Which option best explains how mannitol reduces cerebral edema?

Which option best explains how mannitol reduces cerebral edema?

• A. It increases renal perfusion
• B. It reduces CSF production
• C. It reduces cerebral edema by drawing water out of brain tissue
• D. It causes hypotension

Answer: (C)

Mannitol works by creating an osmotic gradient that pulls water out of swollen brain tissue into the bloodstream. When given IV, it raises plasma osmolality, so water shifts from intracellular and interstitial brain compartments into the vascular space, reducing cerebral edema and intracranial pressure. This is why drawing water out of brain tissue best explains its effect. Other options aren’t the mechanism—increased renal perfusion or reduced CSF production aren’t how mannitol lowers edema, and hypotension is a possible side effect, not the therapeutic action.

Mannitol works by creating an osmotic gradient that pulls water out of swollen brain tissue into the bloodstream. When given IV, it raises plasma osmolality, so water shifts from intracellular and interstitial brain compartments into the vascular space, reducing cerebral edema and intracranial pressure. This is why drawing water out of brain tissue best explains its effect. Other options aren’t the mechanism—increased renal perfusion or reduced CSF production aren’t how mannitol lowers edema, and hypotension is a possible side effect, not the therapeutic action.