A live/attenuated vaccine contains:

• A. Inactivated toxins
• B. Killed organisms
• C. A synthetic peptide
• D. A modified living version of a microorganism

Answer: (D)

Live attenuated vaccines use a weakened form of the microbe that can still replicate in the body. That limited replication presents the immune system with a broad range of antigens in a way that closely mimics natural infection, which tends to provoke strong and long-lasting immunity involving both antibodies and T cells. Because the organism is alive but weakened, it can stimulate a more robust and durable response, often with fewer doses. This contrasts with vaccines that use inactivated toxins (toxoids), which protect against toxins rather than the pathogen itself; vaccines made from killed organisms, which cannot replicate; or vaccines built from just a synthetic peptide or other subunit pieces of the microbe, which usually require more boosters or adjuvants to achieve strong immunity. So the approach described is a modified living version of a microorganism.