Wiki Professional Claim - Relationship Code

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I am fairly new to billing and have not run into this scenario before. Adult child in early 20s is insured under stepparent's plan (child not adopted by stepparent). I am completing the claim in Netwerkes and the "patient's relationship to insured" drop down list only lists 9 options. There is not an option for stepchild. Of those 9, only "Child" and "Other Relationship" would be appropriate options. In this case, would it be correct to select "Child" or "Other Relationship"? TIA :)