How many types of members are explicitly required in an IRB according to the Common Rule?

The correct answer highlights that an Institutional Review Board (IRB) must consist of at least two types of members: a scientist and a nonscientist. This requirement is designed to ensure a diverse perspective during the review process, which is critical for protecting human subjects in research.

The rationale behind having both scientist and nonscientist members is to balance technical expertise with broader ethical perspectives. Scientists are typically knowledgeable about the specific research methods and scientific principles behind the studies being reviewed. In contrast, nonscientist members can contribute insights related to social, cultural, and ethical considerations that may impact participants' rights and welfare.

The Common Rule outlines these membership requirements to protect the interests of human subjects, ensuring that the review process includes varying viewpoints and expertise. By mandating this diversity among board members, the IRB is better equipped to evaluate research protocols comprehensively.

The other options suggest requirements that either overstate or misinterpret the composition needed for an IRB under the Common Rule. For example, citing more than two categories dilutes the primary requirement, and stating only one type overlooks the essential balance of perspectives necessary in ethical review.