This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Pharmacology
[{'content': 'Which drug is not used for erectile dysfunction? \nSelect and explain. \nA. Phenylephrine, B. Apomorphine, C. Yohimbine, D. Vardenafil', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Physiological erection occurs in response to the release of nitric oxide from noradrenergic - nor cholinergic nerves associated with parasympathetic discharge. Nitric oxide acts by activating guanylyl cyclase -| CGMP - cavernosal relaxation resulting in the erection. Thus the drugs which can be used for erectile dysfunction are: 1. PGE-l analogs eg Alprostadil 2. antagonists 3. Sildenafil (PDE - inhibitor) and its congeners tadalafil and vardenafil 4. Apomorphine (releases DA in CNS). Ref; KD Tripathi 8th ed.', 'role': 'assistant'}]