Passage Summary: There is a test that always catches a specific sickness; Justine took the test and it came back positive, so the author is sure she is sick.

Conclusion: Justine definitely has syndrome Q.

Reasoning: The new screening test always returns a positive result for anyone who has the syndrome, and Justine's test result was positive.

Analysis: This is a classic 'Mistaken Reversal' of a conditional statement. The premise tells us that if you have the syndrome, you will test positive (S -> P), but the author concludes that because Justine tested positive, she must have the syndrome (P -> S). This ignores the possibility of 'false positives,' where someone without the syndrome still triggers a positive result. Look for an answer that describes this confusion between a condition that is necessary to get a result and one that is sufficient to guarantee a diagnosis.