Q. A 55-year-old woman presents to the clinic with complaints of flushing, diarrhea, and wheezing for the past several months. These symptoms seem to worsen with stress and exercise. She also mentions unintentional weight loss over the past few months. Physical examination is notable for telangiectasias on her skin and palpable mass in her right lower quadrant. Laboratory evaluation reveals increased levels of a specific substance in her urine. In the context of this patient's clinical presentation, the precursor for the high levels of this specific substance in her urine originates from which of the following amino acids?