Gastroenteritis: This is the most likely diagnosis given the combination of fever, nausea, vomiting, and abdominal pain. Gastroenteritis, often caused by viral or bacterial infections, is a common condition that matches all the symptoms described.

Appendicitis: Although it primarily presents with abdominal pain, nausea, and vomiting, fever can also be a symptom. The pain often starts near the navel and then moves to the lower right abdomen.
Food Poisoning: Similar to gastroenteritis, food poisoning can cause fever, nausea, vomiting, and abdominal pain, usually after consuming contaminated food or drinks.
Urinary Tract Infection (UTI): While UTIs are more common in women, they can cause fever, nausea, vomiting, and abdominal pain, especially if the infection has moved up to the kidneys (pyelonephritis).

Perforated Viscus: A hole in the wall of the stomach or intestine can lead to severe abdominal pain, fever, and vomiting. This is a surgical emergency.
Ectopic Pregnancy: In women of childbearing age, an ectopic pregnancy can cause abdominal pain, nausea, vomiting, and fever if the pregnancy ruptures. This is a life-threatening condition.
Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain, nausea, vomiting, and fever. It requires immediate medical attention.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially causing bowel obstruction. It's rare in adults but can cause abdominal pain, vomiting, and fever.
Mesenteric Lymphadenitis: Inflammation of the lymph nodes in the mesentery (the tissue that attaches the intestines to the back of the abdominal wall) can mimic appendicitis and cause similar symptoms.
Diverticulitis: Inflammation of the diverticula (small pouches) in the wall of the colon can cause abdominal pain, fever, and changes in bowel habits, though nausea and vomiting are less common.