What is a visible sign of advanced exposure to a mustard agent after 5 minutes?

A visible sign of advanced exposure to a mustard agent after 5 minutes is dead and gray skin. Mustard gas is a potent chemical warfare agent that causes severe skin damage. Upon exposure, it induces necrosis, leading to the skin appearing gray and dead in areas where the agent has come into contact. This results from the agent's ability to disrupt cellular structures and cause extensive tissue damage.

In contrast, painful burns and swelling with redness are more indicative of immediate or less severe reactions rather than advanced exposure. Minor skin irritation generally occurs with initial contact and often resolves quickly, which does not align with the severe nature of advanced exposure. Thus, the appropriate recognition of dead and gray skin serves as a critical marker for identifying severe mustard agent exposure.