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The MR is a cytosolic nuclear receptor found in the distal nephron and several other tissues. When activated, it promotes expression of ENaC and Na⁺/K⁺-ATPase, increasing sodium and water reabsorption while increasing potassium and hydrogen excretion. The lesson contrasts aldosterone and synthetic agonists, including fludrocortisone, with MR antagonists such as spironolactone, eplerenone, and finerenone. It connects these mechanisms to uses including adrenal disorders, hypertension, heart failure, and fluid accumulation, while highlighting clinically important risks such as changes in blood pressure, edema, electrolyte disturbance, and endocrine effects. Aldosterone synthase inhibitors and drug-interaction considerations are also introduced, providing a framework for comparing these related agents.