Niacin (Vitamin B3) deficiency

A 15-year-old boy is brought in by his parent to the clinic for evaluation of a skin rash over his chest and arms for the past 2 weeks. The patient has noticed that the rash becomes pruritic and painful to touch when he goes outside to play soccer. He also complains of loose stools over the last week. Three months ago, he was diagnosed with latent tuberculosis and was started on isoniazid. He and his family immigrated from India 9 months ago. Temperature is 37.0°C (98.6°F), pulse is 74/min, respirations are 16/min, and blood pressure is 115/75 mmHg. BMI is 20 kg/m2. Physical examination reveals scaly erythematous patches and hyperpigmentation of the arms, chest and skin below the knees. Motor strength is 5/5 in all four extremities. Sensation is intact throughout. Which of the following amino acids is a precursor of the vitamin that is most likely deficient in this patient?