<prompt> Which drug is not used for erectile dysfunction?PhenylephrineApomorphineYohimbineVardenafil  <response>: 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. 