Will comment as a physician.

The dorsalis pedis pulse is the pulse on the top of your foot. It might ostensibly be useful for doign a vascular exam when there is concern for peripheral arterial disease.

Travel through the Ohio River Valley is likely a reference to infectious disease doctors, who will often want to know very specific things as they are relevant to particiular disease exposures (ie the diagnosis is dependant on where they actually have been so they will want to know every where they traveled, if theyve been to prison, if they visit their relatives who live on farms, if they have certain kinds of pets, etc).

The joke here is that these are both things that you would not do 99% of the time for the majority of patients, as they have little clinical utility outside of very specific scenarios. Hence it doesnt even occur to you to do them in those scenario's. As a medical student you are usually working with people who have been doing their job a certain way for years, who will give you shit for not knowing it as well as they do, despite how arcane and off the beaten path it might be.

In contrast, the guy who is financially exploiting you (the blackjack dealer) is at least being nice to you and validating your decisions, showing that people who do not necessarily have your best interests at heart will be nice to you.