Acute Respiratory Failure And Hypoxemia - USMLE Step 2

SECTION 1

Historical Context & Motivation

The recognition of acute respiratory failure as a distinct clinical entity emerged alongside the development of arterial blood gas analysis and mechanical ventilation in the twentieth century. Before clinicians could measure partial pressures of oxygen and carbon dioxide in arterial blood, respiratory failure was largely a clinical diagnosis based on cyanosis, respiratory distress, and eventual cardiopulmonary collapse. The evolution of critical care medicine fundamentally transformed how we understand, classify, and treat patients whose lungs fail to maintain adequate gas exchange. This history is not merely academic—it underpins the physiological reasoning tested on the USMLE Step 2 and directly informs bedside decision-making in every intensive care unit today.

1952 • Copenhagen Polio Epidemic

Bjørn Ibsen demonstrated that positive-pressure ventilation via tracheostomy could save patients with respiratory paralysis, founding the modern intensive care unit and establishing mechanical ventilation as a life-saving intervention for respiratory failure.

1958 • Arterial Blood Gas Analysis

Poul Astrup and John Severinghaus developed practical electrodes for measuring arterial PaO₂ and PaCO₂, enabling the quantitative definition of respiratory failure and the distinction between hypoxemic and hypercapnic subtypes.

1967 • Description of ARDS

Ashbaugh, Bigelow, Petty, and Levine published the landmark description of acute respiratory distress syndrome (ARDS), establishing diffuse alveolar damage as a major cause of refractory hypoxemic respiratory failure.

2000 • ARDSNet Trial (Low Tidal Volume)

The ARDS Network demonstrated that lung-protective ventilation with 6 mL/kg predicted body weight reduced mortality by 22%, shifting the paradigm toward minimizing ventilator-induced lung injury in acute hypoxemic respiratory failure.

2012 • Berlin Definition of ARDS

The Berlin Definition standardized ARDS classification into mild, moderate, and severe categories based on PaO₂/FiO₂ ratio thresholds, providing a universally accepted framework for clinical trials and bedside stratification of hypoxemic respiratory failure.

These milestones reveal a recurring theme: the capacity to measure and classify respiratory failure has consistently preceded improvements in treatment. Today, the clinician confronting acute respiratory failure must rapidly determine whether the predominant derangement is hypoxemia, hypercapnia, or both, and then identify the underlying pathophysiological mechanism—a skill set central to USMLE Step 2 clinical reasoning.

SECTION 2

Core Principles & Definitions

Acute respiratory failure is defined as the inability of the respiratory system to meet the metabolic demands of the body for oxygen uptake and/or carbon dioxide elimination. This definition immediately reveals two fundamental subtypes. Type I (hypoxemic) respiratory failure is characterized by a PaO₂ less than 60 mmHg on room air with a normal or low PaCO₂, reflecting a primary failure of oxygenation. Type II (hypercapnic) respiratory failure is defined by a PaCO₂ greater than 50 mmHg with respiratory acidosis, indicating failure of ventilation. In practice, many patients exhibit features of both types simultaneously, particularly as disease progresses and respiratory muscles fatigue.

Hypoxemia vs. Hypoxia

Hypoxemia refers specifically to a low partial pressure of oxygen in arterial blood (PaO₂ < 60 mmHg). Hypoxia is the broader concept of inadequate oxygen delivery to tissues, which may occur even with a normal PaO₂ (e.g., carbon monoxide poisoning, severe anemia).

The A-a Gradient

The alveolar–arterial (A-a) oxygen gradient quantifies the difference between alveolar oxygen tension (PAO₂) and arterial oxygen tension (PaO₂). A normal A-a gradient is approximately (Age/4) + 4 mmHg. An elevated gradient indicates a pathological barrier to gas exchange.

PaO₂/FiO₂ Ratio

The P/F ratio normalizes oxygenation across varying levels of supplemental oxygen. A normal P/F ratio is approximately 500. Values below 300 define ARDS (Berlin criteria), and values below 200 indicate severe ARDS.

Five Mechanisms of Hypoxemia

The five classic pathophysiological causes of hypoxemia are: V/Q mismatch, intrapulmonary shunt, diffusion impairment, hypoventilation, and low inspired oxygen (FiO₂). V/Q mismatch is the most common mechanism encountered clinically.

