What is the appearance of a reactive RPR test?

A reactive RPR (Rapid Plasma Reagin) test indicates the presence of antibodies typically associated with syphilis. In a reactive test, the serum often shows characteristic agglutination, which could appear as black clumps or particles suspended in the serum. This agglutination is a result of the reaction between reagin antibodies in the patient's serum and the cardiolipin-cholesterol-lecithin antigen used in the RPR test.

The clarity or color of the liquid can vary, but the defining feature of reactivity is the visible aggregation or clumping that occurs. This is critical for healthcare providers to interpret the results correctly, as a reactive RPR test would warrant further confirmation tests to diagnose a syphilis infection. The other choices do not accurately depict the hallmark appearance of a reactive RPR test, as they lack the specific symptom of clumping that indicates a positive result.