Which of the following elements must be a component of the BSL3 laboratory design?

The correct answer highlights an essential aspect of laboratory safety design in a BSL-3 (Biosafety Level 3) facility. A visual airflow indicator at the lab entry is crucial because it provides immediate, visible confirmation that air is flowing into the laboratory from the surrounding areas, rather than out of it. This is vital for maintaining containment of potentially infectious agents that may be manipulated within the laboratory, ensuring that any harmful pathogens don't escape into the external environment.

While other components might be important for specific safety protocols or efficiencies in a BSL-3 design, the visual airflow indicator specifically addresses the critical requirement for maintaining negative pressure in the laboratory space, which is fundamental to preventing airborne transmission of biohazards. This design feature directly supports the necessary precautions required when working with dangerous pathogens that can be aerosolized.