This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
[{'content': 'Serotonin can be synthesized from \nSelect and explain. \nA. Tryptophan, B. Trypsin, C. Dopa, D. Epinephrine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In bacteria that synthesize tryptophan, high cellular levels of this amino acid activate a repressor protein, which binds to the trpoperon. Binding of this repressor to the tryptophan operon prevents transcription of downstream DNA that codes for the enzymes involved in the biosynthesis of tryptophan Refer kDT 7/e p170', 'role': 'assistant'}]