**Question:** The drug of choice for thoracic actinomycosis is-

A. Penicillin
B. Streptomycin
C. Tetracycline
D. Amoxicillin

**Correct Answer:** A. Penicillin

**Core Concept:**
Actinomycosis is a rare, chronic, suppurative bacterial infection caused by Actinomyces species, particularly Actinomyces israelii. It primarily affects the head and neck region, but can also involve other body sites, including the thorax. Actinomycosis is treated with antibiotics targeting anaerobic bacteria, as the infection thrives in an anaerobic environment.

**Why the Correct Answer is Right:**
Penicillin is a beta-lactam antibiotic, which exerts its antimicrobial effect by inhibiting bacterial cell wall synthesis. Actinomyces species possess a unique cell wall structure, which makes them susceptible to beta-lactam antibiotics. Penicillin G is the most effective penicillin against Actinomyces species.

**Why Each Wrong Option is Incorrect:**

A. Streptomycin: Although effective against various Gram-negative bacteria, Streptomycin is not as effective against anaerobic bacteria like Actinomyces species.
B. Tetracycline: Tetracycline is less potent against anaerobic bacteria, making it an inferior choice for treating thoracic actinomycosis.
D. Amoxicillin: Similar to Tetracycline, Amoxicillin is less effective against anaerobic bacteria, including Actinomyces species.

**Why Each Wrong Option is Incorrect:**
A. Streptomycin: Although effective against various Gram-negative bacteria, Streptomycin is not as effective against anaerobic bacteria like Actinomyces species.
B. Tetracycline: Tetracycline is less potent against anaerobic bacteria, making it an inferior choice for treating thoracic actinomycosis.
D. Amoxicillin: Similar to Tetracycline, Amoxicillin is less effective against anaerobic bacteria, including Actinomyces species.

**Clinical Pearl:**
Thoracic actinomycosis is a rare but important differential diagnosis in cases of chronic, indolent, and granulomatous lung disease. Prompt antibiotic treatment with beta-lactam antibiotics, such as Penicillin, is essential for successful therapy.