The argument tells us that whenever the person has a syndrome, the test yields a positive.
S->P
It then concludes that because a person shows positive, she must have the syndrome.
P->S
This is a logical fallacy of affirming the consequent or mistaken negation, we cannot conclude that just because someone's test result is positive, they definitely have a syndrome, after all it might be a false positive.
(A) accurately describes this flaw, the claim is that S -> P, not that P -> S, therefore, it is the correct answer.