What physical principle explains why a paper disk impregnated with an antibiotic produces a circular zone of inhibition on a lawn of bacteria in the Kirby-Bauer test?

• A. Antibiotic molecules diffuse radially through the agar, creating a concentration gradient; growth is inhibited where local antibiotic concentration exceeds the organism's inhibitory threshold, producing a circular zone whose radius is inversely related to the inhibitory concentration.
• B. Chemical reactions between the disk matrix and agar produce heat that denatures bacterial proteins in a circular pattern around the disk; the larger the heated zone, the more susceptible the bacteria are to thermal inactivation caused by the antibiotic-impregnated disc binder.
• C. Capillary action draws the antibiotic into the agar only along straight lines radiating from the disk, and where this capillary flow is strongest bacteria are inhibited, creating a circular appearance only due to the diffusion of nutrients rather than the antibiotic itself.
• D. Antibiotic molecules are actively transported by bacterial cells away from the disk, and the balance between bacterial uptake and metabolism creates a depletion zone in which bacteria cannot grow because the antibiotic is sequestered by the cells near the disk.

Answer: (A)