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School: Yale
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 +13  (nbme19#4)

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

 +0  (nbme19#13)

This is essentially a formal logic question. Logically speaking, the question asks us to identify a mechanism that tumor suppressors have which proto-oncogenes do not. In other words, what is a mechanism shared by all known tumor suppressors but not shared by any known proto-oncogenes? For that reason, it can’t be phosphorylation; sure, phosphorylation is a mechanism of tumor suppressors but it’s also a mechanism of many known proto-oncogenes.

 +0  (nbme20#15)

The more general principle: endothelia vasodilate in the presence of high CO2; you gotta get rid of that acid somehow! Can’t let it accumulate, as lower pH within a “micro-environment” affects structure/efficiency of enzymes, proteins, etc. The more acidic a local environment, the more you expect nearby vasculature to dilate (as a means of increasing flow rate, thereby ferrying off accumulate acid).

The anesthesiologist can exploit this mechanism. By hyperventilating (blowing off CO2), the brain vasculature senses a low CO2 / “hunky-dory state,” which requires no vasodilation. In other words, the vasculature does not need to continue the ATP-consuming practice of synthesizing Nitric Oxide (NO).

 +0  (nbme20#11)

As described in the question stem, this mutation occurs within an intron (a gene segment which is transcribed [DNA->RNA] but not translated). RNA splicing enzyme(s) grab RNA and “loop it”; an intron is cut out and the exons on either side of the intron are adjoined, like this:

exon1—intron—exon2 => exon1—exon2

Typically, this splicing occurs at the very edges of the intron (what I denoted with the “—” character). But in our case, a mutation within the intron is causing RNA splicing enzyme to recognize a new site: the splicer cuts within the intron (instead of at the very edge, as it should). So, we get something that looks like this:


That’s a totally different mRNA molecule, and it's going to make our β-globin protein look (and behave) awfully strange.

 +0  (nbme20#6)

This is an interesting one. I like to remember it this way: in people with narcolepsy, all the “right kinds” of sleep are happening at all the “wrong times” of day. During the day, when a power nap would typically throw you immediately into REM, this kid is only entering Stage 1 or 2 (lightest sleep = slightest noises jar him back to reality). At night, when he should peacefully drift into Stage 1, 2, and so on, he instead completely zonks out. Classic narcolepsy.

From UpToDate: “Narcolepsy can be conceptualized as a disorder of sleep-wake control in which elements of sleep intrude into wakefulness and elements of wakefulness intrude into sleep.”