A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 75-year-old woman is brought to the emergency department by her daughter because of shortness of breath and a productive cough with blood-tinged sputum for the past 24 hours. Five days ago, she developed muscle aches, headache, fever, and clear rhinorrhea. These symptoms lasted 3 days. She lives in a house with her daughter. Her temperature is 39.3°C (102.8°F), pulse is 118/min, respirations are 22/min, and blood pressure is 100/60 mm Hg. She appears lethargic. Physical examination shows scattered crackles and rhonchi throughout both lung fields. An x-ray of the chest shows bilateral lobar opacities and several small, thin-walled cystic spaces with air-fluid levels within the pulmonary parenchyma. Which of the following is the most likely causal pathogen?
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A
Mycobacterium tuberculosisIncorrect. TB cavitation evolves over weeks to months with night sweats, weight loss, and apical predominance — not 24-hour post-flu necrotizing pneumonia.
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B
Staphylococcus aureusCorrect. Post-influenza pneumonia with rapid progression, hemoptysis, and cavitary lesions (pneumatoceles) classically points to Staphylococcus aureus.
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C
Legionella pneumoniaeIncorrect. Legionella causes lobar pneumonia with GI symptoms and hyponatremia, but does NOT typically cavitate.
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D
Streptococcus agalactiaeIncorrect. Group B Streptococcus (S. agalactiae) is a neonatal pathogen; in adults it primarily causes invasive disease in diabetics and pregnant women — not post-influenza cavitary pneumonia.
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E
Klebsiella pneumoniaeIncorrect. Klebsiella causes lobar pneumonia with currant-jelly sputum and upper-lobe cavitation in alcoholics or diabetics, but it is not the classic post-influenza superinfection that S. aureus is.
↑ Tap an answer to reveal the reasoning
Answer: B. Five days after a viral upper respiratory illness (myalgia, headache, fever, clear rhinorrhea — classic influenza), an elderly woman now develops high fever, productive cough with BLOOD-TINGED sputum, hemodynamic compromise, and chest x-ray showing bilateral lobar opacities WITH THIN-WALLED CYSTIC SPACES/PNEUMATOCELES containing air-fluid levels. This is post-influenza necrotizing pneumonia, classically caused by STAPHYLOCOCCUS AUREUS (including MRSA and Panton-Valentine leukocidin (PVL)-producing strains). The viral infection damages respiratory epithelium and impairs mucociliary clearance, predisposing to bacterial superinfection.
The pathognomonic radiographic clue is pneumatocele/cavitary lesions in a post-influenza setting — S. aureus produces these via toxin-mediated tissue necrosis. Other key bugs:
- Streptococcus pneumoniae: most common bacterial cause of post-influenza pneumonia overall, but causes lobar consolidation WITHOUT cavitation
- Haemophilus influenzae: also a common bacterial superinfection
- Mycobacterium tuberculosis: cavitary disease but indolent course with weight loss/night sweats over weeks-months, not 24 hours
- Legionella: causes pneumonia with GI symptoms and hyponatremia, no cavitation
- Streptococcus agalactiae (GBS): neonatal sepsis/meningitis, not adult post-flu pneumonia
Treatment: empiric vancomycin or linezolid for MRSA coverage plus a beta-lactam for pneumococcus.
Pearl: post-influenza pneumonia with cavitary or hemorrhagic features = think Staph aureus.
**Why each option:**
**A.** TB cavitation evolves over weeks to months with night sweats, weight loss, and apical predominance — not 24-hour post-flu necrotizing pneumonia.
**B.** Correct. Post-influenza pneumonia with rapid progression, hemoptysis, and cavitary lesions (pneumatoceles) classically points to Staphylococcus aureus.
**C.** Legionella causes lobar pneumonia with GI symptoms and hyponatremia, but does NOT typically cavitate.
**D.** Group B Streptococcus (S. agalactiae) is a neonatal pathogen; in adults it primarily causes invasive disease in diabetics and pregnant women — not post-influenza cavitary pneumonia.