Musculoskeletal strain or overuse: This is often the most common cause of chronic flank pain, especially if the patient has a history of heavy lifting, bending, or other strenuous activities that could strain the muscles in the flank area. Normal investigations would not rule out musculoskeletal issues as these are often diagnosed clinically.

Nephrolithiasis (kidney stones) with small or asymptomatic stones: Even if initial investigations like ultrasound or CT scans are normal, small stones might not be visible, or the patient might be in a phase where the stones are not causing obstruction or significant symptoms.
Chronic pyelonephritis or interstitial nephritis: These conditions can cause chronic flank pain and might not always show up on initial imaging studies, especially if the changes are subtle or the disease is in its early stages.
Spinal issues (e.g., lumbar spondylosis, herniated discs): Referral pain from the spine can manifest as flank pain. Normal abdominal and renal investigations would not rule out spinal causes.

Renal cell carcinoma or other renal tumors: Although investigations might initially appear normal, it's crucial to consider the possibility of a renal tumor, especially if the patient has risk factors. Further imaging or follow-up might be necessary.
Pulmonary embolism (if the pain is pleuritic): While less common, if the flank pain is pleuritic (worsens with deep breathing), pulmonary embolism must be considered due to its high mortality rate if untreated.
Aortic aneurysm or dissection: These are life-threatening conditions that might not always be immediately apparent on initial evaluations, especially if the aneurysm is small or the dissection does not significantly alter the aortic caliber.

Amyloidosis: A condition where abnormal proteins (amyloid) accumulate in organs, potentially causing chronic pain, including flank pain, due to renal involvement.
Sarcoidosis: An autoimmune disease that can affect multiple organs, including the kidneys, and cause chronic flank pain among other symptoms.
Ehlers-Danlos syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. It could potentially cause chronic musculoskeletal pain, including flank pain, due to its effects on collagen.