Here is the item again:
Davina, a good friend from my biology class, prefers what is under a guy's
skin to 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 from my biology class, prefers ... 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.