Musculoskeletal strain: This is often the most common cause of acute lumbar and flank pain, resulting from overuse, heavy lifting, or sudden twisting motions. The pain typically worsens with movement and improves with rest.

Kidney stones (nephrolithiasis): Pain from kidney stones can start in the lower back and radiate to the flank, groin, or abdomen. It's often severe and can be accompanied by nausea, vomiting, and urinary symptoms.
Pyelonephritis: An infection of the kidney, which can cause flank pain, fever, and urinary symptoms such as dysuria and frequency.
Degenerative disc disease or herniated disc: These conditions can cause lumbar pain that radiates to the flank, especially if the disc affects nearby nerves.

Abdominal aortic aneurysm: Although less common, an aneurysm can cause severe back or flank pain and is a medical emergency. Risk factors include smoking, hypertension, and atherosclerosis.
Pulmonary embolism: In rare cases, a pulmonary embolism can refer pain to the upper back or flank. This condition is life-threatening and requires immediate medical attention.
Sickle cell crisis: Individuals with sickle cell disease can experience severe pain episodes, including flank pain, due to vaso-occlusive crises.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension, sweating, palpitations, and flank pain.
Spinal infections (osteomyelitis, discitis): Infections of the spine can cause severe back pain that may radiate to the flank, often accompanied by fever and neurological deficits.
Retroperitoneal fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can encase and obstruct structures, leading to flank pain and other symptoms.