**Core Concept**
The question is testing the knowledge of extramedullary relapse in Acute Lymphoblastic Leukemia (ALL), specifically the most common site of such relapse. Extramedullary relapse refers to the recurrence of leukemia in sites outside the bone marrow, such as the central nervous system (CNS), testes, or other organs. In the context of ALL, the CNS is a significant site of extramedullary relapse due to the blood-brain barrier's limited permeability.

**Why the Correct Answer is Right**
The CNS is the most common site of extramedullary relapse in ALL due to the disease's ability to infiltrate the meninges, which are the protective membranes surrounding the brain and spinal cord. This infiltration can occur through the bloodstream or by direct extension from adjacent tissues. The CNS's unique environment and the presence of specific adhesion molecules, such as CD44, facilitate the interaction between leukemic cells and the meningeal microenvironment, leading to relapse. The high frequency of CNS relapse in ALL necessitates regular monitoring and prophylactic measures, including intrathecal chemotherapy and cranial irradiation.

**Why Each Wrong Option is Incorrect**

**Option A:**
This option is incorrect because, although the testes are a site of extramedullary relapse in ALL, they are not the most common site. Testicular relapse typically occurs in post-pubertal males and is often associated with a poorer prognosis.

**Option B:**
This option is incorrect because, although the liver and spleen are secondary sites for leukemia infiltration, they are not as frequently involved as the CNS in ALL relapse.

**Option C:**
This option is incorrect because, although the lymph nodes can be involved in ALL relapse, they are not the most common site of extramedullary relapse. Lymph node involvement is more common in lymphoblastic lymphoma.

**Clinical Pearl / High-Yield Fact**
Regular CNS prophylaxis, including intrathecal methotrexate and cranial irradiation, significantly reduces the risk of CNS relapse in ALL patients. This emphasizes the importance of vigilant monitoring and prompt intervention in suspected cases of CNS involvement.

**Correct Answer:**
C. Central Nervous System