Ureterolithiasis: The presence of a 4 x 3.5 mm stone in the left upper ureter with a high Hounsfield unit (HU) value of 490, which is consistent with a calcified stone, along with mild left hydronephrosis and proximal hydronephrosis, strongly supports this diagnosis. The size and location of the stone are typical for causing obstructive symptoms.

Nephrolithiasis with stone migration: Although the stone is currently located in the ureter, it's possible that it originated in the kidney and migrated down. The presence of hydronephrosis suggests an obstructive process, which could be due to a stone that has moved from the kidney.
Ureteral stricture or kinking: A stricture or kinking of the ureter could cause obstructive symptoms similar to those caused by a stone, including hydronephrosis. However, the presence of a visible stone makes this less likely.

Ureteral tumor (e.g., transitional cell carcinoma): Although less common than stones, a tumor in the ureter could cause obstruction and hydronephrosis. Missing this diagnosis could have severe consequences due to the potential for malignancy.
Sloughed renal papilla: In the context of severe pyelonephritis or other renal infections, a sloughed renal papilla could obstruct the ureter and cause symptoms similar to those of a stone.
Blood clot: In patients with bleeding disorders or those on anticoagulation, a blood clot could form in the ureter and cause obstruction.

Ureteral foreign body: Although rare, it's possible for a foreign body to be present in the ureter, causing obstruction. This could be iatrogenic (e.g., a fragment of a medical device) or due to other unusual circumstances.
Ureteral endometriosis: In rare cases, endometrial tissue can implant in the ureter, causing obstruction and symptoms similar to those of a stone. This condition is more commonly associated with female reproductive organs but can occasionally affect the ureters.