Are you sure it’s meant to be a 16 marker? A 16 marker for this would be ‘discuss the biological approach to OCD’ - that’s where you talk about neural explanations and genetic explanations.
But since the question is asking ‘discuss neural explanations for OCD’ it should be an 8 marker, and that’s where you first describe what it is (the idea that physical and psychological characteristics are determined by the behaviour of the nervous system, in particular the brain and individual neurones etc.), then you evaluate two points (it could be a strength and a weakness or two strengths/two weaknesses)