What happens if I go off topic in the speaking exam?
Firstly, remember your score is based on all of your test, so don't worry if you had issues with just one question.
Going off topic will really depend on the examiner's perception of the situation. If the examiner just feels you did not understand the topic, they should ignore this particular answer as you are not being graded on your listening skills.
However, if the examiner feels that the issue is more connected to you being unable to cohesively join your ideas together, this will affect you fluency and cohesion score.