## **Core Concept**
The patient's presentation suggests a sexually transmitted infection (STI) with systemic symptoms, including a maculopapular rash, lymphadenopathy, and specific lesions around the anal area. The laboratory findings show a mild leukocytosis with a normal differential count. The clinical picture is highly suggestive of secondary syphilis.

## **Why the Correct Answer is Right**
The patient's symptoms, such as the maculopapular rash on the palms and soles, lymphadenopathy, and the presence of hypertrophic, flat, wart-like lesions (condylomata lata) around the anal area, are classic for secondary syphilis. The most useful laboratory test for diagnosing syphilis, particularly in its secondary stage, is the **Rapid Plasma Reagin (RPR) test** or the **Venereal Disease Research Laboratory (VDRL) test**, which are non-treponemal tests. These tests detect antibodies against substances (cardiolipin) released from damaged host cells and spirochetes. They are useful for screening and for monitoring disease activity and response to treatment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a complete blood count (CBC) has been provided and shows mild leukocytosis, it is not specific for diagnosing the condition suggested by the patient's symptoms.
- **Option B:** A specific treponemal test like the **Fluorescent Treponemal Antibody Absorption (FTA-ABS) test** is highly sensitive and specific for syphilis but is not typically used for monitoring treatment response due to its persistence of positivity.
- **Option C:** A **Western blot** is another confirmatory test for syphilis but is not the first line for screening or the most useful in this context compared to non-treponemal tests.
- **Option D:** Without specific details on what this option entails, we proceed with the understanding that non-treponemal tests (like RPR or VDRL) are most useful in this scenario.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that non-treponemal tests (RPR or VDRL) are used for screening and monitoring treatment response in syphilis, as they correlate with disease activity. Treponemal tests (like FTA-ABS) are used for confirmation due to their high specificity but do not correlate with disease activity.

## **Correct Answer:** .