Type I vs. Type II Failure

Type I (oxygenation failure): PaO₂ < 60 mmHg, normal/low PaCO₂. Common causes: pneumonia, ARDS, pulmonary embolism. Type II (ventilatory failure): PaCO₂ > 50 mmHg with acidosis. Common causes: COPD exacerbation, neuromuscular disease, drug overdose.

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KEY TAKEAWAY

Think of the A-a gradient like a quality-control measure in a factory. The lungs are the factory, and their job is to load oxygen onto hemoglobin conveyor belts. The A-a gradient measures how much oxygen is lost between the "loading dock" (alveolus) and the "shipping department" (arterial blood). If the factory is well-run (normal lungs), very little is lost in transit. A widened gradient means something inside the factory—a blocked conveyor (shunt), a slow belt (diffusion impairment), or mismatched supply lines (V/Q mismatch)—is preventing efficient transfer. This single value separates intrinsic lung pathology from extrapulmonary causes of hypoxemia.

SECTION 3

Visual Explanation — Mechanisms of Hypoxemia

The five mechanisms of hypoxemia are organized by their effect on the A-a gradient and their response to supplemental oxygen. The top row displays each mechanism with its characteristic features. The bottom panel highlights the critical clinical distinction: V/Q mismatch responds to supplemental O₂, whereas shunt physiology does not.

The diagram above encapsulates the most frequently tested framework for approaching hypoxemia on USMLE Step 2. The first branch point is the A-a gradient: if it is normal, the hypoxemia is caused by hypoventilation or low inspired oxygen tension, both of which reflect inadequate delivery of oxygen to alveoli rather than impaired transfer across the alveolar–capillary membrane. If the A-a gradient is elevated, intrinsic lung pathology is present, and the next critical question is whether the hypoxemia corrects with supplemental oxygen. Shunt physiology is the hallmark of refractory hypoxemia—blood passes through non-ventilated alveoli and cannot be oxygenated regardless of how much supplemental oxygen is delivered to the ventilated units. This distinction directly guides therapeutic decisions: patients with shunt physiology require positive end-expiratory pressure (PEEP), alveolar recruitment maneuvers, or prone positioning rather than simply increasing FiO₂.

SECTION 4

Mathematical Framework — Gas Exchange Equations

Several key equations allow clinicians to quantify the severity of gas exchange impairment and identify the mechanism of hypoxemia. Mastery of these calculations is essential for USMLE Step 2, where you will be expected to compute the alveolar gas equation, determine the A-a gradient, and interpret the PaO₂/FiO₂ ratio from arterial blood gas data presented in clinical vignettes.

ALVEOLAR GAS EQUATION

PAO₂ = FiO₂ × (P_B − P_H₂O) − (PaCO₂ / R)

Where PAO₂ = alveolar oxygen tension, FiO₂ = fraction of inspired oxygen (0.21 on room air), P_B = barometric pressure (760 mmHg at sea level), P_H₂O = water vapor pressure (47 mmHg at 37°C), PaCO₂ = arterial CO₂ tension, and R = respiratory quotient (typically 0.8).

A-a GRADIENT

A-a gradient = PAO₂ − PaO₂

The expected normal A-a gradient increases with age. A commonly used approximation is: Expected A-a gradient ≈ (Age/4) + 4. Values exceeding this suggest intrinsic pulmonary pathology (V/Q mismatch, shunt, or diffusion impairment).

PaO₂/FiO₂ RATIO (P/F RATIO)

P/F ratio = PaO₂ / FiO₂

Normal P/F ratio ≈ 500. ARDS classification (Berlin Definition): Mild ARDS: P/F 200–300 mmHg; Moderate ARDS: P/F 100–200 mmHg; Severe ARDS: P/F ≤ 100 mmHg. All with PEEP ≥ 5 cm H₂O.

SHUNT EQUATION (SIMPLIFIED)

Qs/Qt = (CcO₂ − CaO₂) / (CcO₂ − CvO₂)

Where Qs/Qt = shunt fraction (proportion of cardiac output bypassing gas exchange), CcO₂ = end-capillary O₂ content, CaO₂ = arterial O₂ content, CvO₂ = mixed venous O₂ content. Shunt fractions > 30% are associated with refractory hypoxemia.

