State Koch's postulates in brief.

• A. A microbe is found in diseased hosts, isolated in pure culture, causes disease when inoculated into a healthy host, and re-isolates from the host
• B. Diseases are caused by imbalances in humors
• C. Microbes are always found in all organisms
• D. Antisepsis can prevent infection

Answer: (A)

Koch's postulates establish causation by linking a specific microbe to a disease through four steps: the microbe should be found in diseased hosts, isolated in pure culture, capable of causing the disease when introduced into a healthy, susceptible host, and then re-isolated from the newly diseased host. The description in this item matches that sequence, making it the best way to summarize Koch’s criteria. This framework underpinned early germ theory and driven many classic demonstrations that a particular organism can cause a specific disease. In practice, there are limitations—some microbes can’t be grown in pure culture, some diseases involve multiple organisms or host factors, and ethical or practical constraints can prevent deliberate infection of hosts; modern approaches often supplement with molecular evidence of virulence factors. The other statements refer to outdated or unrelated ideas (humoral theory, universal presence of microbes, or antisepsis) and don’t describe the method for proving causation.