**Question:** Rituximab is used in all EXCEPT:

A. Chronic lymphocytic leukemia (CLL)
B. Primary central nervous system lymphoma (PCNSL)
C. Myasthenia gravis
D. Acute lymphoblastic leukemia (ALL)

**Core Concept:**
Rituximab is a monoclonal antibody used for the treatment of certain types of lymphoma and autoimmune disorders involving B cells. It is directed against the CD20 antigen expressed on the surface of mature B cells, leading to apoptosis (cell death) of these cells.

**Why the Correct Answer is Right:**
Rituximab is primarily used in the treatment of non-Hodgkin's lymphoma (NHL) and Hodgkin's lymphoma (HL). In the context of the question, it is not indicated for CLL, a type of chronic lymphoproliferative disorder affecting mature B cells, as it is a targeted therapy and does not address the root cause of the disease.

**Why Each Wrong Option is Incorrect:**
A. Chronic lymphocytic leukemia (CLL) is a type of lymphoma, not a lymphoproliferative disorder involving mature B cells. It requires treatment targeting the underlying cellular abnormality.
B. Primary central nervous system lymphoma (PCNSL) is a type of lymphoma and is treated with Rituximab in combination with other therapies.
C. Myasthenia gravis is a neuromuscular disorder, not a lymphoproliferative disorder involving B cells. Rituximab is not indicated for neuromuscular conditions.
D. Acute lymphoblastic leukemia (ALL) is a type of lymphoproliferative disorder involving immature B cells, which is a different type of lymphoproliferative disorder requiring different treatment strategies.

**Clinical Pearl:**
Rituximab is a valuable treatment option in specific lymphoproliferative disorders involving mature B cells, such as non-Hodgkin's lymphoma (NHL) and Hodgkin's lymphoma (HL). It is essential to consider the underlying disease process and target the specific condition when selecting treatment options. In this case, Rituximab is not appropriate for diseases affecting immature or mature B cells, such as CLL, PCNSL, and ALL. Myasthenia gravis is a neuromuscular disorder, not a lymphoproliferative disorder, so Rituximab is not indicated in this condition.