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NBME Answers

free120/Block 1/Question#32

A 20-year-old woman comes to the physician because ...

von Willebrand disease

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 +1  upvote downvote
submitted by imnotarobotbut(22),

This is a bad question. Platelet aggregation time being normal, ok fine I can see that. But VWF stabilizes factor 8 and you'd see an increase in PTT (first line next to VWF in First Aid). Why is their PTT normal?

a1_antitrypsin  Totally agree, and they give you a slight increase in PT instead +  

 +0  upvote downvote
submitted by benwhite_dotcom(37),

von Willebrand disease is by far the most common inherited bleeding diathesis. Frequently, the only laboratory abnormality is increased bleeding time (literally you prick the patient and see how long it takes them to stop bleeding). On Step, bleeding women have VWD. Bleeding boys have hemophilia.

 +0  upvote downvote
submitted by canyon_run(0),

why is the platelet aggregation test is normal in VWD? my problem is that Gp1b and VWfactor have to interact to induce a confirmational change in platelets to release ADP –> ADP binds to adp-receptor and induces Gp2b/3a which enables aggregation via fibrinogen. which would lead to abnormal aggregation? and is the ristocetin assay not a platelet aggregation test? or can it sometimes be normal and sometimes not?

benwhite_dotcom  It can be abnormal as well, depends on the subtype and severity (the wikipedia page does a decent job explaining). The most common subtype of VWD is a quantitative defect, which is often mild/nearly clinically occult and can have essentially normal laboratory testing. This is one of those questions where the labs are really there to exclude the other choices. +1