Final answer:
The second researcher is addressing the issue of external validity, which concerns the generalizability of the medication's effectiveness from rats to humans with Alzheimer's disease.
Step-by-step explanation:
When the second researcher plans to test the medication with humans diagnosed with Alzheimer's disease after it was found to improve memory in rats, they are addressing the issue of external validity. External validity refers to the extent to which the results of a study can be generalized or applied to other populations, settings, or times.
Since the medication was initially tested on rats, the researcher now aims to see if these findings are applicable to humans with Alzheimer's disease—an entirely different species and with a specific medical condition—therefore challenging the study's external validity.