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USMLE Pearl

On exam, when given an ABG on room air, your first step should always be: (1) calculate PAO₂ using the alveolar gas equation, (2) determine the A-a gradient, and (3) classify the mechanism. This three-step approach immediately narrows the differential diagnosis.

SECTION 5

Classification & Etiologies of Acute Respiratory Failure

A systematic classification of acute respiratory failure requires integration of the type of failure (oxygenation vs. ventilation), the A-a gradient, and the clinical context. The following diagram and table provide a comprehensive map of common etiologies organized by their pathophysiological mechanisms, which is the format most frequently encountered on the USMLE.

Diagnostic algorithm for acute respiratory failure. Begin with ABG analysis to classify the type of failure. For Type I failure, the A-a gradient and response to supplemental oxygen identify the underlying mechanism and narrow the differential.

Mechanism	A-a Gradient	Response to O₂	Classic Etiologies
V/Q Mismatch	Elevated	Improves	COPD, asthma, pneumonia, PE, ILD
Shunt	Elevated	Refractory	ARDS, lobar atelectasis, AVM, intracardiac shunt
Diffusion Impairment	Elevated	Improves	Pulmonary fibrosis, emphysema (exercise-induced)
Hypoventilation	Normal	Improves	Opioid overdose, neuromuscular disease, obesity hypoventilation
Low FiO₂	Normal	Improves	High altitude, enclosed fire (O₂ displacement)

Summary of hypoxemia mechanisms with A-a gradient, O₂ responsiveness, and etiologies

SECTION 6

Worked Example — ABG Interpretation in Acute Respiratory Failure

A 58-year-old man with a history of heavy smoking presents to the emergency department with progressive dyspnea over 48 hours. He is febrile (38.8°C), tachypneic (respiratory rate 32/min), and has bilateral crackles on auscultation. Chest radiograph reveals bilateral diffuse opacities. He is on a non-rebreather mask at FiO₂ = 1.0. His ABG shows: pH 7.32, PaCO₂ 33 mmHg, PaO₂ 58 mmHg, HCO₃⁻ 17 mEq/L.

Determine the Type, Mechanism, and Severity of Respiratory Failure

1
Step 1 — Identify the Type of Respiratory Failure

The PaO₂ is 58 mmHg, which is below 60 mmHg, confirming hypoxemia. The PaCO₂ is 33 mmHg (low-normal), not elevated, so there is no primary hypercapnic component. This is Type I (hypoxemic) respiratory failure. The low PaCO₂ reflects compensatory hyperventilation, which is an expected physiologic response to hypoxemia.

Classification: Type I respiratory failure
2
Step 2 — Calculate PAO₂ Using the Alveolar Gas Equation

PAO₂ = FiO₂ × (P_B − P_H₂O) − (PaCO₂ / R). Substituting: PAO₂ = 1.0 × (760 − 47) − (33 / 0.8) = 713 − 41.25 = 671.75 mmHg. The alveolar oxygen tension is approximately 672 mmHg.

PAO₂ ≈ 672 mmHg
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Step 3 — Determine the A-a Gradient

A-a gradient = PAO₂ − PaO₂ = 672 − 58 = 614 mmHg. The expected A-a gradient for a 58-year-old is approximately (58/4) + 4 ≈ 18.5 mmHg. An A-a gradient of 614 mmHg is massively elevated, indicating severe intrinsic pulmonary pathology.

A-a gradient = 614 mmHg (severely elevated)
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Step 4 — Assess Response to Supplemental Oxygen

The patient is already on FiO₂ of 1.0 (100% oxygen via non-rebreather mask) and his PaO₂ is only 58 mmHg. This demonstrates refractory hypoxemia—the hallmark of shunt physiology. In V/Q mismatch, increasing FiO₂ to 1.0 should raise PaO₂ significantly because the ventilated alveoli can compensate. In shunt, blood bypasses gas exchange entirely, so increasing FiO₂ has minimal effect.

Mechanism: Intrapulmonary shunt
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Step 5 — Calculate P/F Ratio and Classify ARDS Severity

P/F ratio = PaO₂ / FiO₂ = 58 / 1.0 = 58 mmHg. According to the Berlin Definition, a P/F ratio ≤ 100 with PEEP ≥ 5 cm H₂O defines severe ARDS. Combined with bilateral opacities on CXR, acute onset, and respiratory failure not fully explained by cardiac failure, this patient meets full Berlin criteria for severe ARDS. Management requires intubation, lung-protective ventilation (6 mL/kg PBW), high PEEP, and consideration of prone positioning.

