Final answer:
The best meaning of "pertinacity" in the passage is determination.
Step-by-step explanation:
The best meaning of "pertinacity" as it is used in the passage is determination. In the context, the narrator's repeated focus on a specific subject convinced Henry that the narrator's disorder had a strong and unwavering determination, rather than being a result of imagination or a lack of concern. The word "pertinacity" implies a persistent and resolute attitude, which aligns with the meaning of determination.
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