Inability to maintain an erection = erectile dysfunction. So now the question is "Why?"
Fatigue, difficulty sleeping, difficulty concentrating is starting to sound like depression. "Difficulty concentrating" might be interpreted as impaired executive function or the beginnings of vascular-related dementia (dementia related to small but numerous cerebral infarcts), but on Step 1 dementia will be blatant (i.e., "lost his way home," "wandering," etc.).
Depression is actually common after a debilitating event like stroke, as you might expect. With depression comes a loss of sexual interest and desire—that is decreased libido.
One can make the argument that a "vascular patient" might have some issues with his "pipes" (arteriosclerosis, parasympathetic/sympathetic dysfunction) and, for this reason, nocturnal erection should be decreased; but note that nothing is mentioned about long-standing vascular disease (no hx of hypertension).
As a result, the best answer choice here is C. (Libido decreased but nocturnal erections normal.) The big question I have is, how the heck does this guy know he's hard when he's asleep!!? :p
The amount of nocturnal erections is decreased, I think (atherosclerotic problem); but I thought that it was a typical case of a patient suffering from depression after a serious illness, therefore => decreased libido.
This is a controversial one, but it seems the consensus is that pt had sxs of major depression, and thus his libido was most likely down. But structurally/blood flow–wise, he was fine, so nocturnal erections were normal. So, concept NBME wants us to realize is that we should screen for depression in pts who complain of sexual dysfunction? Or ask about sex in pts who display sxs of depression, like that patient had in the stem of the Q.