Which osmotic agent pulls water out of the extracellular space of edematous brain tissue?

• A. Furosemide
• B. Hypertonic saline
• C. Mannitol
• D. Propofol

Answer: (C)

The main idea is that reducing brain edema relies on an osmotic therapy that raises the osmolality of the blood, so water is drawn out of the brain tissue into the bloodstream. Mannitol does exactly this when given IV—it's an osmotic agent that increases plasma osmolality, creating a gradient across the brain–blood barrier. Water moves from the edematous brain’s extracellular and intracellular spaces into the vascular space, which reduces brain volume and intracranial pressure. That’s why it’s considered the classic osmotic agent for this purpose.

Hypertonic saline works similarly by increasing serum osmolality, but mannitol is the prototypical example often highlighted for pulling water out of edematous brain tissue. Furosemide is a diuretic that can help remove fluid overall but doesn’t create the osmotic gradient needed to draw water from the brain. Propofol isn’t an osmotic agent; it’s used for sedation and can help lower metabolic demand but doesn’t remove brain water via osmotic effects.

In practice, monitor electrolytes, osmolality, and renal function, and be aware of risks like dehydration, electrolyte shifts, and potential rebound edema.