Heart failure (HF) is a clinical syndrome in which the heart cannot pump enough blood to meet the body's metabolic demands, or can only do so at abnormally elevated filling pressures. It is one of the most common reasons for hospitalization in adults over 65 in the United States, a leading cause of 30-day readmission, and a consistently top-tested topic on the NCLEX-RN. HF is classified by the side of the heart affected — left-sided (most common), right-sided, or biventricular — and by ejection fraction: HFrEF (reduced, EF less than 40%), HFmrEF (mildly reduced, EF 40 to 49%), or HFpEF (preserved, EF 50% or greater). The ejection fraction distinction is clinically critical because it drives pharmacologic management: HFrEF responds to guideline-directed medical therapy (ACE inhibitors or ARNIs, beta-blockers, mineralocorticoid antagonists, and SGLT2 inhibitors), whereas HFpEF is managed largely through volume control and treatment of underlying conditions such as hypertension and atrial fibrillation. The American College of Cardiology and American Heart Association also stage HF (A through D) to emphasize prevention in at-risk people before structural change occurs. For the NCLEX-RN...