A drug blocking aldosterone would most directly reduce the reabsorption of:

1. Trace the hormone to the ion it controls. 2. The adrenal cortex produces aldosterone. 3. Aldosterone directly influences renal function. 4. It stimulates sodium reabsorption. 5. Blocking it would therefore reduce sodium reabsorption. _Source: OpenStax Anatomy and Physiology 2e (CC BY 4.0), Ch 25 'The Urinary System', sections 25.1-25.10_