Which drug is known to decrease cerebrospinal fluid (CSF) production?

The drug that decreases cerebrospinal fluid (CSF) production is omeprazole. This medication is a proton pump inhibitor primarily used to reduce gastric acid production, but it has also been shown to impact CSF dynamics. Omeprazole effectively decreases CSF production by inhibiting the carbonic anhydrase enzyme within the choroid plexus, thereby reducing the secretion of bicarbonate, which is crucial for CSF formation.

Other options listed, while they may have clinical significance in other contexts, do not primarily affect CSF production. For example, furosemide is a loop diuretic that primarily promotes diuresis and is used to treat fluid overload, but it does not directly decrease CSF production. Spironolactone, an aldosterone antagonist, also does not influence CSF dynamics and mainly functions in potassium sparing diuresis and managing hypertension. Mannitol, an osmotic diuretic, is typically used to decrease intracranial pressure by drawing fluid out of the brain, but it does not reduce the actual production of CSF.

Therefore, omeprazole is the drug that directly decreases CSF production through its action on the choroid plexus.