If you find yourself trying to force what you know to fit the question, you’re probably straying into irrelevance. Don’t start with what you know and try to write it down come what may, start with the question and then answer it. You may feel that it’s tragic not to let the examiner see that you know a lot about another subject but we cannot give marks for irrelevant material, however interesting it may be.
Read the question and answer it. Be brutal with yourself so that your examiner won’t have to be!
Quick test for relevance: if I had to defend this material in court, would I be confident in front of the judge? If I had to take part in a debate on television, would I choose to use these points?