What is essential regarding the rubber closure on a vial?

The essential aspect regarding the rubber closure on a vial is that it can be contaminated by flip top caps. The surface of the rubber stopper is particularly sensitive to contamination when the protective flip top is removed. If aseptic technique is not followed, or if the rubber closure is not disinfected properly before needle insertion, there's a risk of introducing contaminants into the vial. This is crucial to understanding because any breach in sterility can compromise the integrity of the medication contained within the vial.

While there are various features of a vial's closure, the other choices don’t accurately reflect the critical points about how rubber closures work in the context of sterile compounding. Long-term sterility is a characteristic of the entire system, including storage and handling practices, rather than just the closure. Although rubber closures are generally removable, this is not a defining characteristic that relates to sterility. Additionally, rubber is the typical material for stoppers, while glass is used for the vial itself, making the choice regarding glass construction not directly relevant to the context of proper closure management and contamination risks.