Dehydration: This is often the most common and easily reversible cause of unexplained renal failure, particularly in elderly patients or those with limited access to fluids. Dehydration can lead to a decrease in renal perfusion, resulting in acute kidney injury (AKI).

Medication-induced nephrotoxicity: Various medications, including NSAIDs, certain antibiotics, and contrast agents, can cause renal failure by inducing nephrotoxicity. The likelihood of this diagnosis depends on the patient's medication history.
Diabetic nephropathy: In patients with a history of diabetes, diabetic nephropathy is a common cause of chronic kidney disease (CKD) and potentially unexplained renal failure.
Hypertension: Uncontrolled hypertension can lead to hypertensive nephrosclerosis, a cause of CKD and renal failure.

Sepsis: Sepsis can cause acute kidney injury (AKI) due to hypoperfusion and the release of inflammatory mediators. Early recognition and treatment are crucial to prevent mortality.
Rhabdomyolysis: This condition, characterized by muscle breakdown, can lead to myoglobinuria and cause AKI. It is essential to identify and treat promptly to avoid severe kidney damage.
Obstructive uropathy: Bilateral urinary tract obstruction or obstruction in a solitary kidney can cause renal failure. Identifying and relieving the obstruction is critical to restore renal function.

Vasculitis: Conditions like ANCA-associated vasculitis or lupus nephritis can cause renal failure but are less common than other diagnoses listed here.
Thrombotic microangiopathies: Disorders such as thrombotic thrombocytopenic purpura (TTP) or hemolytic-uremic syndrome (HUS) are rare but can lead to renal failure.
Inherited disorders: Certain genetic conditions, such as polycystic kidney disease (PKD), can cause renal failure but typically have a more gradual onset and are identified through family history or specific diagnostic tests.