Dehydration leading to Prerenal Acute Kidney Injury (AKI): The patient's acute diarrhea has likely caused significant dehydration, which in turn has led to a decrease in renal perfusion, resulting in the elevated creatinine and BUN levels. The patient's history of hypertension and baseline creatinine of 1.2 also support this diagnosis, as chronic hypertension can lead to chronic kidney disease, making the kidneys more susceptible to acute injury.

Gastroenteritis: The patient's symptoms of acute diarrhea could be due to a viral or bacterial gastroenteritis, which is a common cause of diarrhea in adults.
Medication-induced AKI: The patient's medications for hypertension could be contributing to the decline in renal function, especially if they are ACE inhibitors or ARBs, which can affect renal function, particularly in the setting of dehydration.
Intrinsic Renal Disease: The patient's decline in renal function could be due to an intrinsic renal disease such as acute tubular necrosis (ATN) or glomerulonephritis, although the lack of other symptoms such as hematuria or proteinuria makes this less likely.

Septicemia: Although the patient's other labs are normal, it is crucial not to miss a diagnosis of septicemia, which can cause AKI and has a high mortality rate if not promptly treated.
Obstructive Uropathy: A diagnosis of obstructive uropathy, such as a urinary tract obstruction, could lead to post-renal AKI and needs to be considered, especially in older males who are at higher risk for prostate issues.

Hemolytic Uremic Syndrome (HUS): This is a rare condition characterized by the triad of hemolytic anemia, thrombocytopenia, and AKI, often associated with certain bacterial infections such as E. coli O157:H7.
Vasculitis: A systemic vasculitis, such as ANCA-associated vasculitis, could cause renal dysfunction and gastrointestinal symptoms, although this would be a rare cause of the patient's presentation.