**Question:** Most commonly used treponemal test in diagnosis of syphilis is:

A. Treponemal Pandy's Test
B. Non-Treponemal Tests (e.g., Rapid Plasma Reagin - RPR)
C. HIV Test
D. Serum Sickness

**Correct Answer:** B. Non-Treponemal Tests (e.g., Rapid Plasma Reagin - RPR)

**Core Concept:** Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum. Diagnosis of syphilis involves both Treponemal tests (positive results indicate active infection) and Non-Treponemal tests (negative results indicate active infection, while positive results indicate a past infection).

**Why the Correct Answer is Right:** Non-Treponemal tests, such as Rapid Plasma Reagin (RPR) and Fluorescent Treponemal Antibody-Absorbed Test (FTA-ABS), are used to detect antibodies against Treponema pallidum. These tests are positive in active syphilis and also remain positive after treatment, indicating past infection. This makes them valuable in diagnosing early syphilis and assessing treatment response.

**Why Each Wrong Option is Incorrect:**

A. Treponemal Pandy's Test: This test is specific to Treponema pallidum, but it is less sensitive than Non-Treponemal tests and gives false-negative results in early syphilis.

C. HIV Test: HIV testing is used to diagnose human immunodeficiency virus infection and is not specific for syphilis.

D. Serum Sickness: This is a hypersensitivity reaction to certain drugs or antigens, not a diagnostic test for syphilis.

**Clinical Pearl:** In clinical practice, combining both Treponemal (e.g., Treponemal Pandy's Test or FTA-ABS test) and Non-Treponemal tests (e.g., RPR or VDRL) can help to diagnose syphilis accurately and evaluate treatment response. Negative Treponemal test combined with positive Non-Treponemal test indicates a past syphilis infection, while positive Treponemal test and negative Non-Treponemal test indicate active syphilis. A positive test result should be confirmed with a confirmatory test (e.g., FTA-ABS or TPPA).