Which of the following findings would indicate a complicated UTI?

The presence of white blood cell (WBC) casts in the urine is indicative of a complicated urinary tract infection (UTI). WBC casts suggest that there is inflammation and infection within the kidneys themselves, often related to pyelonephritis, which is a more severe form of UTI that can lead to systemic involvement. This finding demonstrates that the infection is not limited to the lower urinary tract but has potential complications affecting the upper urinary tract, making the condition more serious and complicated.

In contrast, a negative urinalysis result typically suggests that there is no active infection; thus, it would not indicate a complicated UTI. The absence of fever or systemic symptoms may suggest a milder case or even a resolution of infection, which does not align with the concept of a complicated UTI. Similarly, normal renal function usually indicates that renal involvement is minimal or absent, which also suggests a non-complicated situation. Therefore, the presence of WBC casts is the key finding that supports the diagnosis of a complicated urinary tract infection.