Confusable diagnoses · PANCE / PANRE

Community-Acquired Pneumonia vs Influenza

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Community-Acquired Pneumonia and Influenza are easy to mix up on the boards. Here's a side-by-side comparison — presentation, workup, imaging, and first-line treatment — drawn from our full outlines.

Community-Acquired Pneumonia vs Influenza at a glance

  • Community-Acquired Pneumonia: Lower respiratory tract infection acquired outside of healthcare settings.
  • Influenza: Acute viral respiratory illness from influenza A or B with seasonal epidemics and pandemic potential.

Try two board-style questions on Community-Acquired Pneumonia vs Influenza

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Question 1Infectious DiseaseEasy
A 65-year-old immunocompetent female is hospitalized with community-acquired pneumonia (Pneumonia Severity Index (PSI) class IV). She has no recent antibiotics, no healthcare exposure, and no methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas risk factors. Which of the following is the most appropriate empiric antibiotic regimen?
  • ACefazolin plus metronidazole
  • BAzithromycin monotherapy
  • CCeftriaxone plus azithromycin
  • DTrimethoprim-sulfamethoxazole
Reveal answer & full explanation
Correct answer: C — Ceftriaxone plus azithromycin
  • ACefazolin plus metronidazole
  • BAzithromycin monotherapy
  • CCeftriaxone plus azithromycin✓
  • DTrimethoprim-sulfamethoxazole

Why Ceftriaxone plus azithromycin is correct

  • Hospitalized non-ICU community-acquired pneumonia (CAP) per Infectious Diseases Society of America/American Thoracic Society (IDSA/ATS) 2019 guidelines: beta-lactam plus macrolide OR respiratory fluoroquinolone monotherapy
  • Beta-lactam options: ceftriaxone 1 g IV daily, cefotaxime, or ampicillin-sulbactam
  • Macrolide: azithromycin 500 mg IV/PO daily
  • Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg or moxifloxacin 400 mg): equivalent efficacy; avoid if recent fluoroquinolone use
  • Duration: 5 days if clinically improving

Why the others are wrong

  • Cefazolin plus metronidazole — cefazolin is not a recommended CAP beta-lactam (unreliable Haemophilus influenzae coverage), the pairing leaves atypical pathogens (Legionella, Mycoplasma, Chlamydophila) uncovered, and IDSA/ATS 2019 advises against routinely adding anaerobic coverage unless lung abscess or empyema is suspected
  • Azithromycin monotherapy — appropriate only for outpatient low-severity CAP; not adequate for hospitalized PSI class IV
  • Trimethoprim-sulfamethoxazole — not a recommended empiric agent for CAP
Question 2Infectious DiseaseMedium
A 28-year-old male presents with 3 days of fever, myalgia, headache, and dry cough during influenza season. Rapid influenza A test is positive. He is otherwise healthy. Which of the following best describes the mechanism by which oseltamivir reduces influenza duration?
  • AM2 ion channel inhibition
  • BPolymerase acidic protein inhibition
  • CNeuraminidase inhibition
  • DReverse transcriptase inhibition
Reveal answer & full explanation
Correct answer: C — Neuraminidase inhibition
  • AM2 ion channel inhibition
  • BPolymerase acidic protein inhibition
  • CNeuraminidase inhibition✓
  • DReverse transcriptase inhibition

Why Neuraminidase inhibition is correct

  • Oseltamivir (Tamiflu) and zanamivir are neuraminidase inhibitors
  • They prevent release of newly assembled virions from infected cells
  • Effective against both influenza A and B
  • Reduces symptom duration by ~1 day if started within 48 hours of symptom onset; also reduces complications
  • Indications: severe illness, hospitalization, high-risk patients (age ≥65, pregnant, immunocompromised, chronic disease)

Why the others are wrong

  • M2 ion channel inhibition — This is the mechanism of amantadine and rimantadine, which are only active against influenza A, have widespread resistance, and are no longer recommended
  • Polymerase acidic protein inhibition — This is the mechanism of baloxavir (Xofluza), a newer single-dose agent, not oseltamivir
  • Reverse transcriptase inhibition — This is an antiretroviral mechanism used against HIV, not influenza

Additional high-yield points

  • Vaccination is the most effective prevention of influenza
  • Reye syndrome: avoid aspirin in children with influenza
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Side-by-side comparison

FeatureCommunity-Acquired PneumoniaInfluenza
At a glanceLower respiratory tract infection acquired outside of healthcare settings.Acute viral respiratory illness from influenza A or B with seasonal epidemics and pandemic potential.
Classic presentationLobar consolidation with bronchial breath sounds and egophony classically points to S. pneumoniae; bullous myringitis with patchy infiltrates suggests Mycoplasma.; Cough (productive purulent sputum, or dry), fever, chills, rigors; Pleuritic chest pain, dyspnea; Constitutional: fatigue, myalgia, anorexia; Elderly may present atypically:…Abrupt onset (over hours) of fever, chills, myalgia, headache, fatigue; Respiratory: dry cough, sore throat, nasal congestion/rhinorrhea; GI symptoms (vomiting, diarrhea) more common in children; Severe prostration disproportionate to other URI causes; Usually self-limited 3-7 days, but cough and malaise may linger 1-2 weeks; Fever,…
Workup / key labsCBC (leukocytosis with left shift), BMP, lactate, procalcitonin (helps de-escalate antibiotics); Blood cultures × 2 if severe, ICU admission, immunocompromise, or cavitary disease; Sputum Gram stain and culture if severe or specific pathogen suspected; Urine antigens: S. pneumoniae and Legionella (severe CAP, Legionella outbreak,…Often clinical diagnosis during peak season in otherwise well outpatients; Rapid influenza diagnostic tests (RIDT) — point-of-care, modest sensitivity (~50-70%), high specificity; Rapid molecular assays (NAAT/PCR) — higher sensitivity, preferred when treatment decision or hospitalization at stake; RT-PCR — gold standard; multiplex…
ImagingChest radiograph (PA and lateral) — standard first test; chest imaging is REQUIRED to diagnose pneumonia; lobar consolidation, interstitial infiltrate, or cavitation; CT chest if non-resolving, complicated (abscess, empyema), or unclear; Bedside lung ultrasound — acceptable alternative to CXR for diagnosis where expertise exists (ATS…Chest radiograph if hypoxia, focal findings, or worsening symptoms — to exclude primary viral pneumonia or secondary bacterial pneumonia; CT chest if diagnosis uncertain or complicated
First-line treatmentOutpatient, no comorbidities, no recent antibiotics: amoxicillin 1 g TID OR doxycycline 100 mg BID OR macrolide (azithromycin, clarithromycin) only if local pneumococcal macrolide resistance is <25% (most US regions exceed this, so amoxicillin or doxycycline is usually preferred); Outpatient with comorbidities (heart, lung, liver,…Antivirals — start ASAP for symptomatic patients (ideally within 48 h of onset):; Oseltamivir 75 mg BID × 5 days (oral; renal dose adjust) — most widely used; Zanamivir 10 mg (2 inhalations) BID × 5 days — avoid in asthma/COPD (bronchospasm risk); Peramivir 600 mg IV × 1 dose — for those unable to tolerate PO; Baloxavir marboxil 40-80…

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Full outlines

Community-Acquired Pneumonia (CAP) Influenza All 514 diagnoses

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.