COVID-19

Risk Factors

Age >65, diabetes mellitus, hypertension, chronic kidney disease, chronic lung disease, coronary artery disease, obesity (BMI ≥40), cardiomyopathy, immunocompromised states, malignancy, chronic liver disease, cerebrovascular disease

Etiology

SARS-CoV-2, a single-stranded RNA betacoronavirus. Enters host cells via the ACE2 receptor. Transmitted human-to-human via respiratory droplets and aerosols. Notable variants: Alpha (B.1.1.7), Delta (B.1.617), Omicron (B.1.1.529)

Presentation

Acute fever + dry cough + dyspnea. May present with anosmia/ageusia, fatigue, myalgia, sore throat, headache, diarrhea, nausea/vomiting. Elderly/immunocompromised may present atypically: delirium, decreased mobility, no fever

Classic Exam

Tachypnea (RR ≥30/min), $S p O_{2}$ <93% on room air, bilateral crackles on auscultation, signs of respiratory distress (accessory muscle use, nasal flaring). In critical cases: cyanosis, hemodynamic instability, altered mental status

Diagnostics

RT-PCR (nasopharyngeal swab) is the confirmatory test. Chest X-ray/CT: bilateral ground-glass opacities (GGO), consolidation. Labs: lymphopenia, elevated CRP, elevated D-dimer (≥0.7 µg/L), elevated ferritin (≥700 µg/L), elevated IL-6 (≥40 pg/mL), elevated LDH. $P a O_{2} / F i O_{2}$ ratio ≤300 mmHg indicates ARDS

Management

Graded by severity. Mild: supportive + antivirals (Favipiravir or Molnupiravir or Nirmatrelvir/Ritonavir). Moderate: Remdesivir IV preferred + anticoagulation. Severe/Critical: Dexamethasone 6 mg/day + Remdesivir + Tocilizumab (anti-IL-6) + LMWH prophylaxis + escalating oxygen (HFNC, NIV, mechanical ventilation, ECMO)

Keywords

"Anosmia," "ageusia," "bilateral ground-glass opacities," "cytokine storm," "ARDS," "D-dimer elevation," "lymphopenia," "prone positioning," "S-gene target failure (SGTF)" for Omicron, "ROX index," "MIS-C" (in children)

01Pathophysiology

SARS-CoV-2 is a single-stranded RNA betacoronavirus that binds to the angiotensin-converting enzyme 2 (ACE2) receptor on host cells using its spike (S) protein. ACE2 receptors are abundantly expressed on type II pneumocytes in the lung, but also on enterocytes in the gut, myocardial cells, and vascular endothelial cells. This receptor distribution directly explains the multisystem nature of the disease: pneumonia, gastrointestinal symptoms (diarrhea in up to 10% of cases), myocardial injury, and coagulopathy all stem from the virus targeting ACE2-expressing tissues.

After binding, the virus enters the cell and hijacks the RNA-dependent RNA polymerase machinery to replicate. The initial viremic phase (approximately days 1 through 7) is characterized by active viral replication in the upper and lower respiratory tract, corresponding to symptoms of fever, cough, and fatigue. This is the window when antivirals are most effective.

In a subset of patients, particularly those with risk factors, the infection triggers a disproportionate host immune response (often termed a "cytokine storm") around days 7 through 10. This is driven by a surge in pro-inflammatory cytokines, notably IL-6, TNF-alpha, and IL-1. This hyperinflammatory phase is characterized by worsening pneumonia, ARDS, lymphopenia (the immune system exhausting its lymphocyte reserves), neutrophilia, and dramatically elevated acute-phase reactants (CRP, ferritin, D-dimer). The D-dimer elevation reflects the prothrombotic state induced by COVID-19, where endothelial injury and inflammation cause widespread microthrombosis and frank thromboembolic events (reported in up to 31% of hospitalized patients).

The Omicron variant (B.1.1.529) carries over 30 mutations in the spike protein, which increases its affinity for the ACE2 receptor and allows it to replicate 70 times faster in airway cells compared to Delta (per University of Hong Kong data). However, it replicates 10 times slower in lung parenchyma, which correlates with the clinically observed lower severity of Omicron relative to Delta.

