With radio buttons you are supposed to be able to tab to the set of buttons (at which point you will see it focused), then use arrow keys to select the desired button. However, in a quiz question, try these steps (I just reproduced it on school.moodledemo.net):

1 Create a new (or use existing) multiple-choice question with radio buttons.
2 Preview or attempt the question.
3 Try to get to the radio buttons using the keyboard by pressing Tab.

There are 2 invisible tab stops (where there is a tab stop but no visible focus marker anywhere) and then you get past the radio buttons.

The first invisible tab stop actually works, if you know what you're doing - if you tab there then press the 'Down' key, the selection and focus both appear and you can actually use the question. However, keyboard users have no way of knowing this and shouldn't have to play hunt-the-focus. Also, there shouldn't be a second tab stop (there should be no 'useless' tabstops), so it's a fairly bad accessibility bug.

This seems to have been introduced by MDL-5311. If the problem is that we can't display focus because it shows focus on the selected button and there isn't one, then the correct behaviour would probably be that when you tab to the first radio button it should automatically show it as selected.