Final answer:
The most important finding to report to the primary healthcare provider immediately is elevated liver enzyme levels.
Step-by-step explanation:
The most important finding to report to the primary healthcare provider immediately when assessing a 38-year-old client with tuberculosis who is taking rifampin would be elevated liver enzyme levels (option C). Rifampin is known to cause hepatotoxicity, which can manifest as elevated liver enzyme levels. This finding indicates potential liver damage and requires immediate medical attention.