P/F ratio = 58 → Severe ARDS

SECTION 7

Management Strategies & Oxygen Delivery Systems

The management of acute respiratory failure is guided by the underlying mechanism, the severity of gas exchange derangement, and the trajectory of clinical deterioration. An essential first step is selecting the appropriate oxygen delivery device, which determines the achievable FiO₂ and the level of respiratory support. Simultaneously, clinicians must address the underlying cause—treating pneumonia with antibiotics, reversing opioid overdose with naloxone, or anticoagulating pulmonary embolism.

Oxygen Delivery Device	Flow Rate	Approximate FiO₂	Best For
Nasal cannula	1–6 L/min	24–44%	Mild hypoxemia, stable patients
Venturi mask	4–12 L/min	24–50% (precise)	COPD with CO₂ retention (titrate FiO₂)
Non-rebreather mask	10–15 L/min	60–90%	Severe hypoxemia, bridge to intubation
High-flow nasal cannula (HFNC)	30–60 L/min	Up to 100%	Moderate-severe hypoxemia, provides small PEEP effect
Non-invasive ventilation (BiPAP)	Variable	Up to 100%	COPD exacerbation (first-line), cardiogenic pulmonary edema
Mechanical ventilation	Variable	21–100%	ARDS, respiratory arrest, severe failure unresponsive to NIV

Oxygen delivery devices and their clinical applications

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KEY TAKEAWAY

Think of oxygen delivery as a spectrum of escalation, like rungs on a ladder. You start at the bottom (nasal cannula) and climb only as high as the patient requires. The critical decision point is recognizing when a patient on high-flow oxygen or NIV is failing—manifested by worsening tachypnea, use of accessory muscles, declining mental status, or a P/F ratio that continues to drop. Delayed intubation in this setting increases mortality. For COPD exacerbations with hypercapnic failure, BiPAP is the first intervention because it directly augments ventilation and reduces work of breathing, but it must never delay intubation when the patient is deteriorating.

SECTION 8

ARDS, Lung-Protective Ventilation & Advanced Concepts

Acute respiratory distress syndrome (ARDS) represents the most severe end of the hypoxemic respiratory failure spectrum and is a frequent topic on USMLE Step 2. The Berlin Definition (2012) requires all four of the following criteria: (1) acute onset within one week of a known clinical insult or new/worsening respiratory symptoms; (2) bilateral opacities on chest imaging not fully explained by effusions, lobar/lung collapse, or nodules; (3) respiratory failure not fully explained by cardiac failure or fluid overload; and (4) P/F ratio ≤ 300 mmHg with PEEP ≥ 5 cm H₂O. Understanding the evidence-based management of ARDS is essential for clinical reasoning on the exam.

Management Strategy	Evidence / Rationale	Key Parameters
Low tidal volume ventilation	ARDSNet trial (2000): 22% mortality reduction. Prevents volutrauma and biotrauma.	V_T = 6 mL/kg PBW; Plateau pressure ≤ 30 cm H₂O
High PEEP	Recruits collapsed alveoli, improves V/Q matching, reduces shunt fraction. Use PEEP-FiO₂ tables.	PEEP typically 10–24 cm H₂O in moderate-severe ARDS
Prone positioning	PROSEVA trial (2013): mortality benefit in severe ARDS (P/F < 150). Improves dorsal lung recruitment and V/Q matching.	≥ 16 hours/day prone position
Conservative fluid management	FACTT trial (2006): conservative fluids reduced ventilator days without increasing organ failure.	Target CVP < 4 mmHg or PAOP < 8 mmHg
Neuromuscular blockade	ACURASYS trial showed benefit; ROSE trial (2019) did not confirm. Consider early in severe ARDS if ventilator dyssynchrony.	Cisatracurium for 48 hours in early severe ARDS
ECMO	EOLIA trial (2018): consider VV-ECMO as rescue for refractory hypoxemia despite optimal conventional management.	P/F < 80 for > 6 hours or pH < 7.25 with PaCO₂ ≥ 60

Evidence-based ARDS management strategies for USMLE Step 2

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High-Yield USMLE Concept

Do NOT use corticosteroids routinely for ARDS without a specific steroid-responsive etiology (e.g., eosinophilic pneumonia, organizing pneumonia). Permissive hypercapnia (tolerating elevated PaCO₂ to maintain low tidal volumes and plateau pressures) is a cornerstone of ARDS management—prioritize lung protection over normocapnia. The target SpO₂ in ARDS is 88–95%, not 100%.

