Viral Gastroenteritis: This is a common cause of diarrhea in adults, often presenting with sudden onset of symptoms including diarrhea, possibly accompanied by vomiting, fever, and abdominal cramps. The patient's normal kidney function (indicated by BUN <5, Cr 0.5, GFR >90) and normal potassium level (Kcl normal) suggest that dehydration, if present, is not severe enough to significantly impact renal function or electrolyte balance at this point.

Bacterial Gastroenteritis: Similar to viral gastroenteritis but caused by bacteria. Symptoms can be more severe and may include blood in the stool. The diagnosis would depend on stool tests.
Food Poisoning: Caused by consuming contaminated food, symptoms can range from mild to severe and include diarrhea, vomiting, and abdominal cramps.
Irritable Bowel Syndrome (IBS): A chronic condition characterized by recurring abdominal pain associated with a change in bowel habits, such as diarrhea or constipation. However, the sudden onset of symptoms might make this less likely if the patient has no prior history of IBS.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, abdominal pain, and other symptoms. Although less common, missing this diagnosis could lead to significant morbidity.
Clostridioides difficile (C. diff) Infection: Especially if the patient has recently been on antibiotics, C. diff infection can cause severe diarrhea and colitis. It's crucial to consider this diagnosis due to its potential severity and the need for specific treatment.
Ischemic Bowel Disease: Although less likely in a patient with normal renal function and no indication of vascular disease, ischemic bowel can cause sudden onset of abdominal pain and diarrhea, especially in older adults.

Microscopic Colitis: A condition characterized by chronic, watery diarrhea, often in older adults, with a normal appearance of the colon during endoscopy but abnormal findings on microscopic examination of colon tissue.
Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, weight loss, and abdominal pain, among other symptoms. Diagnosis is often delayed due to its rarity and nonspecific symptoms.
Hormone-secreting Tumors: Certain tumors, like VIPomas (which secrete vasoactive intestinal peptide), can cause severe, chronic diarrhea. These are rare and usually considered after other diagnoses have been ruled out.