Nephrolithiasis (Kidney Stones): This is often the most common cause of acute flank pain, typically presenting with severe, colicky pain that may radiate to the groin. The pain is usually accompanied by nausea, vomiting, and sometimes hematuria.

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 underlying urinary tract issues.
Musculoskeletal Strain: Overuse or injury to the muscles in the flank area can cause significant pain, especially if there's a history of heavy lifting or strenuous activity.
Urinary Tract Infection (UTI): While more commonly associated with lower urinary tract symptoms, a UTI can cause flank pain if it ascends to the kidneys.

Appendicitis (if the appendix is retrocecal): Although appendicitis typically presents with right lower quadrant pain, a retrocecal appendix can cause pain that is perceived in the flank.
Pulmonary Embolism: Can cause sudden onset of flank pain if the embolism affects the upper parts of the lung, though this would typically be accompanied by respiratory symptoms.
Aortic Dissection or Aneurysm: These are life-threatening conditions that can cause severe flank pain, among other symptoms, and require immediate medical attention.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension and pain, which might be perceived in the flank.
Renal Cell Carcinoma: A type of kidney cancer that might cause flank pain, though it's more commonly associated with a palpable mass, hematuria, and systemic symptoms like weight loss.
Spinal Pathologies (e.g., disc herniation, spinal stenosis): Can cause flank pain that is referred from the spine, especially if there's nerve root involvement.