**Core Concept**
The question involves the diagnosis and monitoring of syphilis, a sexually transmitted infection caused by *Treponema pallidum*. The key test used to monitor treatment response in syphilis is a nontreponemal test, which detects antibodies against cardiolipin in the serum. These tests are used to track the decline of antibody levels as the infection resolves.

**Why the Correct Answer is Right**
VDRL (Venereal Disease Research Laboratory) is a nontreponemal test that detects antibodies formed in response to cardiolipin. It is widely used to monitor treatment response in syphilis because its titer decreases significantly as the infection is cured. Unlike treponemal tests (like FTA-ABS or TPI), which remain positive after treatment, VDRL titers fall to non-reactive levels in successful therapy. This makes it ideal for serial monitoring.

**Why Each Wrong Option is Incorrect**
Option B: FTA-ABS (Fluorescent Treponemal Antibody Absorption) is a treponemal test that remains positive even after successful treatment. It cannot be used to monitor response because it does not decrease with cure.
Option C: TPI (Treponemal Pallidum Immunodiffusion) is also a treponemal test and remains positive post-treatment. It is not useful for monitoring treatment response.
Option D: RPR (Rapid Plasma Reagin) is a nontreponemal test, similar to VDRL, and is often used in clinical practice. However, VDRL is more standardized and widely accepted in monitoring, especially in resource-limited settings. While RPR is valid, VDRL is the preferred choice in this context.

**Clinical Pearl / High-Yield Fact**
Always use a nontreponemal test (like VDRL or RPR) to monitor treatment response in syphilis. A falling titer to negative or non-reactive is a key indicator of successful therapy. Treponemal tests like FTA-ABS or TPI are confirmatory but not useful for monitoring.

✓ Correct Answer: A. VDRL