Final answer:
The first action a medical assistant should take when a patient exhibits signs of distress during a stress test is to stop the test to prevent further cardiac strain.
Step-by-step explanation:
If a patient becomes increasingly short of breath and tachycardia with abnormalities appearing on their EKG during a stress test, the first action a medical assistant should take is to stop the stress test immediately. This is essential to prevent further strain on the patient's heart and to reduce the risk of a serious cardiac event. Once the test is stopped, further assessments and interventions, such as checking the patient's blood pressure and administering oxygen, can be considered based on the patient's condition.