Q. A 30-year-old woman presents to the emergency department with severe, colicky right flank pain that radiates to the groin. She reports having similar episodes in the past. She has a family history of similar problems. On examination, her vitals are stable. Abdominal examination reveals right renal angle tenderness. A CT scan of the abdomen shows a radio-opaque stone in the right ureter. Urinalysis reveals hexagonal crystals and a positive sodium cyanide nitroprusside test. Which of the following amino acids is most likely to be found in excess in this patient's urine?