Differential Diagnosis

• Single most likely diagnosis
  • Acute pyelonephritis with renal vein thrombosis: The description of a diffusely enlarged left kidney with a heterogeneous T2 signal, striated appearance at the nephrographic phase, and no excretion of contrast in the delayed phase, along with multiple cortical non-enhancing areas, is highly suggestive of acute pyelonephritis. The low signal filling defect at the left renal vein close to the renal hilum supports the presence of renal vein thrombosis, a known complication of acute pyelonephritis, especially in patients with predisposing factors such as nephrotic syndrome or dehydration.
• Other Likely diagnoses
  • Severe hydronephrosis with infection: Although the question stem mentions no PCS (pelvic-calyceal system) obstruction, severe hydronephrosis could potentially cause a similar appearance due to the distension of the renal pelvis and calyces, and if infected, could mimic some features of pyelonephritis. However, the absence of excretion in the delayed phase and the specific mention of no PCS obstruction makes this less likely.
  • Renal infarction: This could present with a similar pattern of non-enhancement and could be associated with renal vein thrombosis. However, the clinical context and the presence of a striated appearance at the nephrographic phase might lean more towards an infectious or inflammatory process rather than a purely ischemic one.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Renal cell carcinoma with venous thrombosis: Although less likely given the acute presentation and the specific imaging findings, renal cell carcinoma can cause a similar appearance, especially if there is associated venous thrombosis. Missing this diagnosis could have significant implications for treatment and prognosis.
  • Sepsis or severe systemic infection: The findings could be part of a more systemic process, and missing a diagnosis of sepsis or a severe systemic infection could be fatal. The presence of acute pyelonephritis could be a sign of a more widespread infection.
• Rare diagnoses
  • Xanthogranulomatous pyelonephritis (XGP): A rare form of chronic pyelonephritis that could present with a similar appearance of a non-functioning kidney and might be associated with venous thrombosis, although this is less common. XGP typically has a more chronic presentation and is associated with long-standing urinary tract infection and obstruction.
  • Renal lymphoma: Although rare, renal lymphoma could present with non-specific findings including renal enlargement and heterogeneous signal on imaging. The presence of venous thrombosis could be a complication of the tumor.