How to answer an exam question such as "how might action potential transmission be affected by multiple sclerosis, a nervous system disease characterised by damage to neuronal myelin sheaths"

5 marking points to hit:Transmission of action potential will be slower in neurons with damaged myelin sheaths (1) because myelin is an electrical insulator (2) so increases the speed of action potential conduction (3). Action potentials jump between regions called "nodes of Ranvier", these are between myelin sheaths and contain high populations of sodium ion channels.
Difficulty with this question: questions that ask about a disease or area of science that is not part of the syllabus and/or that you are not familiar with might cause panic (especially in an exam environment!), but try not to! The main thing is learning to recognise how to apply your knowledge and key concepts to unfamiliar topics - this is not only an important part of exam technique, but will help with university interview preparation and might also help spark some more interest for the real life/scientific applications that your A Level work relates to!