The biggest mistakes that students make in taking exams relate to the following:
Inability to make use of the information given
Ignoring important diagnostic information
Using a single finding as an exclusionary criterion
Expecting a trick when there is none
About half of the knowledge base relates to normal structure and function. That includes vocabulary and definitions - something you could get out of a medical dictionary. If there is a term or a definition you do not know, that is information you cannot use to aid your reasoning process.
There is a temptation to rush forward and pick the first thing you see, or jump to conclusions without thinking the problem through. Force yourself to read the question thoroughly and then analyze ALL the information. Don't miss the 'big hairy thing.'
Ignoring the advice about medicine's analog nature, students may apply 'all or none' criteria to questions with multiple pieces of information, and throw out or ignore key bits of information.
If you start second-guessing yourself, and suspect the situation is more complex than it really is, or that the question has some hidden meaning, then you will fail. High quality questions are constructed with one good answer, and after reading the question stem it should be obvious what that answer is.