Please answer with explanation

If you try to understand what the author was trying you will get it clear, he was saying that he will come across reading some diseases then he will think that he got it and fell frightened, So, the order is read => assume you have it => fear => read =>assume you have it => frightened

Thanks Suri. Please explain a bit more on your reasoning behind option A 

A is the answer, it is pretty simple, R cannot be the closing statement, then options B and C gone, only between option A and D, A makes better sense.