Why this matters for the exam: The two-phase model (viral replication then hyperinflammation) is the foundation for understanding treatment timing. Antivirals work in the first phase; corticosteroids and immunomodulators (Tocilizumab) work in the second. Giving steroids too early (before hypoxia develops) may impair viral clearance; giving antivirals too late (after cytokine storm onset) has limited benefit.

02Classification and Clinical Manifestation

Asymptomatic

Clinical Criteria: Positive SARS-CoV-2 test, no signs or symptoms
Oxygen Status: Normal

Mild

Clinical Criteria: Symptoms of upper respiratory infection (fever, cough, fatigue, myalgia, sore throat, rhinorrhea, anosmia/ageusia). No evidence of pneumonia or hypoxia
Oxygen Status: $S p O_{2}$ >95% on room air

Moderate

Clinical Criteria: Clinical signs of pneumonia (fever, cough, tachypnea) but no signs of severe pneumonia. In children: cough or difficulty breathing + tachypnea without severe distress
Oxygen Status: $S p O_{2}$ >93% on room air

Severe

Clinical Criteria: Clinical pneumonia PLUS at least one of: RR >30/min, severe respiratory distress, or $S p O_{2}$ <93% on room air. In children: cyanosis, $S p O_{2}$ <93%, severe retractions, lethargy, seizure, or inability to drink
Oxygen Status: $P a O_{2} / F i O_{2}$ ≤300 mmHg

Critical

Clinical Criteria: ARDS, sepsis, septic shock, or multiorgan failure requiring mechanical ventilation or vasopressors
Oxygen Status: Refractory hypoxemia

Severity	Clinical Criteria	Oxygen Status
Asymptomatic	Positive SARS-CoV-2 test, no signs or symptoms	Normal
Mild	Symptoms of upper respiratory infection (fever, cough, fatigue, myalgia, sore throat, rhinorrhea, anosmia/ageusia). No evidence of pneumonia or hypoxia	$S p O_{2}$ >95% on room air
Moderate	Clinical signs of pneumonia (fever, cough, tachypnea) but no signs of severe pneumonia. In children: cough or difficulty breathing + tachypnea without severe distress	$S p O_{2}$ >93% on room air
Severe	Clinical pneumonia PLUS at least one of: RR >30/min, severe respiratory distress, or $S p O_{2}$ <93% on room air. In children: cyanosis, $S p O_{2}$ <93%, severe retractions, lethargy, seizure, or inability to drink	$P a O_{2} / F i O_{2}$ ≤300 mmHg
Critical	ARDS, sepsis, septic shock, or multiorgan failure requiring mechanical ventilation or vasopressors	Refractory hypoxemia

Tachypnea thresholds in children:

Age	Respiratory Rate Threshold
<2 months	≥60 breaths/min
2 to 11 months	≥50 breaths/min
1 to 5 years	≥40 breaths/min
>5 years	≥30 breaths/min

MIS-C (Multisystem Inflammatory Syndrome in Children): Fever ≥3 days AND at least 2 of: (a) rash or bilateral non-purulent conjunctivitis or mucocutaneous inflammation, (b) hypotension or shock, (c) myocardial dysfunction/pericarditis/coronary abnormalities (elevated troponin/NT-proBNP), (d) coagulopathy (elevated PT, aPTT, D-dimer), (e) acute GI symptoms (diarrhea, vomiting, abdominal pain). PLUS elevated inflammatory markers (ESR, CRP, or procalcitonin) AND no other bacterial cause AND evidence of COVID-19 (RT-PCR, antigen, or serology positive, or epidemiological contact).

