There are 2 comprehensive exams for the PhD portion. I can describe what they were like based on my school (Medical College of Georgia). They may be different at other schools, and different within departments. They both are 8 hour essay exams. The first one I took before Thanksgiving of my 2nd year. It consisted of all the classes I took during the 1st year, as well as the 3 rotations. I had the option of taking the exam on two days - 4 hours each. Questions were related to lectures covered by faculty within the department only. This has recently changed to encompass all courses in the first year (molecular bio, cell bio, neuroscience, physiology, immunology, biochem). For me I had to review medical histology, cell bio and molecular bio. The 2nd exam focuses on topics related to your research. I took this 2 semesters before defending, but the timeline depends on the department. You sit down with your advisor and come up with various topics related to your research, he/she will present those topics to your committee and they will design questions based on those topics. Some departments are structured such that the 1st day is writing, and the second day is an oral exam. For me, I had a completely written exam, for which I am grateful for.
I hope this helps.
Hany