Nephrotic Syndrome: This condition is characterized by severe swelling (edema), particularly in the flanks, due to kidney damage leading to excessive loss of protein in the urine. The swelling is a result of hypoalbuminemia, which reduces the oncotic pressure, causing fluid to leak out of blood vessels into the interstitial space.

Chronic Kidney Disease (CKD): CKD can lead to fluid retention and swelling, especially in the flanks, as the kidneys' ability to filter and remove excess fluids is impaired.
Heart Failure: Right-sided heart failure can cause fluid to accumulate in the body, leading to edema in the flanks among other areas, due to the heart's reduced ability to pump blood effectively.
Liver Cirrhosis: Cirrhosis can lead to a buildup of fluid in the abdominal cavity (ascites) and peripheral edema, including swelling in the flanks, due to portal hypertension and hypoalbuminemia.

Renal Cell Carcinoma: Although less common, renal cell carcinoma can cause swelling in the flanks due to a tumor in the kidney. Early detection is crucial for effective treatment.
Wilms Tumor (in children): A type of kidney cancer that occurs in children, which can present with a palpable abdominal mass and swelling in the flanks.
Pulmonary Embolism with associated Nephrotic Syndrome or Renal Vein Thrombosis: While less direct, pulmonary embolism can lead to complications that affect kidney function, indirectly causing flank swelling.

Lymphoma: Certain types of lymphoma can cause swelling in the flanks due to lymph node enlargement or direct kidney involvement.
Amyloidosis: A condition characterized by the deposition of amyloid proteins in various tissues, including the kidneys, which can lead to nephrotic syndrome and flank swelling.
Fabry Disease: A rare genetic disorder that can affect the kidneys and lead to flank pain and swelling among other symptoms.