What feature of a vial allows for a closed system during dispensing?

A tight seal is a critical feature of a vial that contributes to forming a closed system during dispensing. This seal prevents the entry of contaminants from the environment, ensuring that the contents of the vial remain sterile and protected from potential microbial contamination during storage and handling. A tight seal also helps maintain the integrity of the formulation, preventing vapors or volatile substances from escaping.

In sterile compounding, maintaining a closed system is essential to ensure the safety and efficacy of the compounded products, especially when preparing injectable medications. By keeping the vial sealed until it is ready for use, the risk of cross-contamination is significantly reduced.

The other options, such as an open flap, a threaded neck, or a detachable lid, do not provide the same level of protection against contamination. An open flap would allow for exposure to environmental contaminants, a threaded neck may or may not ensure a tight closure, and a detachable lid could compromise the sterility of the contents once it is removed. Therefore, the tight seal is the most effective feature that ensures a closed system during dispensing activities.