03Diagnostic Workup

RT-PCR (nasopharyngeal swab)

Role: Confirmatory / Gold Standard
Key Findings: Positive NAAT confirms infection. Positive at day 1; follow-up PCR only for severe/critical cases (day 10+). CT value helps assess infectiousness in persistent positivity

Rapid Antigen Test (RDT-Ag)

Role: Best Initial Screening Test (rapid triage)
Key Findings: Faster, cheaper, but lower sensitivity than PCR. Negative RDT-Ag in symptomatic patients must be followed by PCR

Chest X-ray

Role: Initial imaging
Key Findings: Bilateral infiltrates, multifocal opacities; rules out other causes of respiratory distress

CT Scan Thorax

Role: More sensitive imaging
Key Findings: Bilateral ground-glass opacities (GGO) with or without consolidation, often peripheral and lower-lobe predominant

CBC with differential

Role: Baseline and serial
Key Findings: Lymphopenia (prognostic marker), neutrophilia, thrombocytopenia in DIC

CRP

Role: Inflammatory marker
Key Findings: >75 mg/L suggests severe inflammation; serial monitoring guides therapy

D-dimer

Role: Coagulopathy marker
Key Findings: ≥0.7 µg/L correlates with thrombotic risk and severity; 3-fold increase is a marker for consideration of admission

Ferritin

Role: Inflammatory/iron marker
Key Findings: ≥700 µg/L indicates cytokine storm phase

IL-6

Role: Cytokine marker
Key Findings: ≥40 pg/mL is a threshold for consideration of Tocilizumab

Procalcitonin

Role: Bacterial co-infection
Key Findings: Low in pure viral pneumonia; elevation suggests bacterial superinfection

Fibrinogen

Role: Coagulopathy
Key Findings: >700 mg/dL in severe cases; <2.0 g/L in DIC

Troponin / NT-proBNP

Role: Cardiac injury
Key Findings: Elevation suggests myocarditis or myocardial injury

ABG

Role: Respiratory status
Key Findings: $P a O_{2} / F i O_{2}$ ≤300 = ARDS criteria; progressive lactic acidosis = poor prognosis

PT / aPTT

Role: DIC screening
Key Findings: Prolongation ≥6 seconds of PT scores 2 points on ISTH DIC criteria

Test	Role	Key Findings
RT-PCR (nasopharyngeal swab)	Confirmatory / Gold Standard	Positive NAAT confirms infection. Positive at day 1; follow-up PCR only for severe/critical cases (day 10+). CT value helps assess infectiousness in persistent positivity
Rapid Antigen Test (RDT-Ag)	Best Initial Screening Test (rapid triage)	Faster, cheaper, but lower sensitivity than PCR. Negative RDT-Ag in symptomatic patients must be followed by PCR
Chest X-ray	Initial imaging	Bilateral infiltrates, multifocal opacities; rules out other causes of respiratory distress
CT Scan Thorax	More sensitive imaging	Bilateral ground-glass opacities (GGO) with or without consolidation, often peripheral and lower-lobe predominant
CBC with differential	Baseline and serial	Lymphopenia (prognostic marker), neutrophilia, thrombocytopenia in DIC
CRP	Inflammatory marker	>75 mg/L suggests severe inflammation; serial monitoring guides therapy
D-dimer	Coagulopathy marker	≥0.7 µg/L correlates with thrombotic risk and severity; 3-fold increase is a marker for consideration of admission
Ferritin	Inflammatory/iron marker	≥700 µg/L indicates cytokine storm phase
IL-6	Cytokine marker	≥40 pg/mL is a threshold for consideration of Tocilizumab
Procalcitonin	Bacterial co-infection	Low in pure viral pneumonia; elevation suggests bacterial superinfection
Fibrinogen	Coagulopathy	>700 mg/dL in severe cases; <2.0 g/L in DIC
Troponin / NT-proBNP	Cardiac injury	Elevation suggests myocarditis or myocardial injury
ABG	Respiratory status	$P a O_{2} / F i O_{2}$ ≤300 = ARDS criteria; progressive lactic acidosis = poor prognosis
PT / aPTT	DIC screening	Prolongation ≥6 seconds of PT scores 2 points on ISTH DIC criteria

The Best Initial Test in most clinical settings is the Rapid Antigen Test (RDT-Ag), which provides a result in minutes and is useful for triage and screening in high-prevalence areas. However, negative results in symptomatic patients or close contacts do not rule out infection and must be confirmed with RT-PCR.

