Final answer:
Adverse event reporting is a regulatory challenge of taking on the 'sponsor' role in investigator-initiated studies for a sponsor-investigator that is not the regulatory responsibility of an investigator.
Step-by-step explanation:
A regulatory challenge of taking on the 'sponsor' role in investigator-initiated studies for a sponsor-investigator that is not the regulatory responsibility of an investigator is adverse event reporting. Adverse event reporting is the process of documenting and reporting any unfavorable or undesirable event that occurs during a study. It is the responsibility of the sponsor to ensure that all adverse events are promptly reported to the appropriate regulatory authorities.