Gastroenteritis: This condition is characterized by inflammation of the gastrointestinal tract, often due to viral or bacterial infections, leading to symptoms like fever, abdominal pain, nausea, vomiting, and metabolic acidosis due to dehydration and bicarbonate loss.

Appendicitis: Inflammation of the appendix can cause fever, abdominal pain (often starting around the navel and then moving to the lower right abdomen), nausea, and vomiting. While not directly causing metabolic acidosis, severe cases can lead to sepsis, which may result in acid-base disturbances.
Urinary Tract Infection (UTI) or Pyelonephritis: Especially in females, UTIs can cause significant symptoms including fever, abdominal or flank pain, nausea, and vomiting. Severe infections can lead to sepsis, potentially causing metabolic acidosis.
Food Poisoning: Similar to gastroenteritis, food poisoning from bacterial toxins can lead to rapid onset of symptoms including fever, abdominal cramps, nausea, vomiting, and potentially metabolic acidosis if there's significant dehydration.

Septic Shock: Although less common, any severe infection (e.g., pneumonia, meningitis, severe UTI, or abdominal infections like perforated viscus) can lead to septic shock, characterized by fever, hypotension, and organ dysfunction, including metabolic acidosis. Missing this diagnosis can be fatal.
Diabetic Ketoacidosis (DKA): Presents with hyperglycemia, metabolic acidosis, and increased ketone bodies. Symptoms include polyuria, polydipsia, nausea, vomiting, and abdominal pain. It's crucial to check blood glucose levels in patients with metabolic acidosis.
Intestinal Obstruction: Can cause severe abdominal pain, nausea, vomiting, and if bowel ischemia occurs, metabolic acidosis. This condition requires prompt surgical intervention.

Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease or ulcerative colitis can cause fever, abdominal pain, nausea, vomiting, and potentially metabolic disturbances if there's significant diarrhea or if the patient develops toxic megacolon.
Adrenal Insufficiency: Although rare, adrenal crisis can present with abdominal pain, nausea, vomiting, fever, and metabolic acidosis, especially if triggered by an infection or other stressor.
Methanol or Ethylene Glycol Poisoning: These toxic ingestions can lead to metabolic acidosis, abdominal pain, nausea, and vomiting, among other symptoms. They are critical to diagnose early due to the availability of specific antidotes.