Which of the following correctly defines a clinical trial?

A clinical trial is defined as a research study that tests a drug or treatment in human subjects. This definition emphasizes the fundamental purpose of clinical trials, which is to evaluate the safety, efficacy, and effectiveness of new interventions. Clinical trials are essential in the development of new therapies, as they provide the necessary data to determine how a treatment works in human populations and to identify any potential side effects.

Conducting clinical trials is a critical step in the drug development process, following preclinical testing in laboratories and animal studies. These trials often involve various phases – I, II, and III – which progressively assess the treatment's safety and effectiveness in larger groups of patients.

Other options present concepts related to medical research or treatment evaluation but do not accurately reflect the definition of a clinical trial. For instance, involving only healthy volunteers or addressing existing treatment methods would not encompass the broader spectrum or intent behind clinical trials, which are specifically designed to assess new treatments in diverse patient populations. Therefore, defining a clinical trial correctly as a research study testing a drug or treatment in human subjects captures its essence and purpose effectively.