Infectious Gastroenteritis: The patient's presentation with diarrhea, elevated white blood cell count (WBC), and neutrophilia suggests an infectious cause. The recent onset and absence of other systemic symptoms (e.g., abdominal pain, fever) point towards a gastrointestinal infection.

Inflammatory Bowel Disease (IBD): Although less likely given the acute presentation, the patient's anemia, elevated platelet count, and mild elevation in liver enzymes could be indicative of IBD, such as Crohn's disease or ulcerative colitis.
Malabsorption: The patient's diarrhea and mild anemia could suggest malabsorption, potentially due to celiac disease, pancreatic insufficiency, or bacterial overgrowth.
Medication-induced Diarrhea: Certain medications can cause diarrhea; without a medication list, it's difficult to assess this possibility, but it remains a consideration.

Clostridioides difficile (C. diff) Infection: Given the potential for severe complications, including toxic megacolon and sepsis, C. diff infection must be considered, especially if the patient has recently been hospitalized or exposed to antibiotics.
Ischemic Bowel Disease: Although less common, ischemic bowel disease can present with diarrhea and abdominal pain. The patient's age and mild elevation in lactate (not directly measured but inferred from anion gap and CO2 levels) make this a "do not miss" diagnosis.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, weight loss, and arthralgias. Diagnosis is often delayed due to its rarity and nonspecific symptoms.
Giardiasis: An intestinal infection caused by the Giardia parasite, which can lead to chronic diarrhea, weight loss, and malabsorption. It's more common in travelers or those exposed to contaminated water.
Microscopic Colitis: A condition characterized by chronic, watery diarrhea, often without visible inflammation on endoscopy. It's more common in older adults and can be associated with certain medications.