American Board of Internal Medicine (ABIM) Certification Practice Exam

In suspected influenza pneumonia with cavitary lesions, what is the most likely causative organism?

1. Staphylococcus aureus

2. Streptococcus pneumoniae

3. Mycoplasma pneumoniae

4. Legionella pneumophila

The correct answer is: Staphylococcus aureus

When considering suspected influenza pneumonia with cavitary lesions, Staphylococcus aureus emerges as the most likely causative organism. This is particularly relevant in the context of secondary bacterial pneumonia following influenza infection, where Staphylococcus aureus can become a critical pathogen. This organism is known to complicate influenza cases and is associated with necrotizing pneumonia, which can lead to the formation of cavities in the lung tissue. Cavitary lesions in the setting of pneumonia suggest more aggressive and destructive processes, which are characteristic of infections with Staphylococcus aureus, especially after influenza, when the host's defenses are compromised. The organism can cause significant tissue damage and lung necrosis, resulting in cavities. In contrast, while other pathogens like Streptococcus pneumoniae can cause pneumonia, they are less commonly associated with cavitary lesions. Mycoplasma pneumoniae generally leads to atypical pneumonia and does not typically present with cavitary lesions. Legionella pneumophila tends to cause pneumonia with different clinical and radiologic features and is less likely to cause cavitary lesions in relation to influenza. Therefore, the association of Staphylococcus aureus with cavitary lesions, especially in patients with a background of influenza, underscores its prominence as the causative organism in