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?