Dehydration: This is a common issue in the elderly, especially in those with hypertension who might be on diuretics. Dehydration can lead to a decrease in urine output due to reduced fluid intake or excessive fluid loss.

Chronic Kidney Disease (CKD): Hypertension is a major risk factor for CKD, which can lead to decreased urine output as kidney function declines.
Medication Side Effects (e.g., diuretics, NSAIDs): Certain medications can affect kidney function or fluid balance, leading to decreased urine output.
Urinary Tract Obstruction: This could be due to various causes such as kidney stones, tumors, or benign prostatic hyperplasia in men, though less common in women, it could be due to pelvic tumors or other obstructive processes.
Heart Failure: Reduced cardiac output can lead to decreased renal perfusion, resulting in decreased urine output.

Sepsis: Although not immediately apparent, sepsis can cause acute kidney injury (AKI) leading to decreased urine output. It's crucial to identify and treat sepsis promptly due to its high mortality rate.
Acute Kidney Injury (AKI): Caused by various factors including medications, contrast dye, or ischemia, AKI can rapidly progress and has a high morbidity and mortality if not recognized and treated early.
Post-Renal Acute Kidney Injury: Obstruction in the urinary tract, which could be due to stones, blood clots, or tumors, is a medical emergency requiring prompt intervention.

Nephrotic Syndrome: Characterized by heavy proteinuria, hypoalbuminemia, and edema, it can lead to decreased urine output in advanced stages due to renal damage.
Amyloidosis: A condition where abnormal proteins (amyloid) accumulate in organs, including the kidneys, leading to their dysfunction and potentially decreased urine output.
Vasculitis: Inflammation of the blood vessels supplying the kidneys can lead to renal failure and decreased urine output, though it's less common in the elderly without a history of autoimmune diseases.