Final answer:
During a dental procedure, Trina vomits due to the effects of ether and her nervousness, exacerbated by the dentist's insistence on continuing the dental work despite her distress.
Step-by-step explanation:
The incident that almost caused Raina to vomit occurred during a dental procedure when the dentist, McTeague, insisted on a certain action despite Trina's fear and discomfort. Trina, already affected by the ether and her nervous state, was overwhelmed when McTeague asked her repeatedly if she would allow him to proceed with the dental work. Despite her refusal and clear signs of distress, McTeague persistently approached her, causing her to exclaim, “Oh, I am sick,” and subsequently experience a fit of vomiting. This was a reaction from the ether as well as her nervousness and excitement.