Learn Hardcore Russian: Когда медсестра готовила укол, я отвернулся, потому что не люблю смотреть на шприц. - When the nurse was preparing the injection, I turned away because I do not like looking at the syringe.

Breakdown of Когда медсестра готовила укол, я отвернулся, потому что не люблю смотреть на шприц.

Why is it готовила, not приготовила?

Because готовила is imperfective, and here it describes an action that was in progress in the background:

Когда медсестра готовила укол... = When the nurse was preparing the injection...

The speaker’s action, я отвернулся, happened during that process.

If you used приготовила, it would usually mean the preparation was completed:

Когда медсестра приготовила укол... = When the nurse had prepared the injection...

That changes the timing slightly. In your sentence, the point is that while she was preparing it, the speaker turned away.

What exactly does укол mean here?

Укол literally means something like a prick or a shot, but in medical contexts it usually means an injection.

So:

It does not mean the syringe itself. That is шприц.

So the sentence contains both:

Why is it отвернулся, not ?