Review the item:
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 class and prefers is correct. Davina prefers ... is a main clause. A good friend from my biology class is an appositive, a noun phrase.
In the example above, the appositive is interrupting the flow of the sentence, so it requires a comma both in front and behind it.