**Core Concept**
Pertussis, also known as whooping cough, is a highly contagious respiratory infection caused by the bacterium *Bordetella pertussis*. Effective management of pertussis involves antibiotic treatment, especially in the early stages of the disease, to reduce the duration and severity of illness, as well as transmission to others.

**Why the Correct Answer is Right**
Erythromycin, a macrolide antibiotic, is the preferred treatment for pertussis due to its ability to inhibit protein synthesis in *Bordetella pertussis*. This action effectively reduces the bacterial load, thereby decreasing the duration and severity of symptoms. Erythromycin is typically administered orally and is effective in treating pertussis in both children and adults.

**Why Each Wrong Option is Incorrect**
**Option A:** Ampicillin is not the preferred antibiotic for pertussis as it is not effective against *Bordetella pertussis*.

**Option B:** Gentamicin, an aminoglycoside antibiotic, is not typically used to treat pertussis due to its potential ototoxicity and nephrotoxicity, especially in children.

**Option D:** Penicillin is not effective against *Bordetella pertussis* and is not recommended for the treatment of pertussis.

**Clinical Pearl / High-Yield Fact**
It is essential to initiate antibiotic treatment within the first three weeks of illness to reduce the contagious period and prevent transmission to others.

**Correct Answer Line**
✓ Correct Answer: C. Erythromycin