Appendicitis: The presentation of right-sided abdominal pain in a pediatric patient, along with elevated WBC (12.7), suggests appendicitis as a highly plausible diagnosis. The slightly elevated urea and creatinine could be indicative of dehydration, which is common in patients with appendicitis due to decreased oral intake and possible vomiting.

Constipation: Although not directly indicated by the blood work, constipation is a common cause of abdominal pain in children and can sometimes cause right-sided pain. The absence of significant abnormalities in the blood work does not rule out constipation.
Gastroenteritis: This condition can cause abdominal pain, vomiting, and diarrhea, leading to dehydration, which might explain the mild elevations in urea and creatinine. The elevated WBC could be in response to an infectious cause.
Mesenteric Lymphadenitis: This condition involves inflammation of the lymph nodes in the mesentery and can mimic appendicitis in its presentation. It's often associated with viral infections and could explain the abdominal pain and elevated WBC.

Intussusception: A serious condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply. It's a medical emergency and can present with abdominal pain, vomiting, and sometimes blood in stool. Although the blood work does not specifically point towards this, it's crucial not to miss due to its severity.
Ectopic Pregnancy: Although less likely in a 12-year-old, it's essential to consider in any female patient with abdominal pain, especially if there's a possibility of sexual activity. The blood work provided does not include a pregnancy test, which would be critical in ruling out this condition.
Kidney Stone: Can cause severe abdominal or flank pain and might explain the mild elevations in urea and creatinine if the stone is obstructing urine flow. The absence of other significant findings in the blood work does not rule out this possibility.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with chronic abdominal pain, changes in bowel habits, and sometimes blood in stool. The elevated WBC and mild anemia (indicated by low Hgb and Hct) could support this, but it would be less common in a presentation with acute right-sided abdominal pain.
Hematological Disorders: The abnormal blood cell indices (e.g., low MCV, elevated RDW) suggest a possible underlying hematological issue, such as iron deficiency anemia or thalassemia. However, these would not directly cause acute abdominal pain but are important to address for overall health.
Abdominal Malignancies: Very rare in children but could present with abdominal pain and systemic symptoms like weight loss or fatigue. The lack of significant abnormalities in the blood work makes this less likely, but it remains a consideration in the differential diagnosis due to its severity.