Viral Gastroenteritis: This is the most common cause of acute diarrhea in adults, especially in the elderly. The symptoms of diarrhea, stomach cramping, and nausea for 4 days are consistent with a viral etiology, such as norovirus or rotavirus.

Bacterial Gastroenteritis: Bacteria like Salmonella, Shigella, or Campylobacter can cause similar symptoms. The elderly are more susceptible to bacterial infections due to decreased immune function.
Medication-induced Diarrhea: The patient may be taking medications that can cause diarrhea as a side effect, such as antibiotics, laxatives, or antacids.
Food Poisoning: Consuming contaminated or spoiled food can lead to symptoms of diarrhea, stomach cramping, and nausea.

Clostridioides difficile (C. diff) Infection: This bacterial infection can cause severe diarrhea and is more common in the elderly, especially those who have recently taken antibiotics. Missing this diagnosis can lead to severe complications, including sepsis and death.
Inflammatory Bowel Disease (IBD) Flare: Although less common in the elderly, IBD (e.g., Crohn's disease or ulcerative colitis) can present with similar symptoms and requires prompt treatment to prevent complications.
Ischemic Bowel Disease: Reduced blood flow to the intestines can cause abdominal pain, diarrhea, and nausea. This condition is more common in the elderly and can be life-threatening if not promptly diagnosed and treated.

Giardiasis: A parasitic infection caused by Giardia lamblia, which can lead to chronic diarrhea and malabsorption.
Microscopic Colitis: A condition characterized by inflammation of the colon, which can cause chronic diarrhea and is more common in the elderly.
Neuroendocrine Tumors: Rare tumors that can produce hormones leading to diarrhea, such as VIPomas or carcinoid tumors.