Musculoskeletal strain: This is often the most common cause of back pain radiating to the lower abdomen, especially if the pain is exacerbated by movement or activity. The musculoskeletal system includes muscles, tendons, and ligaments, and strain in these areas can cause referred pain to the abdomen.

Kidney stones (nephrolithiasis): Pain from kidney stones can start in the back and radiate to the front, typically to the groin area. The pain is often severe and can be accompanied by nausea, vomiting, and blood in the urine.
Urinary tract infection (UTI): While UTIs more commonly cause dysuria and frequency, they can also cause lower back pain that radiates to the abdomen, especially if the infection has moved up to the kidneys (pyelonephritis).
Ovarian cysts or torsion: In women, ovarian cysts can cause pain that starts in the lower back and radiates to the front of the abdomen. If the cyst ruptures or the ovary torsions, the pain can become severe and acute.
Diverticulitis: Inflammation of the diverticula in the colon can cause pain in the lower left abdomen that may radiate to the back.

Appendicitis: Although appendicitis typically presents with pain that starts around the navel and then moves to the lower right abdomen, atypical presentations can occur, especially in retrocecal appendicitis, where the pain might radiate to the back.
Ectopic pregnancy: A pregnancy that develops outside the uterus can cause severe abdominal pain that may radiate to the back, along with vaginal bleeding and other symptoms. This is a medical emergency.
Aortic dissection or aneurysm: Severe, tearing pain in the back that radiates to the abdomen can be a sign of an aortic dissection or rupture, which is immediately life-threatening.
Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain that radiates to the back, often accompanied by nausea, vomiting, and elevated pancreatic enzymes.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of severe hypertension, headache, sweating, and abdominal or back pain.
Retroperitoneal fibrosis: A rare condition where fibrotic tissue forms in the retroperitoneum, which can cause back and abdominal pain due to the encasement of structures like the ureters.
Intra-abdominal testicular torsion: A rare condition where the testicle twists inside the abdominal cavity, causing severe pain that may radiate from the back to the abdomen.