What occurs in Phase II of the clinical trials?

Phase II of clinical trials is primarily focused on determining the efficacy of a drug in patients who have the specific disease or condition that the drug is intended to treat. This phase typically involves a larger group of participants who are affected by the illness, allowing researchers to evaluate how well the treatment works and whether it produces the desired therapeutic effects.

At this stage, researchers also continue to assess safety and monitor adverse effects, but the main goal is to establish whether the drug demonstrates sufficient effectiveness before moving on to larger populations in Phase III trials. This efficacy determination helps to provide valuable data that can inform regulatory submissions and future research directions.

The other options mentioned refer to different phases or facets of drug development. Investigating safety typically occurs in Phase I, filing a New Drug Application (NDA) happens after successful clinical trials, and conducting large-scale trials is characteristic of Phase III. Therefore, the correct answer focuses specifically on the crucial aspect of assessing efficacy in the Phase II context.