**Core Concept:** Disc diffusion method is a standard method for determining the susceptibility of microorganisms to antimicrobial agents. It involves placing antimicrobial discs containing known concentrations of antibiotics on an agar plate with a test organism and measuring the zone of inhibition formed around the discs.

**Why the Correct Answer is Right:** The correct answer, **D.** Kirby-Bauer method, is named after the two scientists who independently described a similar technique in the 1960s. This method involves placing antimicrobial discs on agar plates and measuring the zone of inhibition, which is a measure of the antimicrobial agent's potency against the test organism.

**Why Each Wrong Option is Incorrect:**

A. **Gram stain method:** This is a staining technique used to differentiate between Gram-positive and Gram-negative bacteria based on their cell wall structure. It is not a method for determining antimicrobial susceptibility.

B. **MIC method:** MIC stands for Minimum Inhibitory Concentration, which is a quantitative method for determining the susceptibility of microorganisms to antimicrobial agents. It involves exposing microorganisms to a series of antimicrobial concentrations and observing the lowest concentration that prevents visible growth. The disc diffusion method is a qualitative method based on zones of inhibition.

C. **Campbell-Wu method:** This is another qualitative method for determining antimicrobial susceptibility, but it uses a different approach than the Kirby-Bauer method (e.g., measuring the growth inhibition zone surrounding a single disc).

**Clinical Pearl:** Disc diffusion method is a simple, rapid, and cost-effective method for determining the susceptibility of microorganisms to antimicrobial agents. It is widely used in clinical microbiology laboratories to guide the choice of antibiotics for treating infections.