Nephrolithiasis (Kidney Stones): This is often the first consideration for flank pain due to its prevalence and the characteristic severe, colicky pain it causes, which can radiate to the groin. The pain is typically accompanied by nausea, vomiting, and sometimes blood in the urine.

Pyelonephritis: An infection of the kidney, which can cause flank pain, fever, and other urinary symptoms. It's more common in women and those with a history of urinary tract infections.
Musculoskeletal Strain: Given the location, strain or injury to the muscles in the flank area can cause significant pain, especially if there's been recent physical activity or trauma.
Diverticulitis: Inflammation of a diverticulum in the colon, which can cause left lower quadrant pain that might be perceived as flank pain, especially if the affected diverticulum is near the spleen or kidney.

Abdominal Aortic Aneurysm: Although less common, an aneurysm of the abdominal aorta can cause flank pain and is a medical emergency. Risk factors include smoking, hypertension, and atherosclerosis.
Pulmonary Embolism: In some cases, a pulmonary embolism can refer pain to the flank area, especially if the embolism is peripherally located. This is a life-threatening condition requiring immediate intervention.
Splenetic Rupture or Infarct: These conditions can cause severe left upper quadrant or flank pain and are emergencies, often associated with trauma or underlying conditions like sickle cell disease.

Pancreatitis: While more commonly associated with mid-abdominal pain, pancreatitis can cause flank pain, especially if the pancreatic tail is involved.
Retroperitoneal Fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can encase and obstruct structures like the ureters, leading to flank pain.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension and pain, which might be perceived in the flank area due to the tumor's location.