Gastroenteritis: This is a common condition characterized by inflammation of the gastrointestinal tract, often due to viral or bacterial infections. Symptoms include abdominal cramps, vomiting, diarrhea, and fever. It's highly prevalent among adolescents and can be triggered by contaminated food or water, or spread through close contact with an infected person.

Appendicitis: Although more common in younger males, appendicitis can occur in females and presents with abdominal pain (initially diffuse, then localizing to the right lower quadrant), nausea, vomiting, and fever. The pain often starts near the navel and then moves.
Menstrual Cramps (Dysmenorrhea): For a 16-year-old female, menstrual cramps are a common cause of abdominal pain. They can be severe enough to cause nausea and vomiting, especially if the cramps are intense.
Food Poisoning: Similar to gastroenteritis, food poisoning from consuming contaminated or spoiled food can lead to sudden onset of abdominal cramps, vomiting, and diarrhea.
Urinary Tract Infection (UTI): While UTIs are more common in adults, they can occur in adolescents. Symptoms include dysuria, frequent urination, and abdominal pain, which can sometimes be accompanied by vomiting.

Ectopic Pregnancy: Although less likely in a 16-year-old, an ectopic pregnancy must be considered, especially if there's a possibility of sexual activity. Symptoms can include abdominal pain, vomiting, and missed periods.
Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply. It's more common in younger children but can occur in adolescents, presenting with severe abdominal pain, vomiting, and sometimes bloody stools.
Ovarian Torsion: A gynecological emergency where the ovary twists around its ligaments, cutting off blood supply. It presents with sudden, severe abdominal pain, often accompanied by nausea and vomiting.
Appendicitis in Atypical Locations: Appendicitis can sometimes present with atypical symptoms if the appendix is located in an unusual position (e.g., retrocecal), which might delay diagnosis.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with chronic abdominal pain, vomiting, diarrhea, and weight loss. While not common, they should be considered in the differential diagnosis, especially if symptoms persist or worsen over time.
Celiac Disease: An autoimmune reaction to gluten, leading to small intestine inflammation and symptoms like abdominal pain, diarrhea, and vomiting after consuming gluten-containing foods.
Porphyria: A group of disorders that can cause abdominal pain, nausea, vomiting, and neurological symptoms due to defects in heme biosynthesis.