Which drug is administered intravenously as an NK1 antagonist?

Fosaprepitant is the correct answer because it is the intravenous formulation of aprepitant, which functions as a neurokinin-1 (NK1) receptor antagonist. NK1 antagonists are primarily used to prevent nausea and vomiting induced by chemotherapy. Fosaprepitant is specifically designed to be given via IV, providing a route of administration that can be beneficial in settings where oral medications may not be ideal, such as in patients experiencing nausea or those undergoing treatment where fast onset of action is required.

The other drugs listed do not meet these criteria. Aprepitant is the oral version of the NK1 antagonist and not administered intravenously. Darbepoetin alfa is an erythropoiesis-stimulating agent used to treat anemia and does not function as an NK1 antagonist. Syndros, while it does have antiemetic properties, is not an NK1 antagonist but rather a formulation of the cannabinoid dronabinol used for similar purposes.