Final answer:
Creatine phosphokinase (CPK), specifically the CPK-MB isoenzyme, is the serum protein that rises soonest after myocardial cell injury, and is crucial in the diagnosis of a heart attack with elevated levels lasting for 1-2 days post-infarction.
Step-by-step explanation:
The serum protein that rises soonest after myocardial cell injury is Creatine phosphokinase (CPK). CPK is the first enzyme to exhibit increased levels within a short time in the blood plasma following a myocardial infarction or heart attack. Specifically the isoenzyme MB form of CPK is indicative of heart damage as this isoenzyme is predominantly found in the heart muscle.
Estimation of CPK-MB is particularly important for the diagnosis of a heart attack. The concentration of CPK maintains a higher level in the plasma for 1-2 days post-infarction. An initial spike in CPK levels can be 10 times the normal concentration, and if subsequent attacks occur the levels can increase dramatically, even up to 300-fold with repeated incidents, which can be a lethal situation.