The Most Accurate (Confirmatory) Test is the RT-PCR (NAAT) from a nasopharyngeal swab. A single positive PCR confirms the diagnosis. If the first PCR is negative in a symptomatic patient, repeat on the following day. For the Omicron variant, a key lab finding is the S-Gene Target Failure (SGTF) or "S-Gene Dropout" on PCR, caused by the del69-70 mutation in the spike protein; this is classified as a probable Omicron case, with definitive sequencing required for confirmation.

Follow-up PCR is not required for asymptomatic, mild, or moderate cases. It is reserved for severe and critical patients only, performed after day 10 of a positive swab. Persistent positivity after clinical improvement often reflects detection of inactive viral RNA fragments (non-infectious), which can persist up to 12 weeks. The CT (Cycle Threshold) value helps assess whether persistent positivity reflects active replication or residual nucleic acid.

For coagulopathy assessment, the ISTH recommends checking D-dimer, PT, and platelet count on all COVID-19 patients. The ISTH DIC scoring system uses platelet count, D-dimer/FDP, PT prolongation, and fibrinogen level to distinguish overt DIC (score ≥5) from non-overt DIC (score <5).

Imaging: Chest X-ray is the initial study; CT thorax is indicated for cases of worsening severity or diagnostic uncertainty. The classic CT pattern is bilateral, peripheral, and basilar ground-glass opacities with or without consolidation, which helps differentiate COVID-19 pneumonia from bacterial lobar pneumonia.

04Management and Treatment

Asymptomatic

Antiviral: None
Anti-inflammatory: None
Anticoagulation: None
Oxygen: None
Supportive: Vitamin C 500 mg q6-8h (14 days), Vitamin D 1000-5000 IU/day (14 days), self-isolation 10 days

Mild

Antiviral: Favipiravir (1600 mg q12h day 1, then 600 mg q12h days 2-5) OR Molnupiravir (800 mg q12h x 5 days) OR Nirmatrelvir/Ritonavir (300/100 mg q12h x 5 days)
Anti-inflammatory: None
Anticoagulation: None
Oxygen: None
Supportive: Vitamin C, Vitamin D, paracetamol PRN, self-isolation 10 days + 3 days symptom-free

Moderate

Antiviral: Remdesivir (200 mg IV day 1, then 100 mg IV days 2-5 or 2-10) preferred; alternatives as above
Anti-inflammatory: None
Anticoagulation: LMWH or UFH per physician assessment
Oxygen: Supplemental O2 if needed
Supportive: Vitamin C 200-400 mg IV q8h, Vitamin D, monitoring labs (CBC, CRP, LFT, RFT)

Severe

Antiviral: Remdesivir (same dosing); alternatives as above
Anti-inflammatory: Dexamethasone 6 mg/day x 10 days (or equivalent: methylprednisolone 32 mg, hydrocortisone 160 mg) + Tocilizumab 8 mg/kg IV (single dose, may repeat x1 if no improvement, min. 12 hours apart, max 800 mg/dose)
Anticoagulation: LMWH standard dose (enoxaparin 0.4 mL SC daily) or UFH 5000 IU SC q12h
Oxygen: HFNC or NIV, escalate to intubation if failing
Supportive: Vitamin C IV, Vitamin B1 IV, Vitamin D, fluid management

Critical

Antiviral: Remdesivir; alternatives as above
Anti-inflammatory: Dexamethasone + Tocilizumab; consider Baricitinib 4 mg PO daily (up to 14 days)
Anticoagulation: Intermediate dose LMWH (enoxaparin 0.4 mL SC q12h) or therapeutic UFH
Oxygen: Mechanical ventilation (lung-protective: TV 4-8 mL/kg, Pplat <30 cmH2O, prone 12-16h/day); ECMO if refractory
Supportive: Vasopressors (norepinephrine first-line, target MAP >65 mmHg)

