I have struggled with this; the term "mental block" is exactly how it feels to me, too. I don't know how I got through enough revision for all of my subjects in Summer!
Maybe mixing up the studying would help? That's how I approached it with my exams. For example, I would do some studying of the subject I was interested in, and then some studying of the subject I wasn't, and then go back to the one I was interested in etc. It's a bit easier when you know that you are going to go back to the subject you like after awhile. Also, planning the revision(/work) with a timetable or something might help break it down and to have a visual aid showing you when you are going to have to study the subject(s) you are not interested in can help prepare you for it.
Also, lots of short breaks? That may help too, to get away from the biochemistry a lot and to break it into smaller bits.
That's all I can think of right now.
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"There is no wealth like intelligence and no poverty harsher than ignorance."