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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Influenza classically has abrupt onset fever, chills, myalgias, headache, cough, sore throat, and profound fatigue
- Treat hospitalized, severe, progressive, or high-risk patients with oseltamivir as soon as possible, even if >48 hours after onset
- Otherwise healthy outpatients benefit most if antivirals are started within 48 hours
- Major complications include viral pneumonia, secondary bacterial pneumonia, asthma/COPD exacerbation, myositis, myocarditis, and encephalitis
- Post-influenza necrotizing pneumonia suggests Staphylococcus aureus, including MRSA
Overview
Influenza is caused by influenza A or B viruses and spreads by respiratory droplets, aerosols, and contact. Antigenic drift causes seasonal epidemics; antigenic shift in influenza A can cause pandemics. Diagnosis is often clinical during seasonal outbreaks, but testing guides antiviral and infection-control decisions.
Epidemiology
Seasonal influenza causes substantial annual morbidity, hospitalization, and deaths in the United States, with highest risk in adults age >=65, young children, pregnant and postpartum patients, long-term care residents, immunocompromised patients, and people with chronic disease.
Clinical Features
Symptoms
Progressive or acute dyspnea depending on severity and underlying cause
Cough, chest discomfort, fatigue, or exercise limitation
Fever, weight loss, night sweats, or hemoptysis when infection or malignancy is present
Syncope, confusion, severe hypoxemia, or rapidly worsening respiratory distress
Symptoms may be absent when the condition is detected incidentally
Signs
Abnormal breath sounds, crackles, wheeze, dullness, or reduced air entry depending on pathology
Tachypnea, tachycardia, or oxygen desaturation when clinically significant
Cyanosis, hypotension, altered mental status, or respiratory exhaustion
Clubbing, lymphadenopathy, cachexia, or signs of chronic disease when present
Physical examination can be normal in early or mild disease
Investigations
First-line
Focused history and examinationIdentify timing, exposures, smoking, travel, medications, immune status, occupational risks, and red flags
Pulse oximetryRapid assessment of oxygenation and severity
Chest imagingChest X-ray is often initial; CT is used when more anatomic detail, malignancy risk, complications, or alternative diagnosis must be clarified
Second-line
Laboratory testingCBC, CMP, inflammatory markers, cultures, viral testing, autoimmune tests, or disease-specific markers depending on presentation
Pulmonary function testingUseful for chronic dyspnea, obstructive/restrictive patterns, DLCO, and treatment response
Microbiology or pathologySputum studies, cytology, biopsy, or fluid analysis when infection, cancer, or inflammatory disease is suspected
Specialist
Specialist referralPulmonology, infectious disease, oncology, sleep medicine, critical care, or thoracic surgery depending on diagnosis and severity
Advanced testingBronchoscopy, PET-CT, sleep study, HRCT, echocardiography, or interventional radiology procedures when indicated
1
Initial management
- Assess severity, oxygenation, hemodynamics, airway risk, and need for hospital or ICU care
- Treat immediately reversible threats such as hypoxemia, bronchospasm, sepsis, PE, pneumothorax, or respiratory failure
- Use diagnosis-specific guideline therapy rather than empiric escalation without a working differential
2
Definitive treatment
- Treat the underlying cause according to US guideline recommendations
- Use imaging, microbiology, pathology, physiology, and risk stratification to guide therapy
- Escalate to procedural or specialist management when medical therapy is insufficient or diagnosis remains uncertain
3
Follow-up and prevention
- Arrange follow-up imaging or functional testing when recommended
- Address smoking cessation, vaccination, occupational exposure, medication toxicity, and comorbidity optimization
- Educate patients on red flags including worsening dyspnea, hemoptysis, syncope, hypoxemia, or persistent fever
Complications
- Respiratory failure: Severe disease can progress to hypoxemia, hypercapnia, or need for ventilatory support
- Secondary infection: Damaged or obstructed lung is prone to bacterial infection
- Pulmonary hypertension: Chronic hypoxemia or parenchymal disease can increase pulmonary vascular resistance
- Delayed diagnosis: Incidental or nonspecific presentations may hide malignancy, TB, PE, or ILD
- Treatment toxicity: Antimicrobials, corticosteroids, anticoagulants, chemotherapy, or procedures can cause harm
USMLE Step 2 CK Exam Tips
- 1Abrupt fever + severe myalgias + cough during winter = influenza
- 2High-risk or hospitalized patient gets oseltamivir even if symptoms began >48 hours ago
- 3Healthy outpatient gets greatest antiviral benefit within 48 hours
- 4Post-influenza pneumonia with cavitation or rapid necrosis = Staphylococcus aureus, including MRSA
- 5Do not give aspirin to children with influenza because of Reye syndrome
- 6Zanamivir can cause bronchospasm; avoid in asthma or COPD
- 7Recurrent fever after initial improvement suggests secondary bacterial pneumonia
practicetest your knowledge on influenzaApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — respiratory and beyond.
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