Learn Hardcore Japanese: kinou isya ni situmon wo simasita ga, kotae wo mada moratte imasen. - Yesterday I asked the doctor a question, but I have not received an answer yet.

The particle に marks the target or recipient of an action. In 医者に質問をしました, 医者 is the person receiving the question—so you use に to show “to the doctor.”

Why is the particle を used after 質問?

質問 is a noun that pairs with the verb する (“to do”), forming 質問をする (“to ask a question”). The particle を indicates 質問 is the direct object of する, so it must be 質問をしました.

What does が mean at the end of しましたが?

Here, が is a conjunction meaning “but.” When you attach が to the end of a clause—質問をしましたが—it links to the next clause and conveys “I asked …, but ….”

What role does まだ play in this sentence?

まだ means “still” or “not yet” when used with a negative verb. In まだもらっていません, it modifies the negative state “haven’t received,” so it means “haven’t gotten it yet.”

What is going on with もらっていません grammatically?

もらう means “to receive.”
1) もらって is the te-form, connecting to
2) いる in its negative form いません, which expresses a continuing state.
So もらっていません literally means “(I) am not in the state of having received (it),” i.e. “haven’t received.”

Could I replace 質問をしました with 聞きました?

Yes, 聞く (“to ask/hear”) can often replace 質問する in casual speech:
昨日医者に聞きましたが…
However, 質問をする emphasizes the formal act of posing a question, while 聞く is more general (“I asked/heard”).

Can I say 答えはまだです instead of 答えをまだもらっていません?

You could say 答えはまだです, which simply means “the answer isn’t ready yet.”
But 答えをまだもらっていません focuses on the action of “receiving” the answer from the doctor: “I haven’t gotten the answer yet.” They’re both natural but carry slightly different nuances.