Nephrolithiasis (Kidney Stones): This condition is the most common cause of colicky flank pain, especially if the pain radiates to the groin and is accompanied by nausea, vomiting, and hematuria. The pain is typically severe and intermittent, corresponding to the movement of the stone through the urinary tract.

Pyelonephritis: An infection of the kidney, which can cause flank pain, fever, and other symptoms like dysuria and frequency. The pain is usually constant and can be accompanied by systemic signs of infection.
Musculoskeletal Strain: Strain or injury to the muscles of the back can cause flank pain, especially if the pain is exacerbated by movement or position.
Urinary Tract Infection (UTI): While more commonly associated with lower urinary tract symptoms, upper UTIs can cause flank pain.

Abdominal Aortic Aneurysm: Although less common, a ruptured abdominal aortic aneurysm can present with severe, sudden-onset flank or back pain and is a medical emergency.
Ectopic Pregnancy: In women of childbearing age, ectopic pregnancy can cause severe, sudden flank pain, often accompanied by vaginal bleeding and is a life-threatening condition if not promptly diagnosed and treated.
Appendicitis (atypical presentation): While appendicitis typically presents with right lower quadrant pain, atypical presentations can occur, including flank pain, especially if the appendix is retrocecal.

Papillary Necrosis: A condition where part of the kidney tissue dies due to lack of blood supply, often associated with diabetes, analgesic abuse, or sickle cell disease, and can cause flank pain.
Renal Infarction: Sudden loss of blood supply to the kidney, which can cause severe flank pain and is a medical emergency.
Retroperitoneal Hemorrhage or Tumor: Bleeding or a tumor in the retroperitoneal space can cause flank pain, among other symptoms, and requires prompt diagnosis and treatment.