Urinary Tract Infection (UTI): The most common complication following ureteroscopy is a urinary tract infection. The procedure involves inserting a scope into the ureter, which can introduce bacteria into the urinary system, leading to infection. Symptoms such as fever are consistent with a UTI.

Sepsis: While less common than UTI, sepsis is a potential complication if the infection spreads beyond the urinary tract. Fever is an early sign of sepsis, making it a consideration in a post-ureteroscopy patient.
Ureteral perforation or injury: Although rare, if the ureter was perforated or injured during the procedure, it could lead to infection, abscess formation, or peritonitis, all of which could present with fever.

Septic shock: A severe form of sepsis, septic shock is life-threatening and requires immediate intervention. Early signs can be subtle, but fever in the context of recent ureteroscopy warrants consideration of this diagnosis to ensure prompt treatment.
Perinephric abscess: An abscess formation around the kidney can occur as a complication of ureteroscopy, especially if there was an injury to the ureter or kidney during the procedure. Fever would be a presenting symptom, and missing this diagnosis could lead to severe consequences.

Fungal infection: While much less common than bacterial infections, fungal infections can occur, especially in immunocompromised patients. The presentation would be similar to a bacterial UTI, with fever being a key symptom.
Systemic inflammatory response syndrome (SIRS) due to non-infectious causes: Rarely, a non-infectious cause such as a reaction to the irrigation fluid used during ureteroscopy or another non-infectious complication could lead to a systemic inflammatory response, presenting with fever among other symptoms.