Which of the following describes a nonstandardized extract?

A nonstandardized extract is typically an preparation of biological substances that do not have a universally accepted standard for comparison. In this context, PNU/mL stands for "Pneumococcal Units per milliliter," which is derived from a specific extract but does not conform to an internationally accepted standard. This makes PNU/mL a fitting description for a nonstandardized extract.

The other units mentioned generally either represent standardized measures or are not commonly used as a designation for nonstandardized extracts. AU/mL (Arbitrary Units per milliliter) and BAU/mL (Binding Antibody Units per milliliter) are often used in the context of assays with specific calibrations that allow for standardization. Similarly, NAU/mL (neutralizing antibody units per milliliter) also tends to align with standardized measurements. Therefore, PNU/mL stands out as the best choice for describing a nonstandardized extract due to its lack of universal standardization.