Final answer:
The most appropriate first step for the teacher to take is to determine whether the student has adequate vocabulary and background knowledge to support comprehension of the textbooks.
Step-by-step explanation:
The most appropriate first step for the teacher to take in order to determine the cause of the student's difficulty is to Determine whether the student has adequate vocabulary and background knowledge to support comprehension of the textbooks. Assessing the student's level of vocabulary and background knowledge is crucial in understanding why their comprehension fluctuates widely, even if they can read orally with accuracy on grade level. This step will help identify if the student needs additional support in building their vocabulary and background knowledge to improve their comprehension of grade-level textbooks.