Nephrotic Syndrome: This condition is characterized by severe proteinuria, hypoalbuminemia, and edema. The edema can manifest as soft tissue swelling in various parts of the body, including the flanks, due to the loss of albumin and subsequent decrease in oncotic pressure.

Heart Failure: Congestive heart failure can lead to fluid overload and subsequent edema. The edema can be distributed in a dependent manner, which might include the flanks, especially if the patient is bedridden or has limited mobility.
Liver Cirrhosis: Cirrhosis can cause hypoalbuminemia and lead to edema. Additionally, cirrhosis is often associated with ascites, which can also contribute to the appearance of flank swelling due to the accumulation of fluid in the abdominal cavity.
Renal Failure: Acute or chronic renal failure can lead to fluid retention and electrolyte imbalances, resulting in edema. The swelling can be seen in the flanks among other areas.

Abdominal Aortic Aneurysm (AAA): Although AAA might not directly cause bilateral flank soft tissue swelling, a ruptured AAA can lead to retroperitoneal hemorrhage, which might be mistaken for or cause swelling in the flanks. This is a medical emergency.
Pheochromocytoma: This rare tumor of the adrenal gland can cause episodic hypertension and might lead to fluid retention and swelling, although this would be less common and more indirect.

Lymphoma: Certain types of lymphoma can cause renal impairment or lead to the accumulation of fluid in the body, including the flanks, due to lymphatic obstruction or systemic disease.
Amyloidosis: This condition involves the deposition of abnormal proteins (amyloid) in various tissues and organs, including the kidneys, which can lead to nephrotic syndrome and subsequent edema.
Sarcoidosis: A systemic granulomatous disease that can affect multiple organs, including the kidneys, leading to renal failure and edema. It can also cause lymphadenopathy and other systemic symptoms.