Here is the sentence again:
Davina, a good friend from my biology class, has more interest in what is
under a guy's skin than what is under his
clothes, for this reason, she is
pursuing a degree in radiography.
The comma between clothes and for is making a comma splice. At this spot, there are two complete sentences joined only with a wimpy comma. Davina, a good friend of mine, has more interest ... begins the first complete sentence. For this reason, she is pursuing a degree in radiography is the second complete sentence.
To fix this problem, you could put a period after clothes and capitalize the F in for.