Severity	Antiviral	Anti-inflammatory	Anticoagulation	Oxygen	Supportive
Asymptomatic	None	None	None	None	Vitamin C 500 mg q6-8h (14 days), Vitamin D 1000-5000 IU/day (14 days), self-isolation 10 days
Mild	Favipiravir (1600 mg q12h day 1, then 600 mg q12h days 2-5) OR Molnupiravir (800 mg q12h x 5 days) OR Nirmatrelvir/Ritonavir (300/100 mg q12h x 5 days)	None	None	None	Vitamin C, Vitamin D, paracetamol PRN, self-isolation 10 days + 3 days symptom-free
Moderate	Remdesivir (200 mg IV day 1, then 100 mg IV days 2-5 or 2-10) preferred; alternatives as above	None	LMWH or UFH per physician assessment	Supplemental O2 if needed	Vitamin C 200-400 mg IV q8h, Vitamin D, monitoring labs (CBC, CRP, LFT, RFT)
Severe	Remdesivir (same dosing); alternatives as above	Dexamethasone 6 mg/day x 10 days (or equivalent: methylprednisolone 32 mg, hydrocortisone 160 mg) + Tocilizumab 8 mg/kg IV (single dose, may repeat x1 if no improvement, min. 12 hours apart, max 800 mg/dose)	LMWH standard dose (enoxaparin 0.4 mL SC daily) or UFH 5000 IU SC q12h	HFNC or NIV, escalate to intubation if failing	Vitamin C IV, Vitamin B1 IV, Vitamin D, fluid management
Critical	Remdesivir; alternatives as above	Dexamethasone + Tocilizumab; consider Baricitinib 4 mg PO daily (up to 14 days)	Intermediate dose LMWH (enoxaparin 0.4 mL SC q12h) or therapeutic UFH	Mechanical ventilation (lung-protective: TV 4-8 mL/kg, Pplat <30 cmH2O, prone 12-16h/day); ECMO if refractory	Vasopressors (norepinephrine first-line, target MAP >65 mmHg)

Asymptomatic and Mild Cases: Managed with home isolation for 10 days from the date of the positive diagnostic specimen (asymptomatic) or from symptom onset plus 3 days symptom-free (mild). Treatment is purely supportive with Vitamin C, Vitamin D, and symptomatic therapy. Antiviral therapy is indicated for mild cases, particularly those with risk factors for progression (diabetes, hypertension, chronic lung disease, obesity, heart disease, CKD, malignancy, cerebrovascular disease, chronic liver disease). Criteria for home isolation include: age <45, no comorbidities, asymptomatic or mild symptoms, access to a separate room and bathroom.

Antiviral options for mild cases:

Favipiravir (200 mg tablets): Loading dose 1600 mg every 12 hours on day 1, then 600 mg every 12 hours on days 2 through 5.
Molnupiravir (200 mg capsules): 800 mg every 12 hours for 5 days. Contraindicated in pregnancy, breastfeeding, and children <18 years. Mechanism: a prodrug converted to hydroxycytidine that competes with viral RNA polymerase, causing lethal mutagenesis.
Nirmatrelvir/Ritonavir (Paxlovid) (150 mg/100 mg combination): Nirmatrelvir 2 tablets every 12 hours + Ritonavir 1 tablet every 12 hours for 5 days. Indicated for adults and children ≥12 years (>40 kg) with mild to moderate disease and high-risk factors. Must be given within 5 days of symptom onset. Contraindicated in pregnancy and breastfeeding. Critical drug interaction: Cannot be co-administered with amiodarone, carbamazepine, rifampin, lovastatin, simvastatin, sildenafil, oral midazolam, colchicine, and many other CYP3A-dependent drugs due to risk of life-threatening toxicity.

Moderate Cases: Require hospitalization. Remdesivir is the preferred antiviral: 200 mg IV on day 1 followed by 100 mg IV daily on days 2 through 5 (can be extended to day 10). Not recommended if eGFR <30 mL/min/1.73m2 except when the benefit outweighs the risk in severe situations. Anticoagulant prophylaxis is considered based on individualized risk assessment.

