Ureterolithiasis: This is the most likely diagnosis given the presence of a 0.4 cm obstructing stone in the right distal ureter, which is causing mild hydronephrosis and stranding of the perirenal fat. The symptoms and imaging findings are classic for a ureteral stone.

Pyelonephritis: The presence of mild hydronephrosis and stranding of the perirenal fat could also suggest an infectious process such as pyelonephritis, especially if there are symptoms like fever or flank pain.
Ureteral obstruction due to other causes: Although a stone is identified, other causes of ureteral obstruction like blood clots, tumors, or extrinsic compression should be considered, especially if the stone is small and the obstruction seems disproportionate.

Ureteral rupture or perforation: Although less likely, the presence of fluid along the right retroperitoneum could indicate a ureteral rupture or perforation, which is a surgical emergency. This diagnosis must be considered and ruled out promptly.
Sepsis due to obstructive uropathy: If the obstruction leads to infection, it could result in sepsis, a life-threatening condition that requires immediate intervention.

Retroperitoneal fibrosis: This is a rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can cause ureteral obstruction. It would be an unusual cause of the symptoms and findings described but should be considered if other diagnoses are ruled out.
Malignant ureteral obstruction: Tumors of the ureter or surrounding tissues could cause obstruction and present similarly to a stone. Although rare, this diagnosis has significant implications for treatment and prognosis.