A 6-year-old boy from rural Maharashtra presents with a symmetric, scaly erythematous rash around the neck in a **C/V pattern** (marked **A** in the diagram). The rash is photodistributed and worsens in summer. His family subsists primarily on a corn-based diet. On examination, he also has chronic diarrhea and behavioral changes. Which of the following is the most likely diagnosis and the primary mechanism underlying the photosensitive distribution of this rash?

Explanation

## Why "Pellagra due to niacin deficiency; impaired NAD/NADP-dependent DNA repair leads to photosensitivity" is right The **Casal Necklace** (marked **A**) — a symmetric, photodistributed dermatitis in the **C/V pattern** around the neck — is pathognomonic for **pellagra**, caused by niacin (vitamin B3) deficiency. The mechanism is that niacin is essential for synthesis of NAD and NADP coenzymes, which are critical for DNA repair pathways. Without adequate niacin, cells cannot repair UV-induced DNA damage, resulting in photosensitive dermatitis in sun-exposed areas. The clinical presentation of the "4 D's" (Dermatitis, Diarrhea, Dementia, Death) confirms pellagra. In this Indian context, a corn-based diet is a recognized risk factor, as niacin in corn is bound and unavailable unless the corn is treated with alkali (nixtamalization), a practice not universal in rural India. Treatment is niacinamide 100 mg three times daily. ## Why each distractor is wrong - **Dermatitis herpetiformis due to gluten sensitivity; IgA-mediated immune response to sun exposure**: DH presents with intensely pruritic vesicles and papules, typically on extensor surfaces (elbows, knees, buttocks), NOT as a symmetric necklace pattern around the neck. The rash is not photodistributed in the classic C/V pattern. DH is associated with celiac disease, not corn-based diets or the systemic 4 D's of pellagra. - **Hartnup disease with secondary niacin deficiency; tryptophan malabsorption impairs endogenous niacin synthesis**: While Hartnup disease IS a cause of pellagra-like dermatitis (due to impaired neutral amino acid transport and loss of tryptophan, which is a precursor for endogenous niacin synthesis), the question stem describes a patient with a corn-based diet and no mention of family history or recurrent episodes triggered by sun exposure alone. Hartnup disease is autosomal recessive and rare; the dietary history makes primary niacin deficiency the more direct diagnosis. Additionally, Hartnup disease typically presents with intermittent photosensitive dermatitis and cerebellar ataxia, not the classic Casal Necklace pattern. - **Porphyria cutanea tarda due to hepatic enzyme deficiency; photosensitive blistering from porphyrin accumulation**: PCT presents with blistering, erosions, and scarring on sun-exposed areas (hands, face), NOT a symmetric necklace pattern. The rash is bullous, not the scaly erythematous dermatitis of the Casal Necklace. PCT is associated with hepatitis C, HIV, or alcohol abuse, not a corn-based diet. The systemic symptoms (diarrhea, dementia) are not features of PCT. **High-Yield:** The **Casal Necklace** is pathognomonic for pellagra; the photodistribution reflects impaired NAD/NADP-dependent DNA repair in niacin deficiency. Always think "4 D's" and dietary risk factors (corn, alcoholism, malabsorption, Hartnup disease, carcinoid syndrome, INH use) when you see this rash. [cite: Harper 32e Ch 44; Park 26e]