preliminary examination vs exam

preliminary examination and exam both are nouns.

Word	Noun	Adjective	Verb	Adverb
preliminary examination	Yes	No	No	No
exam	Yes	No	No	No

As nouns, exam is a hypernym of preliminary examination; that is, exam is a word with a broader meaning than preliminary examination:

preliminary examination: an examination taken by graduate students to determine their fitness to continue
exam: a set of questions or exercises evaluating skill or knowledge

Other hypernyms of preliminary examination include examination, test.

preliminary examination (noun)	exam (noun)
an examination taken by graduate students to determine their fitness to continue	a set of questions or exercises evaluating skill or knowledge