Acute Tubular Necrosis (ATN): This is a common cause of acute kidney injury (AKI) in older adults, which can be triggered by various factors such as medication, infection, or contrast agents. The fact that the renal function returns to normal after a month suggests a reversible cause, making ATN a plausible explanation.

Dehydration or Hypovolemia: Prolonged dehydration can lead to a significant drop in renal function, which can recover once the patient is rehydrated.
Medication-Induced Nephrotoxicity: Certain medications, such as NSAIDs, can cause nephrotoxicity, leading to a decline in renal function, which can improve once the offending agent is discontinued.
Urinary Tract Obstruction: Obstruction of the urinary tract, such as kidney stones or benign prostatic hyperplasia, can cause a decline in renal function, which can resolve once the obstruction is relieved.

Vasculitis: Conditions like ANCA-associated vasculitis or polyarteritis nodosa can cause renal dysfunction and must be considered, as they can be life-threatening if left untreated.
Glomerulonephritis: Post-streptococcal glomerulonephritis or other forms of glomerulonephritis can cause a sudden decline in renal function and require prompt diagnosis and treatment.
Atherosclerotic Renal Artery Stenosis: This condition can cause renal ischemia, leading to a decline in renal function, and is essential to diagnose, as it can be treated with angioplasty or stenting.

Thrombotic Microangiopathy: Conditions like thrombotic thrombocytopenic purpura (TTP) or hemolytic-uremic syndrome (HUS) can cause renal dysfunction, but are relatively rare and often associated with other systemic symptoms.
Sarcoidosis: This autoimmune condition can cause renal dysfunction, but is less common and often presents with other systemic symptoms, such as lung or skin involvement.
Amyloidosis: This condition can cause renal dysfunction, but is relatively rare and often associated with other systemic symptoms, such as weight loss or fatigue.