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COVID-19 for USMLE Step 1

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COVID-19 for the USMLE Step 1 Exam
Etiology
  • SARS-CoV-2:
    • COVID-19 is caused by SARS-CoV-2, an enveloped, single-stranded RNA virus from the Coronaviridae family.
    • The virus enters human cells by binding to the ACE2 receptor via its spike protein. ACE2 is expressed on the epithelial cells of the lungs, intestines, heart, and vasculature, contributing to multi-organ involvement.
  • Variants:
    • Variants arise due to mutations in the spike protein, leading to differences in transmissibility and immune evasion. Examples include the Delta and Omicron variants.
Transmission
  • Respiratory Droplets:
    • COVID-19 is primarily spread through respiratory droplets when infected individuals cough, sneeze, or talk. Transmission can also occur through aerosols in poorly ventilated areas and through contact with contaminated surfaces (fomites).
  • Incubation Period:
    • The incubation period ranges from 2-14 days, with a median of 4-5 days.
Pathophysiology
  • Viral Entry:
    • SARS-CoV-2 uses the spike protein to bind to ACE2 receptors on host cells, primarily in the lungs. After entering the cell, the virus replicates, causing cell damage, inflammation, and impaired lung function.
  • Immune Response and Cytokine Storm:
    • In severe cases, a dysregulated immune response leads to a cytokine storm, characterized by excessive release of pro-inflammatory cytokines like IL-6 and TNF-α, resulting in tissue damage, ARDS (acute respiratory distress syndrome), and multi-organ failure.
  • Thrombosis:
    • COVID-19 is associated with a hypercoagulable state, with increased risk of thromboembolic events such as deep vein thrombosis (DVT) and pulmonary embolism (PE).
Clinical Features
  • Mild to Moderate Symptoms:
    • Common symptoms include:
    • Fever
    • Cough (dry or productive)
    • Fatigue
    • Myalgias
    • Headache
    • Anosmia (loss of smell) and ageusia (loss of taste)
    • Sore throat and rhinorrhea
    • Gastrointestinal symptoms (diarrhea, nausea) are less common.
  • Severe Disease:
    • Progression to severe disease occurs in approximately 15-20% of cases. Features include:
    • Dyspnea and hypoxia (SpO2 < 94%)
    • Pneumonia
    • ARDS in critical cases.
Diagnosis
  • PCR Testing:
    • The gold standard for diagnosis is RT-PCR from nasopharyngeal or oropharyngeal swabs, which detects viral RNA.
  • Antigen Testing:
    • Rapid antigen tests can detect viral proteins but are less sensitive than PCR, especially in asymptomatic or early-stage infections.
  • Chest Imaging:
    • Chest X-ray or CT scan may show ground-glass opacities in patients with pneumonia.
Management
Mild to Moderate Disease
  • Supportive Care:
    • Mild cases can be managed with hydration, rest, and antipyretics (e.g., acetaminophen for fever).
    • Monoclonal antibodies (e.g., sotrovimab) may be used in high-risk patients to prevent progression.
Severe Disease
  • Hospitalization:
    • Patients with hypoxia or respiratory distress may require hospitalization for oxygen therapy.
  • Corticosteroids:
    • Dexamethasone (6 mg daily) is recommended for patients with severe COVID-19 requiring oxygen or mechanical ventilation, as it reduces mortality by modulating the immune response.
  • Antivirals:
    • Remdesivir is an antiviral used in hospitalized patients to reduce the duration of illness, especially those requiring supplemental oxygen.
  • Anticoagulation:
    • Prophylactic anticoagulation is recommended due to the increased risk of thrombosis in hospitalized patients.
Complications
  • ARDS (Acute Respiratory Distress Syndrome):
    • ARDS is a severe form of lung injury leading to respiratory failure, requiring mechanical ventilation.
  • Thromboembolism:
    • Increased risk of thrombotic events such as pulmonary embolism and stroke due to the hypercoagulable state induced by SARS-CoV-2.
  • Multi-organ Failure:
    • In severe cases, dysregulated immune response and cytokine storm can lead to multi-organ failure, including kidney, liver, and heart dysfunction.
Prevention
  • Vaccination:
    • COVID-19 vaccines (e.g., mRNA vaccines like Pfizer and Moderna) are highly effective in preventing severe disease, hospitalization, and death. Booster doses are recommended to maintain immunity, particularly against emerging variants.
  • Non-Pharmacologic Interventions:
    • Masking, social distancing, and hand hygiene are effective strategies for reducing transmission, particularly in high-risk settings.
Key Points
  • COVID-19 is caused by the SARS-CoV-2 virus, which primarily affects the respiratory system and is transmitted through respiratory droplets and aerosols.
  • Common symptoms include fever, cough, fatigue, and loss of smell or taste. Severe cases may lead to ARDS, pneumonia, and multi-organ failure.
  • Diagnosis is confirmed by RT-PCR, while rapid antigen tests are useful but less sensitive. Imaging may show ground-glass opacities in severe cases.
  • Management includes supportive care for mild cases, with dexamethasone and remdesivir used for severe disease requiring hospitalization. Anticoagulation is necessary in hospitalized patients due to the risk of thromboembolic events.
  • Vaccination is the most effective preventive measure, reducing the risk of severe illness and death.

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