Final answer:
Mrs. Frank loses her self-control for the first time because of her husband's infidelity.
Step-by-step explanation:
Mrs. Frank loses her self-control for the first time because of her husband's infidelity. In the given excerpt, Mrs. Frank is shown to have stains on her cheek, indicating her husband's betrayal. This revelation causes her to lose her self-control and confront the issue voluntarily.