The correct word is MRI. The physician misspeaks. She does say MRA but probably means MRI.. The two procedures are not interchangeable. Earlier in the report she does refer to MRI and again later in the report says, "If the MRI is again normal, ...".
To flag a word, you would leave a blank line within the sentence like this, __________, place a sticky note on the report, referencing the inaudible word. This alerts the grader to a "flagged" word. The transcriptionist on the job would flag this to the attention of the dictator.
Hope this helps.