**Core Concept**
Denileukin diftitox is a recombinant protein that combines the IL-2 receptor-binding domain with the enzymatic domain of diphtheria toxin. This unique structure allows it to selectively target and kill cancer cells that express the IL-2 receptor.

**Why the Correct Answer is Right**
Denileukin diftitox binds to the IL-2 receptor on the surface of cancer cells. The IL-2 receptor is a heterotrimeric complex consisting of three subunits: α, β, and γ. Denileukin diftitox specifically binds to the α subunit, which is often overexpressed on the surface of cancer cells. This binding event triggers the internalization of the receptor, leading to the activation of the diphtheria toxin's enzymatic domain and subsequent cell death.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the mechanism of action of denileukin diftitox.
**Option B:** While the IL-4 receptor is a cytokine receptor, it is not the primary target of denileukin diftitox.
**Option C:** The CD25 receptor is another name for the IL-2 receptor α subunit, which is the correct target of denileukin diftitox.

**Clinical Pearl / High-Yield Fact**
Denileukin diftitox is used to treat cutaneous T-cell lymphoma (CTCL), a type of cancer that originates from T cells. Its mechanism of action provides a unique example of how targeted therapies can selectively kill cancer cells while sparing normal cells.

**Correct Answer: C. CD25 receptor**