Sentence:  <WSD>Antirequisites</WSD> affect the availability of a course in the opposite direction; if the <WSD>antirequisite</WSD> of course A has been taken, then the student cannot enroll in course A. 
Meaning of the word marked with <WSD> tags:  A previous course (or learning experience) that overlaps in content with a course offering such that students may not take that course offering for academic credit.