Musculoskeletal strain: This is often the most common cause of flank pain due to the large number of muscles in the back that can be strained, leading to pain in the flank area. Activities that involve heavy lifting, bending, or sudden movement can cause this type of strain.

Urinary tract infection (UTI): While more commonly associated with dysuria and frequent urination, UTIs can sometimes present with flank pain, especially if the infection has moved up the urinary tract to the kidneys (pyelonephritis).
Kidney stones: Small stones may not cause an obstruction but can still cause pain, typically sharp and severe, in the flank area as they move through the urinary tract.
Constipation: Severe constipation can cause flank pain due to the proximity of the colon to the flank area and the potential for referred pain.

Pyelonephritis: An infection of the kidney, which can be severe and requires prompt antibiotic treatment. It often presents with flank pain, fever, and other systemic symptoms.
Appendicitis (if appendix is retrocecal): Although appendicitis typically presents with right lower quadrant pain, a retrocecal appendix (located behind the cecum) can cause right flank pain.
Pulmonary embolism: In rare cases, a pulmonary embolism can cause referred pain to the flank area, especially if the embolism is large and affects blood flow significantly.
Aortic dissection or aneurysm: These are life-threatening conditions that can cause severe, tearing flank pain and require immediate medical attention.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension and pain, sometimes in the flank area.
Renal cell carcinoma: A type of kidney cancer that can cause flank pain, although it is more commonly associated with a palpable mass and hematuria.
Spinal infections or tumors: Infections like osteomyelitis or discitis, or tumors of the spine, can cause flank pain due to their proximity to the flank area.