Community-Acquired Pneumonia
Pulmonology
Illness script · Pulmonology
Community-Acquired Pneumonia
Acute lower respiratory tract infection acquired outside hospital, causing alveolar consolidation and impaired gas exchange.
This illness script for Community-Acquired Pneumonia covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Most common in elderly, infants, immunocompromised, and smokers
- Streptococcus pneumoniae = most common bacterial cause overall
- Atypical pathogens (Mycoplasma, Chlamydophila, Legionella) in younger, healthier adults
- Legionella: air-conditioning/water towers, hyponatremia clue
- Aspiration risk: alcoholism, seizure, dysphagia, altered consciousness
- Viral (influenza, COVID-19, RSV) increasingly recognized in adults
02
Presentation
- Classic triad: fever, productive cough, pleuritic chest pain
- Dyspnea, tachypnea, tachycardia common on exam
- Lobar consolidation signs: dullness to percussion, increased tactile fremitus, egophony
- Atypical (walking) pneumonia: gradual onset, dry cough, headache, minimal exam findings
- Legionella: high fever, GI symptoms, confusion, relative bradycardia
- Timeline: acute (<1 week) for typical; subacute (1–3 weeks) for atypical
03
Pathophysiology
- Pathogen reaches lower airways via aspiration of oropharyngeal flora or inhalation
- Alveolar macrophage response fails → neutrophil recruitment → consolidation
- Alveolar fluid + debris → V/Q mismatch → hypoxemia
- Host inflammatory cascade → systemic SIRS/sepsis in severe cases
04
Diagnostics
- CXR first-line: lobar or patchy infiltrate (may lag symptoms early)
- Sputum Gram stain + culture before antibiotics (not always necessary outpatient)
- Procalcitonin/CBC support bacterial vs. viral distinction but not diagnostic
- Urine Legionella antigen (serogroup 1) and pneumococcal antigen for severe/hospitalized CAP
- CT chest if CXR negative but high suspicion, or concern for obstruction/abscess
- PSI/PORT score or CURB-65 guides inpatient vs. outpatient disposition
05
Management
- Outpatient healthy (no comorbidities): amoxicillin OR doxycycline OR azithromycin monotherapy
- Outpatient with comorbidities: respiratory fluoroquinolone (levofloxacin) OR beta-lactam + macrolide
- Inpatient non-ICU: beta-lactam + macrolide OR respiratory fluoroquinolone
- ICU/severe CAP: beta-lactam + azithromycin OR beta-lactam + fluoroquinolone; add MRSA/Pseudomonas coverage if risk factors
- Admit if CURB-65 ≥2 (confusion, BUN >20, RR ≥30, BP <90/60, age ≥65)
- Follow-up CXR in 6–8 weeks for smokers/elderly to exclude underlying malignancy
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Pulmonary Embolism
PE causes pleuritic pain + hypoxia but CXR typically clear and no fever/consolidation
Acute Bronchitis
Bronchitis has normal CXR without consolidation; productive cough without systemic illness
Lung Abscess
Abscess shows air-fluid level on CXR; fever persists >1–2 weeks despite antibiotics
Tuberculosis
TB features upper-lobe cavitation, night sweats, weight loss, exposure history, chronic course
Keep reading
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- COPD ExacerbationAcute worsening of COPD symptoms beyond normal day-to-day variation, typically triggered by infection or environmental irritants.
Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.