That's exactly what I expected. You are in the beginning of your prep. That's why you have this confusion and lack of confidence.
This is perfectly normal, questions at the end of each Kaplan's chapter frequently refer to a material that is not in the chapter and that material you'll cover when you read the other chapters.
Do not open Kaplan Qbook until you finish a whole book. For example finish all of the biochemistry book then do the biochemistry blocks in Kaplan Qbook. This way you'll see that you are doing much better.
Also, don't jump from book to book. Focus on one book, finish it then go to the next. The best sequence when reading Kaplan notes is this
1- Anatomy 2- Physiology 3- Biochemistry 4- Microbiology 5- Pathology 6- Pharmacology 7- Behavioral sciences.
Read more
Do not look at your friends and what they are reading. By the time you finish all Kaplan books you'll feel much better.
Another thing, as you do more and more questions in the future you'll see your performance getting better and better as you learn all the tricks and topics start to be repeated.
Also, you may need to read Kaplan notes twice and perhaps three times before you have a good grasp of the USMLE Step 1 topics. See
this thread that discusses how many times you should repeat Kaplan notes.
Biochemistry is
one of the most difficult subjects and I suggest you steer away from it for the time being. Don't let it frustrate you.
Kaplan notes is perfectly enough for Step 1 preparation see this thread
http://www.usmle-forums.com/usmle-s.../786-usmle-step-1-preparation-books-plan.html
Good luck in your prep and don't hesitate to vent out your fears and confusions here with us
