Functional abdominal pain: This is a common cause of abdominal pain in adolescents, often related to stress, anxiety, or irritable bowel syndrome (IBS). The diagnosis is typically made based on clinical presentation and the absence of other findings.

Constipation: A frequent cause of abdominal pain in adolescents, often due to poor bowel habits, low fiber diet, or inadequate fluid intake.
Menstrual cramps (dysmenorrhea): Abdominal pain associated with menstruation, caused by uterine contractions and inflammation.
Gastroesophageal reflux disease (GERD): Stomach acid reflux into the esophagus can cause abdominal pain, especially after eating.
Urinary tract infection (UTI): Bacterial infection of the urinary system, which can cause abdominal pain, dysuria, and frequent urination.

Appendicitis: Inflammation of the appendix, which can cause severe abdominal pain, nausea, and vomiting. If missed, it can lead to perforation and peritonitis.
Ectopic pregnancy: A pregnancy that develops outside the uterus, which can cause abdominal pain, vaginal bleeding, and shock. Although less likely in a 16-year-old, it is crucial to consider in sexually active females.
Ovarian torsion: Twisting of the ovary, which can cause severe abdominal pain, nausea, and vomiting. Prompt diagnosis and treatment are essential to preserve ovarian function.
Intussusception: Telescoping of a portion of the intestine into another, which can cause abdominal pain, vomiting, and bloody stools.

Inflammatory bowel disease (IBD): Chronic inflammation of the gastrointestinal tract, which can cause abdominal pain, diarrhea, and weight loss.
Pancreatitis: Inflammation of the pancreas, which can cause severe abdominal pain, nausea, and vomiting.
Hepatitis: Inflammation of the liver, which can cause abdominal pain, jaundice, and fatigue.
Abdominal malignancies: Rare tumors that can cause abdominal pain, weight loss, and other systemic symptoms.