My downstairs water pressure just dropped significantly. What's the usual culprit?

Sudden pressure loss downstairs often points to a specific issue with that line or its fixtures, not the main supply. First, check any dedicated shut-off valves for the downstairs area; ensure they're fully open. Next, unscrew and clean the aerators on all affected faucets downstairs, as mineral buildup is a common culprit. If those simple steps don't restore pressure, you might have a blockage in the branch line or a failing pressure-balancing valve in a shower, which would require a professional to diagnose and fix properly.