Is there such a thing as too much of an answer?

For example, “6 marks – discuss the ethical issues around such and such”

Lets say it’s the last question of the exam and I have plenty of time left and I have lots of points come to mind for this question. Can I write an answer that looks like it could be for a 10 mark question?
In other words, am I going to be marked down for making 4 or 5 points when they only wanted 3 points? Will they choose my best points to mark?

Thank you,
Dan