**Core Concept**
RAVULIZUMAB is a humanized monoclonal antibody that targets the complement protein C5, thereby inhibiting the formation of the membrane attack complex (MAC) and preventing complement-mediated damage. It is used to treat certain conditions involving excessive activation of the complement system.

**Why the Correct Answer is Right**
Ravulizumab works by binding to C5 and preventing its cleavage into C5a and C5b. This blocks the formation of the MAC, which is a critical component of the complement system that can cause cell lysis and tissue damage. By inhibiting the complement system, ravulizumab is effective in treating conditions such as paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome (aHUS), and certain types of immunomodulatory disorders. The drug's mechanism of action is distinct from that of eculizumab, another C5 inhibitor, and offers a longer duration of action due to its half-life of approximately 13 days.

**Why Each Wrong Option is Incorrect**
**Option A:** While ravulizumab is used to treat certain hematological disorders, it is not primarily used for the treatment of sickle cell disease.
**Option B:** The drug is not primarily used for the treatment of autoimmune disorders such as lupus or rheumatoid arthritis, although it may be used in certain cases to modulate the immune response.
**Option C:** Ravulizumab is not a chemotherapy agent and is not used to treat cancer.

**Clinical Pearl / High-Yield Fact**
Ravulizumab has a unique dosing regimen, with a single loading dose of 10 mg/kg, followed by maintenance doses of 2.5 mg/kg every 8 weeks. This dosing strategy allows for a prolonged duration of action and minimizes the risk of adverse effects.

**Correct Answer: D. Paroxysmal Nocturnal Hemoglobinuria (PNH).**