Final answer:
The 75-year-old patient's symptoms of right arm weakness, slurred speech, and altered mental status are indicative of a stroke, which is caused by an interruption of the blood supply to the brain.
Step-by-step explanation:
The patient exhibits right arm weakness, slurred speech, and changes in his usual mental status, which are classic symptoms associated with a stroke. A stroke occurs when the blood supply to part of the brain is interrupted or reduced, preventing brain tissue from getting oxygen and nutrients. Strokes can lead to symptoms like muscle weakness (hemiparesis), speech difficulties, and sensory disturbances such as numbness or a pins and needles sensation. Based on the age of the patient and the presented symptoms, it is unlikely that the symptoms are due to seizure, diabetic ketoacidosis, or opioid overdose which present with different clinical signs.