Severe and Critical Cases: This is where the two-pronged approach (antiviral + anti-inflammatory) becomes essential.

Dexamethasone 6 mg/day (oral or IV) for 10 days, or methylprednisolone 32 mg, or hydrocortisone 160 mg. Steroids are indicated ONLY in patients requiring supplemental oxygen or ventilation. Do not give steroids in mild disease without hypoxia (this is a classic exam trap).
Tocilizumab (anti-IL-6): 8 mg/kg IV single dose (max 800 mg). Given early when the patient enters the severe phase (typically ≥1 week after symptom onset). Criteria: SOFA score still <3, but CURB-65 >2, or $S p O_{2}$ <93%, or RR >30/min, or bilateral multilobar infiltrates, with IL-6 ≥40 pg/mL AND at least one of: D-dimer ≥0.7 µg/L, lymphocytes <800 x $10^{9}$ /L, ferritin ≥700 µg/L, fibrinogen >700 mg/dL, or CRP >75 mg/L. A second dose can be given if no improvement (minimum 12-hour interval).
Baricitinib (Janus Kinase inhibitor): 4 mg orally daily for up to 14 days in severe/critical cases, combined with corticosteroids. Not to be used simultaneously with Tocilizumab (increased risk of secondary infections). Dose adjustment required for eGFR <60 mL/min/1.73m2 and hepatic impairment.

Oxygen Therapy Escalation:

Initiate $O_{2}$ when $S p O_{2}$ <93%. Start with nasal cannula, titrate up to non-rebreather mask (NRM) at 15 L/min, target $S p O_{2}$ 92-96% (>94% in pregnant patients).
If no improvement in 1 hour, escalate to HFNC: Start at flow 30 L/min, $F i O_{2}$ 40%. Titrate flow by 5-10 L/min increments. Monitor with the ROX index = ( $S p O_{2} / F i O_{2}$ ) / respiratory rate. ROX >4.88 at hours 2, 6, and 12 indicates safe ventilation; ROX <3.85 indicates high risk of intubation.
If HFNC fails in 1-2 hours, trial NIV (BiPAP: inspiratory pressure 12-14 cmH2O, PEEP 6-12 cmH2O, $F i O_{2}$ 40-60%). If total inspiratory pressure >20 cmH2O is required to achieve target tidal volume, consider immediate intubation.
Awake prone positioning combined with HFNC or NIV (2 hours, twice daily) improves oxygenation and reduces need for intubation in mild-to-moderate ARDS.
Invasive mechanical ventilation: Lung-protective strategy with tidal volume 4-8 mL/kg predicted body weight, plateau pressure <30 cmH2O, driving pressure <15 cmH2O. In moderate-severe ARDS, use higher PEEP protocol (ARDSnet table). Prone positioning for 12-16 hours/day if refractory hypoxemia persists.
ECMO is considered when: $P a O_{2} / F i O_{2}$ <80 mmHg for >6 hours, $P a O_{2} / F i O_{2}$ <50 mmHg for >3 hours, or pH <7.25 with $P a C O_{2}$ >60 mmHg for >6 hours. Absolute contraindications include Clinical Frailty Scale ≥3, mechanical ventilation >10 days, and severe chronic comorbidities.

Anticoagulation:

Moderate/Severe cases: Standard prophylactic dose LMWH (enoxaparin 0.4 mL SC once daily) or UFH 5000 IU SC every 12 hours.
Critical cases: Intermediate dose LMWH (enoxaparin 0.4 mL SC every 12 hours). Contraindications: active bleeding, platelet count <25,000/mm3, HIT, severe hepatic impairment, hemorrhagic stroke.
Post-discharge or stable thromboembolic complications: Rivaroxaban (DOAC) for extended prophylaxis >10 days.
Fondaparinux (2.5 mg SC once daily) is an alternative, preferred if history of HIT, but not recommended in critical patients with renal insufficiency.

