**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by the production of a toxin that can cause respiratory and cardiac complications. The key to treating diphtheria lies in neutralizing this toxin.

**Why the Correct Answer is Right**
Antitoxin is a serum-based treatment that contains antibodies against the diphtheria toxin. When administered, it binds to the toxin and prevents it from causing further damage to the body. This is crucial in reducing the severity of the disease and preventing complications such as myocarditis and respiratory failure. The antitoxin does not, however, treat the underlying bacterial infection, which is why antibiotics are also necessary in the treatment of diphtheria.

**Why Each Wrong Option is Incorrect**
**Option B:** Tetracycline is a broad-spectrum antibiotic that is effective against a wide range of bacterial infections, but it is not the primary treatment for diphtheria.
**Option C:** Erythromycin is a macrolide antibiotic that is effective against Corynebacterium diphtheriae, but it does not neutralize the toxin and is not the most important element in the treatment of diphtheria.
**Option D:** Penicillin is a beta-lactam antibiotic that is effective against many bacterial infections, but it does not neutralize the diphtheria toxin and is not the primary treatment for diphtheria.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that antitoxin should be administered as soon as possible after the diagnosis of diphtheria to reduce the risk of complications and improve outcomes.

**✓ Correct Answer: A. Antitoxin**