**Core Concept**
A spinal tap, also known as a lumbar puncture, is performed to collect cerebrospinal fluid (CSF) for diagnostic purposes. In leukemia, the rationale behind performing a spinal tap is to evaluate for central nervous system (CNS) involvement, which is a significant prognostic factor and affects treatment decisions.

**Why the Correct Answer is Right**
The CNS is a common site for leukemia relapse, and CNS involvement is associated with a poorer prognosis. Performing a spinal tap allows for the detection of leukemia cells in the CSF, which is an indicator of CNS involvement. This information is crucial for determining the need for intrathecal chemotherapy, cranial radiation, or other treatments aimed at preventing CNS relapse.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not provide a clear rationale for performing a spinal tap in leukemia. While it may be true that a spinal tap is performed to evaluate for CNS involvement, it does not explain why this is particularly relevant in leukemia.
**Option B:** This option is incorrect because it does not specifically address the rationale for performing a spinal tap in leukemia. While a spinal tap may be performed to evaluate for infection or other conditions, this is not the primary reason in the context of leukemia.
**Option C:** This option is incorrect because it does not accurately describe the rationale for performing a spinal tap in leukemia. While a spinal tap may be performed to evaluate for other conditions, this is not the primary reason in the context of leukemia.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that CNS involvement in leukemia is a significant prognostic factor and affects treatment decisions. Performing a spinal tap to evaluate for CNS involvement is a critical step in managing patients with leukemia.

**Correct Answer:** C. To evaluate for central nervous system involvement.