Standardization of antibiotic potency: In the 1944 definition, one “International Unit” (IU) of penicillin activity is equivalent to what mass of crystalline penicillin G (approximately 98% purity)?

Introduction / Context:
International Units (IU) provide a standardized measure of biological activity, ensuring consistency across batches and laboratories. For penicillin, potency depends on antimicrobial activity rather than mass alone, and the IU anchors that activity to a reference substance of defined purity.

Given Data / Assumptions:

• The reference is penicillin G (benzylpenicillin), the archetypal penicillin used in early standards.
• Purity of the reference crystal is approximately 98%.
• The historical IU definition is widely reproduced in pharmacopeias and microbiology texts.

Concept / Approach:
The 1944 standard equated 1 IU of penicillin activity to 0.6 micrograms of highly pure penicillin G. This mass-to-activity mapping allows dosing and quality control by bioassay or equivalently calibrated physicochemical methods, bridging biological potency with manufacturing specifications.

Step-by-Step Solution:
Identify the active reference: crystalline penicillin G.Recall the canonical conversion: 1 IU ≈ 0.6 micrograms at ~98% purity.Select the exact matching option and rule out alternatives with different masses, purities, or penicillin types.

Verification / Alternative check:
Pharmacopeial monographs and historical standards list the 0.6 microgram equivalence for penicillin G activity.

Why Other Options Are Wrong:
Options that specify penicillin F or different mass/purity do not match the official historical definition. Lower purity alters mass-to-IU equivalence and is therefore incorrect.

Common Pitfalls:
Confusing milligrams and micrograms; overlooking that purity is part of the definition.

Final Answer:
0.6 micrograms of crystalline penicillin G of about 98% purity