Beyond ARDS, it is important to recognize the concept of ventilator-induced lung injury (VILI), which encompasses volutrauma (overdistension from excessive tidal volumes), barotrauma (alveolar rupture from high pressures), atelectrauma (shear forces from cyclic opening and closing of collapsed alveoli), and biotrauma (systemic inflammatory response triggered by mechanical injury to alveolar epithelium). Lung-protective ventilation targets each of these mechanisms: low tidal volumes limit volutrauma, plateau pressure limits prevent barotrauma, and adequate PEEP prevents atelectrauma. This pathophysiological framework makes the ventilator settings in ARDS logical rather than arbitrary.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A patient presents with hypoxemia (PaO₂ 50 mmHg) and hypercapnia (PaCO₂ 70 mmHg) after a heroin overdose. The A-a gradient is calculated to be 12 mmHg (normal for age). What is the primary mechanism of this patient's hypoxemia, and why is the A-a gradient normal?

Show answer

The primary mechanism is hypoventilation due to opioid-induced respiratory depression. The A-a gradient is normal because the lungs themselves are intrinsically healthy—there is no parenchymal disease, V/Q mismatch, or shunt. The hypoxemia occurs because elevated PaCO₂ directly reduces alveolar oxygen tension (PAO₂) through the alveolar gas equation: as CO₂ accumulates in the alveoli, it displaces oxygen. Since the gas exchange membrane is intact, the difference between alveolar and arterial oxygen remains appropriately small. Treatment is naloxone (opioid reversal) and ventilatory support.

Question 2: BASIC CALCULATION

A 40-year-old woman breathing room air (FiO₂ = 0.21) at sea level has an ABG showing PaO₂ = 70 mmHg and PaCO₂ = 40 mmHg. Calculate the A-a gradient and determine if it is normal or elevated for her age.

Show answer

First, calculate PAO₂ using the alveolar gas equation: PAO₂ = 0.21 × (760 − 47) − (40 / 0.8) = 0.21 × 713 − 50 = 149.7 − 50 = 99.7 mmHg. The A-a gradient = PAO₂ − PaO₂ = 99.7 − 70 = 29.7 mmHg. The expected A-a gradient for a 40-year-old is approximately (40/4) + 4 = 14 mmHg. The measured gradient of approximately 30 mmHg is significantly elevated, indicating intrinsic pulmonary pathology such as V/Q mismatch, shunt, or diffusion impairment. Further workup should include chest imaging and assessment of response to supplemental oxygen.

Question 3: INTERMEDIATE

A 65-year-old man with COPD presents with worsening dyspnea. ABG on 2 L nasal cannula (estimated FiO₂ = 0.28): pH 7.28, PaCO₂ 68 mmHg, PaO₂ 55 mmHg, HCO₃⁻ 31 mEq/L. Is this acute, chronic, or acute-on-chronic respiratory failure? What is the most appropriate next intervention?

Show answer

This is acute-on-chronic respiratory failure. The elevated bicarbonate (31 mEq/L) indicates chronic metabolic compensation for chronic hypercapnia (Type II failure), but the pH of 7.28 shows that the current PaCO₂ of 68 mmHg exceeds what the kidneys have compensated for—implying an acute rise in PaCO₂ superimposed on chronic retention. For every 10 mmHg acute rise in PaCO₂, pH falls by 0.08, and for chronic rise, pH falls by 0.03. The degree of acidemia here suggests an acute component. The most appropriate next intervention is non-invasive positive pressure ventilation (BiPAP), which is first-line for acute hypercapnic respiratory failure in COPD exacerbations, shown to reduce intubation rates and mortality. Simultaneously, treat the underlying trigger (bronchodilators, systemic corticosteroids, antibiotics if infectious exacerbation).