Pediatric Dosing:

Vitamin C: age-tiered (1-3 years: max 400 mg/day; 4-8 years: max 600 mg/day; 9-13 years: max 1.2 g/day; 12-18 years: max 1.8 g/day) + Zinc 20 mg/day.
Antibiotics when bacterial co-infection suspected: Ceftriaxone IV 50-100 mg/kg/day, plus Azithromycin 10 mg/kg if atypical pneumonia suspected.
MIS-C: Combination of IVIG + corticosteroids is the treatment of choice. Anti-IL-6 and anticoagulants considered with multidisciplinary input.

Isolation Completion Criteria:

Asymptomatic: 10 days from diagnostic specimen date (no follow-up PCR needed).
Mild/Moderate: 10 days from symptom onset + minimum 3 days symptom-free (no follow-up PCR needed).
Severe/Critical: Follow-up RT-PCR must be negative x1 + minimum 3 days symptom-free. If PCR remains persistently positive with declining CT values and clinical improvement, the treating physician can discharge based on clinical assessment (persistent positivity likely reflects non-viable viral RNA).

05Differential Diagnosis and Distractors

Influenza

Why It Is Similar: Acute fever, cough, myalgia, respiratory symptoms, can cause viral pneumonia and ARDS
Key Discriminator: Influenza has a shorter incubation (1-4 days vs. 2-14 days for COVID-19). Anosmia/ageusia is uncommon in influenza. Influenza PCR positive, SARS-CoV-2 PCR negative. COVID-19 has more prominent lymphopenia and D-dimer elevation

Bacterial Community-Acquired Pneumonia (CAP)

Why It Is Similar: Fever, cough, dyspnea, pulmonary infiltrates
Key Discriminator: CAP shows lobar consolidation (not bilateral GGO). Procalcitonin is elevated (low in pure COVID-19). Sputum culture positive for bacteria. Neutrophilic leukocytosis instead of lymphopenia

MERS-CoV

Why It Is Similar: Both are betacoronaviruses causing severe pneumonia
Key Discriminator: MERS is associated with camel exposure and Middle East travel. Much higher case fatality rate (~35%). Confirmed by MERS-CoV RT-PCR

Kawasaki Disease (vs. MIS-C in children)

Why It Is Similar: Fever ≥5 days, rash, conjunctivitis, mucocutaneous changes, coronary abnormalities
Key Discriminator: Kawasaki typically affects children <5 years; MIS-C peaks at ages 6 to 12 years. MIS-C has prominent GI symptoms (abdominal pain, diarrhea) and shock/myocardial dysfunction, less common in classic Kawasaki. MIS-C has elevated D-dimer and coagulopathy markers. MIS-C has positive SARS-CoV-2 serology or epidemiological link

Pulmonary Embolism

Why It Is Similar: Acute dyspnea, hypoxemia, elevated D-dimer, tachycardia
Key Discriminator: PE causes pleuritic chest pain and typically unilateral leg swelling (DVT). CT pulmonary angiography shows filling defect, not bilateral GGO. D-dimer elevated in both, but CT pattern and Wells score differentiate them

Acute Exacerbation of COPD

Why It Is Similar: Dyspnea, cough, wheezing in elderly with comorbidities
Key Discriminator: History of chronic productive cough, smoking history. Obstructive pattern on spirometry. Chest imaging shows hyperinflation, not new bilateral GGO. SARS-CoV-2 PCR negative

Toxic Shock Syndrome (vs. MIS-C)

Why It Is Similar: Fever, rash, hypotension, multiorgan involvement
Key Discriminator: TSS caused by Staphylococcal or Streptococcal toxins, culture-positive. No epidemiological link to COVID-19. No coronary artery abnormalities

