Which of Koch's Postulates Would Be Especially Difficult to Satisfy

Multiple Choice

Which of Koch's postulates would be especially difficult to satisfy when diagnosing an infection caused by a chlamydia?

A) The microorganism is always found in individuals with the disease.
B) The microorganism can be taken from the host and grown in pure culture.
C) A sample of the culture produces the same disease when injected into a new, healthy host.
D) The newly infected host yields a new, pure culture of microorganisms identical to those obtained in the original diseased individual.
E) Chlamydias meet all the requirements to validate Koch's postulates.