A patient tested positive for syphilis by the RPR method but negative on the FTA-ABS test. Which routine laboratory test is most likely to be abnormal?

In this clinical scenario, the patient has a positive RPR (Rapid Plasma Reagin) test but a negative FTA-ABS (Fluorescent Treponemal Antibody Absorption) test. This pattern typically indicates a reactive screening test for syphilis due to non-specific antibodies but a confirmation test that does not show evidence of syphilis, potentially reflecting a false-positive result.

The activated partial thromboplastin time (APTT) is a test that measures the intrinsic pathway of coagulation and can be prolonged in various conditions, including those associated with antiphospholipid syndrome. In syphilis infections, patients may develop antiphospholipid antibodies, which can lead to a prolonged APTT. Thus, in the context of a syphilis screening with a positive RPR and negative FTA-ABS, it is plausible for the APTT to be abnormal due to the presence of these antibodies.

The other choices pertain to different laboratory tests that do not directly correlate with the serological status provided in this case. Antismooth muscle antibodies are typically associated with autoimmune liver diseases, not syphilis. Aspartate aminotransferase (AST) is often evaluated in liver function tests and its