-Morphine is considered the drug of choice in STEMI because it is a potent analgesic, which is a pain reliever. It acts directly on the (CNS) Central Nervous System which decreases the feeling of pain.
-It is mainly prescribed to treat severe, acute and crushing pain, such as the experienced due to Myocardial Infarction, labor or acute pulmonary Edema. It is used when the pain associated with certain conditions cannot be alleviated with oxygen and nitrates.
In conclusion, Morphine is used to treat the severe pain caused by the St-elevated myocardial infarction due to it's potent analgesic properties.