmation **High-Yield:** Laboratory findings in pellagra: - **Serum niacin level** (low) - **Urinary N-methylnicotinamide** (low — main niacin metabolite) - **Elevated tryptophan levels** (substrate accumulates when NAD synthesis is blocked) - **Elevated 3-hydroxyanthranilic acid** (tryptophan metabolite) ### Treatment **Key Point:** Niacin deficiency is one of the few vitamin deficiencies that responds dramatically to supplementation: - **Nicotinamide 100–500 mg daily** (preferred; does not cause flushing) - **Nicotinic acid 50–100 mg daily** (causes vasodilation/flushing) - Dermatitis resolves in days; diarrhea in 1–2 weeks; CNS symptoms take weeks to months **Warning:** If CNS symptoms (dementia) are allowed to progress, they may become irreversible. [cite:Park 26e Ch 8; Harrison 21e Ch 375]