Mesenteric Lymphadenitis: This condition is a common cause of abdominal pain, especially in the right lower quadrant (RLQ) and right upper quadrant (RUQ), and can be accompanied by fever. The negative CT scan does not rule out this diagnosis, as mesenteric lymphadenitis can be subtle and not always visible on imaging.

Viral Gastroenteritis: Although the UA is negative, viral gastroenteritis can cause abdominal pain, fever, and does not always result in a positive urinalysis. The absence of a positive HCG and negative CT scan supports this possibility.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with chronic or acute abdominal pain, fever, and can affect any part of the gastrointestinal tract. The negative CT and other tests do not rule out IBD, as diagnosis often requires endoscopy and biopsy.
Intestinal Obstruction: Early or partial obstruction might not be evident on a CT scan, especially if it's not a complete obstruction. Symptoms can include abdominal pain and fever, although the absence of significant findings on CT makes this less likely.

Appendicitis: Despite the negative CT scan, appendicitis is a critical diagnosis not to miss due to its potential for severe complications if untreated. Atypical presentations or early disease might not show typical findings on imaging.
Ectopic Pregnancy: Although the HCG is negative, very early ectopic pregnancies might not have detectable levels of HCG yet. This diagnosis is critical due to its potential for severe morbidity and mortality.
Ovarian Torsion: This is another emergency that requires prompt diagnosis and treatment. It can present with acute abdominal pain and might not always have specific findings on CT scans or other initial tests.

Hepatic or Splenic Infarction: These conditions can present with upper abdominal pain and fever but are less common. The negative CT scan makes these diagnoses less likely, but they could be considered if other tests or clinical findings suggest them.
Abdominal Migraine: A rare condition that can cause severe abdominal pain, often accompanied by other migraine symptoms. It's more common in children but can occur in adults.
Familial Mediterranean Fever: A genetic disorder that causes recurrent episodes of fever and abdominal pain, among other symptoms. It's rare and typically diagnosed in individuals of Mediterranean descent.