MSRA-0600

A 45-year-old woman presents to the Emergency Department with a 2-week history of dull pain under her tongue while eating. This resolves after she finishes eating and has been gradually getting worse. Over the previous three days, this was also accompanied by swelling of the floor of her mouth. She is afebrile and all of her parameters are stable.
Which of the following is the first-line investigation that needs to be done?