Actually Cliff is right, the real boards almost never have questions like this.....they will almost never ask you "what the most likely cause is..." without giving you some tidbit of evidence pointing to one diagnosis (given that the questions are a paragraph long, they need to fill them up with some info)...They expect you to know concepts in epidemiology and prevalence, but rarely test you directly on prevalence statistics of diseases...I.e. they won't ask you the most common cause of diarrhea as a general question, but might ask you the most common cause in someone on a camping trip, or in a 19 monther in the industrialized world having copious watery diarrhea.