**Core Concept**
Elek's gel precipitation test is a laboratory diagnostic tool used to detect the presence of diphtheria toxin, a potent exotoxin produced by Corynebacterium diphtheriae, the causative agent of diphtheria. This test relies on the specific precipitation reaction between the diphtheria toxin and its antitoxin.

**Why the Correct Answer is Right**
The Elek's gel precipitation test works by placing a sample of the suspected bacterial culture in a well within a gel medium and then applying a specific antitoxin to an adjacent well. If the culture is indeed producing diphtheria toxin, a visible line of precipitation will form in the gel, indicating a positive result. This reaction is specific to the diphtheria toxin and its corresponding antitoxin, making it a reliable diagnostic tool for identifying C. diphtheriae infections. The test is particularly useful in cases where the bacterial culture is not yet available or when laboratory facilities are limited.

**Why Each Wrong Option is Incorrect**
**Option A:** Gonococcus is the causative agent of gonorrhea, and its diagnosis typically involves Gram staining, culture, or nucleic acid amplification tests. Elek's gel precipitation test is not used for diagnosing gonococcal infections.
**Option C:** H. influenzae, or Haemophilus influenzae, is a bacterium that can cause various infections, including meningitis and otitis media. While it is a significant pathogen, the Elek's gel precipitation test is not used for its diagnosis.
**Option D:** Anthrax is caused by Bacillus anthracis, and its diagnosis typically involves culture, PCR, or serological tests. Elek's gel precipitation test is not used for diagnosing anthrax infections.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Elek's gel precipitation test is specific to the diphtheria toxin and its antitoxin, making it a reliable diagnostic tool for identifying C. diphtheriae infections. This test can be particularly useful in resource-limited settings where more advanced laboratory facilities are not available.

**✓ Correct Answer: B. Diphtheria**