**Core Concept**
Rituximab is a monoclonal antibody targeting the CD20 antigen on B lymphocytes, leading to their depletion and subsequent immunosuppressive effects. It is used in the treatment of various autoimmune and lymphoproliferative disorders.

**Why the Correct Answer is Right**
Rituximab is not typically used in the treatment of Paroxysmal Nocturnal Hemoglobinuria (PNH), a rare acquired disorder characterized by the destruction of red blood cells due to the absence of CD55 and CD59 on their surface. This is because rituximab targets B cells, which are not primarily involved in the pathogenesis of PNH. In contrast, rituximab is effective in treating conditions like Non-Hodgkin's Lymphoma (NHL), Rheumatoid Arthritis (RA), and Systemic Lupus Erythematosus (SLE), where B cells play a significant role in disease progression.

**Why Each Wrong Option is Incorrect**
**Option A:** Rituximab is indeed used in the treatment of NHL, as it can deplete malignant B cells and improve patient outcomes.
**Option C:** Rituximab is also used in RA, where it can reduce B cell-mediated inflammation and slow disease progression.
**Option D:** Rituximab is used in SLE, particularly in patients with severe disease or those who have failed other treatments, to reduce B cell activity and alleviate symptoms.

**Clinical Pearl / High-Yield Fact**
Rituximab's mechanism of action is distinct from other immunosuppressive agents, making it a valuable option for treating autoimmune diseases where B cell depletion is beneficial. However, its use is not without risks, including increased susceptibility to infections.

**✓ Correct Answer: B. PNH**