What is the main term and essential modifier in the alphabetic index

Morgan Smith had an acute myocardial infarction, commonly referred to as a heart attack

Main term is acute myocardial infarction

Essential modifier is heart attack

Explanation:

It should be understood that essential modifiers appear next to a lead term or as subterms indented below lead terms in the alphabetical index and do affect the selection of target code. They describe essential differences in site, etiology or type of disorder and must appear in the clinical statement for the code to be assigned.