Acute pyelonephritis, also known as an upper urinary tract infection, occurs when certain pathogens enter the urinary system and cause inflammation in areas such as the renal pelvis and calyces. Patients typically exhibit symptoms such as fever, back pain, and may also experience visible blood in the urine. The condition often has a severe onset. The common pathogens causing this type of infection primarily include bacteria, fungi, and viruses, among which the vast majority of cases are caused by bacterial infections. Furthermore, Gram-negative bacilli constitute the majority of these bacteria, with Escherichia coli being the most common.