Diarrhea, Dermatitis, Dementia: What One Cause of All?

A 45-year-old man presents with a chief complaint of worsening diarrhea over the past several months. A scaly hyperpigmented rash is noted on the dorsum of his hands, neck, and shins (Figure). He has recently noted more difficulty sleeping and problems with his memory. Which of the following vitamin deficiencies may explain his symptoms?

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