Question 4: APPLIED

A 52-year-old woman with pneumonia is intubated and mechanically ventilated with FiO₂ = 0.6, PEEP = 10 cm H₂O, tidal volume = 350 mL (6 mL/kg PBW). Her ABG shows PaO₂ = 75 mmHg, PaCO₂ = 52 mmHg, pH 7.30. The plateau pressure is 28 cm H₂O. She meets Berlin criteria for ARDS. Calculate her P/F ratio, classify ARDS severity, and describe your next management steps.

Show answer

P/F ratio = PaO₂ / FiO₂ = 75 / 0.6 = 125 mmHg. This classifies as moderate ARDS (P/F 100–200 with PEEP ≥ 5). Current ventilator settings are appropriate: tidal volume at 6 mL/kg PBW and plateau pressure of 28 cm H₂O (below the 30 cm H₂O threshold). The PaCO₂ of 52 with pH 7.30 represents permissive hypercapnia, which is acceptable in ARDS to maintain lung-protective ventilation—do not increase tidal volume to normalize CO₂. Next steps: consider increasing PEEP using a PEEP-FiO₂ table to recruit alveoli and reduce shunt fraction, while monitoring plateau pressure. If the P/F ratio drops below 150 despite optimization, initiate prone positioning for ≥ 16 hours/day per the PROSEVA protocol. Maintain conservative fluid management and treat the underlying pneumonia with appropriate antibiotics.

Question 5: CRITICAL THINKING

A 30-year-old woman with no prior medical history presents with acute dyspnea and hypoxemia (PaO₂ 52 mmHg on room air). CXR is completely clear. The A-a gradient is elevated at 45 mmHg. When placed on 100% oxygen via non-rebreather, her PaO₂ rises only to 110 mmHg (expected > 500 mmHg on 100% O₂ in healthy lungs). She has a fixed split S2 on auscultation. What is the most likely diagnosis, the mechanism of her hypoxemia, and why does her PaO₂ partially but not fully correct with supplemental oxygen?

Show answer

The most likely diagnosis is an atrial septal defect (ASD) with right-to-left shunting, suggested by the fixed split S2 (pathognomonic for ASD), clear chest radiograph (excluding parenchymal disease), young healthy patient, and elevated A-a gradient with poor response to 100% O₂. The mechanism is intracardiac right-to-left shunt: deoxygenated blood from the right atrium crosses directly into the left atrium, bypassing the pulmonary capillary bed entirely. This shunted blood mixes with oxygenated blood, producing systemic hypoxemia. The reason PaO₂ partially corrects but does not normalize with 100% O₂ is that oxygen enriches the blood passing through ventilated alveoli (raising their O₂ content), but the shunted fraction receives no oxygen at all. The final arterial PaO₂ represents a weighted mixture of fully oxygenated pulmonary venous blood and completely deoxygenated shunted blood. As shunt fraction increases, the contribution of shunted blood dominates and PaO₂ becomes increasingly refractory. This case also illustrates Eisenmenger physiology if the shunt has reversed from left-to-right to right-to-left due to pulmonary hypertension.

SUMMARY

Acute Respiratory Failure & Hypoxemia — Summary

Summary

Acute respiratory failure is the inability of the respiratory system to maintain adequate gas exchange, classified as Type I (hypoxemic, PaO₂ < 60 mmHg) or Type II (hypercapnic, PaCO₂ > 50 mmHg). The five mechanisms of hypoxemia are V/Q mismatch (most common), intrapulmonary shunt (refractory to supplemental O₂), diffusion impairment, hypoventilation, and low inspired oxygen. The A-a gradient (calculated via the alveolar gas equation) is the key branch point: a normal gradient implicates hypoventilation or low FiO₂, while an elevated gradient points to intrinsic lung disease.

The P/F ratio stratifies severity and defines ARDS per the Berlin Definition (mild 200–300, moderate 100–200, severe ≤ 100). Management of ARDS centers on lung-protective ventilation (6 mL/kg PBW, plateau pressure ≤ 30 cm H₂O), high PEEP for alveolar recruitment, prone positioning for severe cases, conservative fluid management, and permissive hypercapnia to prioritize lung protection over normocapnia. For Type II failure in COPD, BiPAP is first-line therapy. The response to supplemental oxygen differentiates V/Q mismatch (corrects) from shunt (refractory)—this distinction is the single most commonly tested concept in respiratory failure on the USMLE.