**Core Concept**
Acute lymphoblastic leukemia (ALL) prognosis is influenced by various factors, including age, initial white blood cell count, and specific genetic abnormalities. The outcome of ALL treatment depends on the presence of these risk factors, which can be used to stratify patients into different risk categories.

**Why the Correct Answer is Right**
A poor prognostic indicator in ALL is the presence of the MLL gene rearrangement. This genetic abnormality is often associated with a higher risk of treatment failure and relapse. The MLL gene is a transcription factor that regulates cell growth and differentiation, and its rearrangement can lead to the production of abnormal proteins that drive leukemia cell proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** Age at diagnosis is a prognostic factor in ALL, but older children tend to have a better prognosis than infants. Therefore, being an infant is not a poor prognostic indicator.

**Option B:** Initial white blood cell count is a prognostic factor in ALL, with higher counts associated with a poorer prognosis. However, the MLL gene rearrangement is a more specific and significant risk factor than high initial white blood cell count.

**Option C:** The presence of the ETV6-RUNX1 fusion gene is actually a good prognostic indicator in ALL, associated with a high cure rate and low risk of relapse.

**Option D:** The presence of the BCR-ABL fusion gene is associated with a poorer prognosis in ALL, but it is not as common as the MLL gene rearrangement.

**Clinical Pearl / High-Yield Fact**
The MLL gene rearrangement is a common genetic abnormality in infant ALL, and its presence should prompt aggressive treatment and close monitoring for relapse.

**Correct Answer: C. The MLL gene rearrangement is a poor prognostic indicator in acute lymphoblastic leukemia.**