​The main rule to remember of the ICD coding rules,which says that the reason for the patient visit (encounter) is coded first is known as the:

A) ​indexing rule
B) ​reason rule
C) ​supporting rule
D) ​classification rule

ICD Coding Rules

The standards and guidelines for using the International Classification of Diseases system to categorize and code patient diagnoses.

Reason Rule

Refers to the purpose or reason for doing a test or procedure, an insurance company criteria for reimbursement; the reason for the patient visit (encounter) is coded first. This is the primary diagnosis; any other issues the patient presents with (comorbidities) are coded next, in order of importance. This includes chronic conditions that must be considered or will be affected by the treatment for the primary diagnosis.

Indexing Rule

Guidelines or principles used in organizing, categorizing, and locating information or data systematically.