Differential	Why It Is Similar	Key Discriminator
Influenza	Acute fever, cough, myalgia, respiratory symptoms, can cause viral pneumonia and ARDS	Influenza has a shorter incubation (1-4 days vs. 2-14 days for COVID-19). Anosmia/ageusia is uncommon in influenza. Influenza PCR positive, SARS-CoV-2 PCR negative. COVID-19 has more prominent lymphopenia and D-dimer elevation
Bacterial Community-Acquired Pneumonia (CAP)	Fever, cough, dyspnea, pulmonary infiltrates	CAP shows lobar consolidation (not bilateral GGO). Procalcitonin is elevated (low in pure COVID-19). Sputum culture positive for bacteria. Neutrophilic leukocytosis instead of lymphopenia
MERS-CoV	Both are betacoronaviruses causing severe pneumonia	MERS is associated with camel exposure and Middle East travel. Much higher case fatality rate (~35%). Confirmed by MERS-CoV RT-PCR
Kawasaki Disease (vs. MIS-C in children)	Fever ≥5 days, rash, conjunctivitis, mucocutaneous changes, coronary abnormalities	Kawasaki typically affects children <5 years; MIS-C peaks at ages 6 to 12 years. MIS-C has prominent GI symptoms (abdominal pain, diarrhea) and shock/myocardial dysfunction, less common in classic Kawasaki. MIS-C has elevated D-dimer and coagulopathy markers. MIS-C has positive SARS-CoV-2 serology or epidemiological link
Pulmonary Embolism	Acute dyspnea, hypoxemia, elevated D-dimer, tachycardia	PE causes pleuritic chest pain and typically unilateral leg swelling (DVT). CT pulmonary angiography shows filling defect, not bilateral GGO. D-dimer elevated in both, but CT pattern and Wells score differentiate them
Acute Exacerbation of COPD	Dyspnea, cough, wheezing in elderly with comorbidities	History of chronic productive cough, smoking history. Obstructive pattern on spirometry. Chest imaging shows hyperinflation, not new bilateral GGO. SARS-CoV-2 PCR negative
Toxic Shock Syndrome (vs. MIS-C)	Fever, rash, hypotension, multiorgan involvement	TSS caused by Staphylococcal or Streptococcal toxins, culture-positive. No epidemiological link to COVID-19. No coronary artery abnormalities

06Traps and High-Yield Pearls

The most common way students get COVID-19 questions wrong is by confusing the timing of therapies with the phase of illness. The exam frequently tests whether you know that dexamethasone is indicated ONLY in patients requiring supplemental oxygen or mechanical ventilation (severe/critical disease). Giving steroids to a mild or moderate patient who is not hypoxic is incorrect because it impairs viral clearance without addressing the inflammatory process (which has not yet begun). The inverse error is also tested: failing to give steroids to a patient who is clearly in the hyperinflammatory phase with progressive hypoxia.

A second frequent trap involves Nirmatrelvir/Ritonavir (Paxlovid) drug interactions. A vignette may describe a COVID-19 patient on amiodarone for atrial fibrillation or simvastatin for hyperlipidemia, and the question asks for the next best step. Prescribing Paxlovid in these patients without stopping the interacting drug is a dangerous error. The exam tests whether you recognize the CYP3A interaction list.

Third, students often miss MIS-C in children. The trap is a vignette with a child presenting several weeks after a respiratory illness with fever, abdominal pain, rash, hypotension, and elevated troponin. Students select sepsis or Kawasaki disease, but the combination of COVID-19 epidemiological link + gastrointestinal symptoms + myocardial dysfunction + coagulopathy + elevated inflammatory markers points to MIS-C. Remember that MIS-C occurs in the post-infectious period (2-6 weeks after acute COVID-19) and treatment is IVIG plus corticosteroids, not antivirals.

Finally, know the ROX index for HFNC monitoring: $ROX = (SpO_2/FiO_2) \div RR$ . A ROX index <3.85 at 2, 6, or 12 hours predicts HFNC failure and need for intubation. This is a quantitative pearl that can appear as a calculation in a vignette describing a deteriorating patient on HFNC.

The core competency being tested in COVID-19 questions is your ability to stratify disease severity, match treatment intensity to the correct phase, and recognize complications (coagulopathy, cytokine storm, MIS-C, persistent positivity) that alter management.

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