Under what circumstances is urine usually sterile?

The correct answer is that urine is usually sterile except for certain carriers. In healthy individuals without any infections, urine in the bladder is typically free of bacteria and other microorganisms, making it sterile. This sterility can be compromised under certain circumstances, such as when an individual is a carrier of specific pathogens, or if there is an existing infection in the urinary tract.

Pathogens may exist in certain carriers without causing overt symptoms, influencing the sterility of urine. For example, individuals who harbor bacteria in their urinary tracts without experiencing any signs of infection can produce non-sterile urine.

Normal conditions refer to the state of health where no infections are present, and even in such cases, the urine remains sterile unless affected by carriers of specific pathogens. Therefore, while it is true that urine is sterile during normal health, the introduction of carriers or infections is what can disrupt this state, confirming that urine is usually sterile except for certain carriers.