Pyelonephritis: This is often the most common cause of perinephric fat stranding due to the spread of infection from the kidney to the surrounding fat. The presence of fever, flank pain, and urinary symptoms supports this diagnosis.

Nephrolithiasis: Kidney stones can cause obstruction and subsequent inflammation, leading to perinephric fat stranding. The presence of a stone on CT and symptoms like severe flank pain (renal colic) are indicative.
Renal Trauma: Trauma to the kidney can result in perinephric hematoma and fat stranding. A history of recent trauma is crucial for this diagnosis.
Renal Cell Carcinoma: Tumors can directly invade the perinephric fat or cause secondary changes like hemorrhage or infection, leading to stranding. A solid renal mass on CT is suggestive.

Pyclonephritis with Abscess Formation: Although a complication of pyelonephritis, the formation of an abscess requires urgent drainage and antibiotic therapy. It's crucial not to miss due to its potential for severe morbidity.
Renal Infarction: This can cause perinephric fat stranding due to ischemia and potential hemorrhage. It's a medical emergency requiring prompt intervention to salvage renal function.
Ureteral Obstruction: Obstruction can lead to pyelonephritis or direct invasion of the perinephric space by a tumor or stone, emphasizing the need for timely diagnosis to prevent renal damage.

Retroperitoneal Fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can encase and obstruct structures, including the ureters. It presents with non-specific symptoms and can be a diagnostic challenge.
Lymphoma: Can involve the kidneys and perinephric space, causing stranding due to tumor infiltration or secondary effects like obstruction or hemorrhage.
Sarcoidosis: Although rare, sarcoidosis can involve the kidneys and cause perinephric fat stranding due to granulomatous inflammation. It's part of a systemic disease with multi-organ involvement.