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COVID-19 for USMLE Step 3

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COVID-19 for the USMLE Step 3 Exam
Etiology
  • SARS-CoV-2:
    • COVID-19 is caused by SARS-CoV-2, a novel coronavirus that uses its spike protein to bind to the ACE2 receptor on host cells. ACE2 is expressed in various organs, including the lungs, heart, and blood vessels, explaining the multisystem involvement in severe disease.
  • Variants:
    • Variants such as Delta and Omicron have mutations in the spike protein, affecting transmissibility, immune evasion, and disease severity.
Transmission
  • Respiratory Droplets and Aerosols:
    • COVID-19 primarily spreads through respiratory droplets during close contact, coughing, or talking. Aerosol transmission occurs in poorly ventilated areas.
    • Surface contact transmission is less common but possible if contaminated surfaces are touched, followed by contact with the mouth, nose, or eyes.
  • Incubation Period:
    • The incubation period ranges from 2-14 days (median 4-5 days).
Pathophysiology
  • Viral Entry and Replication:
    • The virus binds to ACE2 receptors on epithelial cells of the respiratory tract. Viral replication leads to cell damage, triggering an immune response and inflammation.
  • Immune Dysregulation:
    • Severe COVID-19 may result in a cytokine storm, with excessive production of inflammatory cytokines (e.g., IL-6, TNF-α), causing widespread inflammation, multi-organ damage, and acute respiratory distress syndrome (ARDS).
  • Coagulopathy:
    • COVID-19 induces a prothrombotic state, increasing the risk of venous and arterial thromboembolism, such as deep vein thrombosis (DVT) and pulmonary embolism (PE).
Clinical Features
  • Mild to Moderate Symptoms:
    • Common symptoms include:
    • Fever
    • Dry cough
    • Fatigue
    • Anosmia and ageusia (loss of smell and taste)
    • Myalgias, headache, sore throat
    • Gastrointestinal symptoms (diarrhea, nausea) in some cases.
  • Severe Disease:
    • Severe cases include:
    • Dyspnea and hypoxia (SpO2 < 94%)
    • Pneumonia
    • Progression to ARDS, with respiratory failure in critical cases.
Diagnosis
  • RT-PCR Testing:
    • RT-PCR from nasopharyngeal or oropharyngeal swabs is the gold standard for diagnosing COVID-19 by detecting viral RNA.
  • Antigen Testing:
    • Rapid antigen tests provide quicker results but have lower sensitivity, especially in asymptomatic individuals or early in infection.
  • Imaging:
    • Chest X-ray and CT scans may show ground-glass opacities and consolidations, especially in patients with pneumonia or ARDS.
Management
Mild to Moderate Disease
  • Supportive Care:
    • Treatment includes hydration, antipyretics for fever, and rest. Most mild cases can be managed at home.
  • Monoclonal Antibodies:
    • Sotrovimab and other monoclonal antibodies can be used in high-risk patients to prevent disease progression.
Severe Disease
  • Hospitalization:
    • Patients with severe COVID-19, especially those with hypoxia or respiratory failure, may require hospitalization for oxygen therapy.
  • Corticosteroids:
    • Dexamethasone (6 mg daily for 10 days) is recommended for patients requiring supplemental oxygen or mechanical ventilation, reducing mortality by dampening the inflammatory response.
  • Antivirals:
    • Remdesivir is an antiviral agent used in hospitalized patients to reduce the duration of illness and improve outcomes, particularly in those requiring oxygen therapy.
  • Anticoagulation:
    • Prophylactic anticoagulation is essential in hospitalized patients due to the increased risk of thromboembolism. Therapeutic anticoagulation is used in cases of confirmed thrombotic events (e.g., DVT, PE).
Complications
  • ARDS (Acute Respiratory Distress Syndrome):
    • Severe lung inflammation can lead to respiratory failure requiring mechanical ventilation and intensive care.
  • Thromboembolic Events:
    • COVID-19 increases the risk of pulmonary embolism, stroke, and deep vein thrombosis.
  • Multi-organ Failure:
    • Severe systemic inflammation can result in failure of the lungs, kidneys, liver, and heart.
Prevention
  • Vaccination:
    • COVID-19 vaccines (e.g., Pfizer, Moderna) are effective in preventing severe disease, hospitalization, and death. Booster doses are recommended to enhance protection against variants.
  • Non-Pharmacologic Measures:
    • Masking, social distancing, and hand hygiene remain key strategies to reduce transmission, especially in high-risk settings.
Key Points
  • COVID-19 is caused by SARS-CoV-2, transmitted through respiratory droplets and aerosols, with an incubation period of 2-14 days.
  • Symptoms range from mild (fever, cough, fatigue) to severe (dyspnea, ARDS, multi-organ failure).
  • Diagnosis is confirmed by RT-PCR, while antigen tests are useful but less sensitive. Imaging may reveal ground-glass opacities in severe cases.
  • Treatment includes supportive care for mild cases, dexamethasone for severe cases requiring oxygen, and remdesivir for hospitalized patients. Anticoagulation is critical due to the risk of thromboembolism.
  • Vaccination is the most effective preventive measure, along with non-pharmacologic strategies like masking and social distancing.

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