Appendicitis: This is often the first consideration for right lower abdominal pain, especially if the pain is severe, constant, and accompanied by fever, nausea, and vomiting. The pain typically starts near the navel and then moves to the right lower abdomen.

Kidney Stone (Nephrolithiasis): Pain from a kidney stone can radiate to the lower back and abdomen. It's usually severe and can be accompanied by nausea, vomiting, and blood in the urine.
Urinary Tract Infection (UTI): While more common in women, UTIs can cause lower abdominal and back pain in anyone. Symptoms include frequent urination, burning sensation while urinating, and cloudy or foul-smelling urine.
Diverticulitis: Inflammation of the diverticula in the digestive tract can cause pain, usually on the left side but can occur on the right, accompanied by changes in bowel habits, fever, and nausea.
Ovarian Cyst (in females): Rupture or torsion of an ovarian cyst can cause severe lower abdominal pain that may radiate to the back, often accompanied by nausea and vomiting.

Ectopic Pregnancy (in females): A pregnancy that develops outside the uterus can cause severe abdominal pain, often on one side, and can be life-threatening if not promptly treated.
Ruptured Aortic Aneurysm: Although more common in older adults with a history of smoking, a ruptured abdominal aortic aneurysm can cause sudden, severe back and abdominal pain and is immediately life-threatening.
Pulmonary Embolism: While primarily causing chest pain and shortness of breath, a pulmonary embolism can sometimes cause referred pain to the abdomen or back, especially if the embolism is large.
Intestinal Obstruction: Blockage of the intestine can cause severe abdominal pain, vomiting, and constipation. It requires immediate medical attention to prevent tissue death.

Psoas Abscess: An abscess in the psoas muscle can cause lower back and abdominal pain, often accompanied by fever and difficulty moving the hip.
Endometriosis (in females): While not typically an acute condition, endometriosis can cause chronic pelvic pain that may radiate to the lower back and abdomen, especially during menstruation.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic abdominal pain, changes in bowel habits, and weight loss, but can occasionally present acutely.
Retroperitoneal Hemorrhage: Bleeding into the space behind the peritoneum can cause severe back and abdominal pain, often due to trauma or anticoagulation therapy.