**Core Concept**
The question is testing the ability to identify the immunophenotypic characteristics of acute lymphoblastic leukemia (ALL) with a specific clinical presentation. The underlying principle is the association between a mediastinal mass and a particular subtype of ALL.

**Why the Correct Answer is Right**
The presence of a large mediastinal mass in a patient with ALL is highly suggestive of T-cell acute lymphoblastic leukemia (T-ALL). T-ALL often presents with a mediastinal mass due to the accumulation of T-cell blasts in the thymus. Immunophenotyping of T-ALL blasts typically shows expression of T-cell markers such as CD2, CD3, CD4, CD5, and CD7, with variable expression of CD8.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because B-cell acute lymphoblastic leukemia (B-ALL) is not typically associated with a mediastinal mass. B-ALL blasts usually express B-cell markers such as CD19, CD20, and CD22, but not T-cell markers.
**Option B:** This option is incorrect because acute myeloid leukemia (AML) is a distinct entity from ALL, with different immunophenotypic characteristics and a different clinical presentation.
**Option C:** This option is incorrect because acute promyelocytic leukemia (APL) is a subtype of AML characterized by the expression of CD13 and CD33, but not T-cell markers.

**Clinical Pearl / High-Yield Fact**
The presence of a mediastinal mass in a patient with ALL should raise suspicion for T-ALL, which requires prompt recognition and treatment to avoid complications.

**Correct Answer:** C. T-cell acute lymphoblastic leukemia (T-ALL)