**Core Concept**
Rituximab is a chimeric monoclonal antibody targeting CD20, a surface protein expressed on B lymphocytes. It is primarily used in B-cell malignancies due to its ability to induce antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC).

**Why the Correct Answer is Right**
Rituximab binds specifically to CD20 on B cells, leading to direct cell lysis via ADCC and CDC, and also depletes B cells. It is a cornerstone in the treatment of **non-Hodgkin’s lymphoma (NHL)**, especially in follicular lymphoma and diffuse large B-cell lymphoma. Its mechanism is most effective in B-cell-derived tumors, making it highly relevant in NHL.

**Why Each Wrong Option is Incorrect**
Option B: Gastrointestinal Stromal Tumors (GISTs) are typically driven by PDGFRA or PDGFRB mutations and are treated with tyrosine kinase inhibitors like imatinib, not rituximab.
Option C: Chronic Myeloid Leukemia (CML) is managed with tyrosine kinase inhibitors (e.g., imatinib), as it involves the BCR-ABL fusion gene; rituximab has no role here.
Option D: Acute Myeloid Leukemia (AML) is a myeloid disorder with no significant CD20 expression; treatment relies on chemotherapy and targeted agents, not rituximab.

**Clinical Pearl / High-Yield Fact**
Rituximab is a key component in the treatment of B-cell lymphomas and is often used in combination with chemotherapy (e.g., R-CHOP). Always remember: **CD20 is B-cell specific**, so rituximab is only effective in B-cell malignancies.

✓ Correct Answer: A. Non-Hodgkin's lymphoma