**Core Concept**
Acute lymphocytic leukemia (ALL) is a type of cancer characterized by the uncontrolled proliferation of immature lymphocytes. Prognostic factors in ALL are crucial for determining the likelihood of treatment success and overall survival. These factors can be broadly categorized into good and bad prognostic signs.

**Why the Correct Answer is Right**
In the context of ALL, **high white blood cell count (WBC) at diagnosis** is considered a bad prognostic sign. This is because a high WBC count indicates a more aggressive disease with a higher likelihood of resistance to chemotherapy. The mechanisms underlying this association are not fully understood, but it is thought to be related to the presence of a larger number of leukemic cells that are more resistant to chemotherapy-induced apoptosis. The WBC count is also a reflection of the overall tumor burden, with higher counts indicating a greater tumor load.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Age at diagnosis** is an important prognostic factor in ALL, but it is not the correct answer. Children diagnosed with ALL between the ages of 1-9 have a better prognosis than those diagnosed at an older age (greater than 10 years) or at a younger age (less than 1 year).
* **Option B:** **Central nervous system (CNS) involvement at diagnosis** is a bad prognostic sign in ALL, but it is not the correct answer. CNS involvement requires a separate consideration and is not directly related to the WBC count.
* **Option C:** **Response to initial induction chemotherapy** is a good prognostic sign in ALL, not a bad one. Patients who achieve complete remission after the first course of chemotherapy have a better prognosis than those who do not.

**Clinical Pearl / High-Yield Fact**
In children with ALL, the WBC count at diagnosis is an important prognostic factor that can influence treatment decisions. A high WBC count is associated with a worse prognosis and may require more intensive treatment regimens.

**Correct Answer:** B. CNS involvement at diagnosis.