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COVID-19 for USMLE Step 2

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COVID-19 for the USMLE Step 2 Exam
Etiology
  • SARS-CoV-2:
    • COVID-19 is caused by SARS-CoV-2, an enveloped, single-stranded RNA virus from the Coronaviridae family. It enters cells by binding to the ACE2 receptor via the spike protein, leading to widespread infection.
    • Multiple variants (e.g., Delta, Omicron) have emerged, showing increased transmissibility and varying degrees of immune escape.
Transmission
  • Respiratory Droplets and Aerosols:
    • COVID-19 primarily spreads through respiratory droplets when infected individuals cough, sneeze, or talk. Aerosol transmission occurs in poorly ventilated spaces, and contact with contaminated surfaces (fomites) can also spread the virus.
  • Incubation Period:
    • The incubation period is 2-14 days (median of 4-5 days).
Pathophysiology
  • Viral Entry and Replication:
    • SARS-CoV-2 binds to ACE2 receptors in the lungs, heart, and other organs, allowing viral entry and replication. Viral replication damages epithelial cells, leading to inflammation and impaired gas exchange.
  • Immune Response:
    • Mild disease elicits an appropriate immune response, but in severe cases, a cytokine storm (excessive release of cytokines like IL-6 and TNF-α) can cause widespread tissue damage, including ARDS and multi-organ failure.
  • Hypercoagulability:
    • COVID-19 is associated with a prothrombotic state, leading to an increased risk of venous and arterial thromboembolism (e.g., deep vein thrombosis, pulmonary embolism).
Clinical Features
  • Mild to Moderate Disease:
    • Symptoms include:
    • Fever
    • Cough (dry or productive)
    • Fatigue
    • Myalgias
    • Loss of smell and taste (anosmia and ageusia)
    • Sore throat and headache
    • Gastrointestinal symptoms (nausea, diarrhea) are less common.
  • Severe Disease:
    • Severe cases may progress to:
    • Dyspnea (shortness of breath)
    • Hypoxia (SpO2 < 94%)
    • Pneumonia
    • ARDS in critical cases, leading to respiratory failure.
Diagnosis
  • PCR Testing:
    • RT-PCR from nasopharyngeal or oropharyngeal swabs is the gold standard for detecting viral RNA.
  • Antigen Testing:
    • Rapid antigen tests detect viral proteins but have lower sensitivity compared to PCR, especially in asymptomatic or early cases.
  • Chest Imaging:
    • Chest X-ray or CT scan in severe cases may show ground-glass opacities and consolidations, consistent with viral pneumonia.
Management
Mild to Moderate Disease
  • Supportive Care:
    • Includes hydration, antipyretics (e.g., acetaminophen for fever), and rest. Most cases can be managed at home.
    • In high-risk patients, monoclonal antibodies (e.g., sotrovimab) may be used early to prevent progression.
Severe Disease
  • Hospitalization:
    • Patients with hypoxia or respiratory distress may require hospitalization for oxygen therapy (nasal cannula, high-flow oxygen, or mechanical ventilation in severe cases).
  • Corticosteroids:
    • Dexamethasone (6 mg daily for 10 days) is recommended for patients with severe or critical illness requiring oxygen or mechanical ventilation, as it reduces mortality by dampening the inflammatory response.
  • Antivirals:
    • Remdesivir is an antiviral used in hospitalized patients with moderate to severe disease, reducing the duration of illness in those requiring supplemental oxygen.
  • Anticoagulation:
    • Prophylactic anticoagulation is recommended for all hospitalized patients due to the risk of thromboembolism. Full-dose anticoagulation may be considered for patients with confirmed thrombotic events.
Complications
  • ARDS:
    • Acute respiratory distress syndrome is a life-threatening complication requiring intensive care and mechanical ventilation.
  • Thromboembolism:
    • Increased risk of pulmonary embolism, stroke, and deep vein thrombosis due to the hypercoagulable state.
  • Multi-organ Failure:
    • Severe cases may lead to failure of multiple organ systems, including the kidneys, liver, and heart, often due to cytokine storm.
Prevention
  • Vaccination:
    • COVID-19 vaccines, including mRNA vaccines (e.g., Pfizer-BioNTech, Moderna) and viral vector vaccines (e.g., Johnson & Johnson), are effective in preventing severe disease, hospitalization, and death. Booster doses are recommended to maintain immunity.
  • Non-Pharmacologic Measures:
    • Masking, social distancing, and hand hygiene are essential for reducing transmission, especially in high-risk environments.
Key Points
  • COVID-19 is caused by the SARS-CoV-2 virus, transmitted primarily through respiratory droplets and aerosols, with an incubation period of 2-14 days.
  • Clinical features range from mild (fever, cough, fatigue) to severe (dyspnea, ARDS, multi-organ failure).
  • Diagnosis is confirmed with RT-PCR, and imaging may show ground-glass opacities in severe cases.
  • Management includes supportive care, dexamethasone for severe disease, and remdesivir in hospitalized patients. Anticoagulation is critical in hospitalized patients due to the risk of thromboembolic events.
  • Vaccination is the most effective preventive measure, with booster doses recommended to enhance immunity.

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