**Core Concept**
Legionnaires' Disease is a type of atypical pneumonia caused by the bacterium *Legionella pneumophila*. The treatment of choice for this infection typically involves macrolide antibiotics due to their intracellular activity and ability to penetrate into the alveolar macrophages where the bacteria reside.

**Why the Correct Answer is Right**
Azithromycin is a macrolide antibiotic that is effective against *Legionella pneumophila* due to its ability to inhibit protein synthesis at the ribosomal level. This mechanism of action allows azithromycin to accumulate within the alveolar macrophages, where the bacteria are located, thereby exerting its bacteriostatic effects. Additionally, azithromycin has a long half-life, which enables once-daily dosing and improves patient compliance.

**Why Each Wrong Option is Incorrect**
**Option A:** Amoxicillin is a beta-lactam antibiotic that is primarily effective against Gram-positive and some Gram-negative bacteria. It does not have sufficient intracellular activity to effectively target *Legionella pneumophila*.
**Option C:** Penicillin is another beta-lactam antibiotic that is typically used to treat infections caused by *Streptococcus pneumoniae*, *Staphylococcus aureus*, and other Gram-positive bacteria. It is not effective against *Legionella pneumophila*.
**Option D:** Streptomycin is an aminoglycoside antibiotic that is primarily used to treat tuberculosis and certain types of Gram-negative bacterial infections. It is not the treatment of choice for Legionnaires' Disease.

**Clinical Pearl / High-Yield Fact**
When treating atypical pneumonias such as Legionnaires' Disease, it is essential to use antibiotics that have intracellular activity, such as macrolides (e.g., azithromycin, clarithromycin), fluoroquinolones (e.g., levofloxacin, moxifloxacin), or doxycycline. These antibiotics can effectively target the bacteria within the alveolar macrophages, thereby reducing the risk of treatment failure.

**✓ Correct Answer: B. Azithromycin**