**Core Concept**
In childhood acute lymphoblastic leukemia (ALL), prognosis is influenced by factors such as age, white blood cell count, blast percentage, and site of involvement. Testicular involvement is a known marker of aggressive disease and resistance to therapy, reflecting poor outcomes.

**Why the Correct Answer is Right**
Testicular involvement in childhood ALL is associated with a higher risk of relapse and treatment failure. It indicates disseminated disease and may reflect a more aggressive biological phenotype. The presence of testicular infiltration is linked to increased risk of extramedullary disease and resistance to chemotherapy, leading to worse survival rates. This is particularly relevant in boys, where testicular involvement is a well-documented poor prognostic sign.

**Why Each Wrong Option is Incorrect**
Option A: A total leukocyte count of 4000–100,000 is common in ALL and not a poor prognostic factor; high counts (>100,000) are associated with worse outcomes, but this range is not considered high.
Option B: Age < 2 years is actually a **favorable** prognostic factor in childhood ALL due to better response to therapy and lower relapse rates.
Option D: Blasts in peripheral smear indicate active disease but are not independently predictive of poor outcome; they are part of the diagnostic criteria, not a standalone poor prognostic factor.

**Clinical Pearl / High-Yield Fact**
Testicular involvement in childhood ALL is a strong indicator of poor prognosis and requires intensified therapy. Always consider this in pediatric patients, especially boys, and it may prompt changes in treatment intensity.

✓ Correct Answer: C. Testicular involvement