A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 65-year-old woman is brought to the emergency department by her daughter for fever and cough. She just returned from a cruise trip to the Bahamas with her family 5 days ago and reports that she has been feeling ill since then. She endorses fever, productive cough, and general malaise. Her daughter also mentions that the patient has been having some diarrhea but reports that the rest of her family has been experiencing similar symptoms. Physical examination was significant for localized crackles at the right lower lobe. Laboratory findings are as follows:
Serum
Na+: 130 mEq/L
K+: 3.9 mEq/L
Cl-: 98 mEq/L
HCO3-: 27 mEq/L
Mg2+: 1.8 mEq/L
What findings would you expect in this patient?
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A
Broad-based budding on fungal sputum cultureIncorrect. Broad-based budding describes Blastomyces dermatitidis, a fungal pneumonia from soil exposure in Mississippi/Ohio river valleys, not this presentation.
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B
Gram-negative rod on chocolate agar with factors V and XIncorrect. Chocolate agar with factors V and X grows Haemophilus influenzae, which causes pneumonia in COPD patients but doesn't typically cause hyponatremia and GI symptoms after water exposure.
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C
Gram-negative on silver stainCorrect. Legionella pneumophila stains poorly on Gram stain and is best visualized with silver stain; it causes pneumonia with hyponatremia and diarrhea after water-aerosol exposure.
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D
Gram-positive diplococci on Gram stainIncorrect. Gram-positive diplococci on Gram stain identifies Streptococcus pneumoniae, which would not explain the cruise-ship water exposure, GI symptoms, and hyponatremia pattern.
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E
Acid-fast bacilli on Ziehl-Neelsen stainIncorrect. Acid-fast bacilli identify mycobacteria such as M. tuberculosis, which causes chronic indolent cavitary pneumonia rather than the acute cruise-ship pneumonia with hyponatremia and diarrhea seen here.
↑ Tap an answer to reveal the reasoning
Answer: C. An elderly woman who recently traveled (cruise ship — classic exposure source via aerosolized water from cooling systems, hot tubs, fountains) and now has high fever, cough, GI symptoms (diarrhea), hyponatremia (Na 130), and lower-lobe pneumonia has Legionnaires disease, caused by Legionella pneumophila.
Legionella is a small, fastidious gram-negative rod that stains poorly with routine Gram stain and is best visualized with silver stain (Dieterle, Warthin-Starry). It grows on buffered charcoal yeast extract agar with cysteine and iron. Clinical clues include the triad of high fever, relative bradycardia, and GI symptoms (especially diarrhea), plus hyponatremia (from SIADH), elevated transaminases, and pneumonia. Diagnosis is confirmed by urinary antigen test for serogroup 1. Treatment: macrolide (azithromycin) or fluoroquinolone (levofloxacin).
Distinguishers: gram-negative rod on chocolate agar with factors V and X is Haemophilus influenzae; gram-positive diplococci is Streptococcus pneumoniae; broad-based budding on fungal culture is Blastomyces. The pearl: water exposure + pneumonia + GI symptoms + hyponatremia = Legionella → silver stain.
**Why each option:**
**A.** Broad-based budding describes Blastomyces dermatitidis, a fungal pneumonia from soil exposure in Mississippi/Ohio river valleys, not this presentation.
**B.** Chocolate agar with factors V and X grows Haemophilus influenzae, which causes pneumonia in COPD patients but doesn't typically cause hyponatremia and GI symptoms after water exposure.
**C.** Correct. Legionella pneumophila stains poorly on Gram stain and is best visualized with silver stain; it causes pneumonia with hyponatremia and diarrhea after water-aerosol exposure.
**D.** Gram-positive diplococci on Gram stain identifies Streptococcus pneumoniae, which would not explain the cruise-ship water exposure, GI symptoms, and hyponatremia pattern.