**Core Concept**
Syphilis is a sexually transmitted infection caused by the bacterium *Treponema pallidum*. The infection can progress through various stages, including primary, secondary, latent, and tertiary syphilis. Early detection and treatment are crucial to prevent long-term complications.

**Why the Correct Answer is Right**
After completing 2 courses of complete therapy, the primary goal is to confirm the resolution of the infection. The investigation of choice is the **RPR (Rapid Plasma Reagin) or VDRL (Venereal Disease Research Laboratory) test**, which detects the presence of antibodies against *Treponema pallidum*. These tests are non-treponemal and measure the level of antibodies against the lipoidal material released from damaged host cells and the treponeme itself.

**Why Each Wrong Option is Incorrect**

**Option A:** **Darkfield microscopy** is a technique used to visualize *Treponema pallidum* in fluid samples from early syphilis lesions. However, it is not used for detection after treatment, as the infection has already been cleared.

**Option B:** **T. pallidum** PCR (Polymerase Chain Reaction) is a molecular diagnostic test that can detect the genetic material of *Treponema pallidum*. While it is highly sensitive, it is not typically used for routine post-treatment surveillance.

**Option C:** **Blood cultures** are not a reliable method for diagnosing syphilis, as *Treponema pallidum* is difficult to culture in vitro.

**Clinical Pearl / High-Yield Fact**
Non-treponemal tests like RPR and VDRL can remain positive for many years after successful treatment, and their titers may not directly correlate with disease activity. However, a significant decrease in titers over time can indicate successful treatment.

**Correct Answer:** A. RPR or VDRL test.