What does theophylline increase in the kidneys to promote diuresis?

Theophylline is a drug that serves multiple purposes, one of which is to promote diuresis, or increased urine production. It does this primarily by increasing glomerular filtration.

When theophylline is administered, it affects the kidneys in a way that enhances the filtration process at the glomeruli, which are the tiny filtering units within the kidneys. The increased glomerular filtration rate (GFR) results from the drug's ability to relax blood vessels through vasodilation, enhancing blood flow to the kidneys and promoting the filtering of blood into urine.

Although other factors such as renal blood flow can indirectly influence diuresis, the direct action of theophylline is primarily on the glomeruli, increasing their ability to filter the blood more effectively. This increase in glomerular filtration leads to greater urine output, making it the correct answer.