If you want to test micro only, I suggest micro by itself. Testing by system leaves you to narrowly picking answer choices based on your current knowledge of the system. For instance, if you know you're in the renal section with micro and you're reading about a patient with pulmonary HTN and painless hepatitis, you're going to expect Schistosoma haematobium as the answer before even considering any other answers. This is a horrible example but I think you get the idea.
