Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

Inhaling the entire vital capacity (VC)

Inhaling the inspiratory reserve volume (IRV)

Exhaling the entire vital capacity (VC)

Exhaling the expiratory reserve volume (ERV)

Breath holding maneuver at functional residual capacity (FRC)

A previously healthy 64-year-old woman comes to the physician because of a dry cough and progressively worsening shortness of breath for the past 2 months. She has not had fever, chills, or night sweats. She has smoked one pack of cigarettes daily for the past 45 years. She appears thin. Examination of the lung shows a prolonged expiratory phase and end-expiratory wheezing. Spirometry shows decreased FEV1:FVC ratio (< 70% predicted), decreased FEV1, and a total lung capacity of 125% of predicted. The diffusion capacity of the lung (DLCO) is decreased. Which of the following is the most likely diagnosis?

Chronic obstructive pulmonary disease

A 68-year-old man comes to the emergency room with difficulty in breathing. He was diagnosed with severe obstructive lung disease a few years back. He uses his medication but often has to come to the emergency room for intravenous therapy to help him breathe. He was a smoker for 40 years smoking two packs of cigarettes every day. Which of the following best represents the expected changes in his ventilation, perfusion and V/Q ratio?

Low ventilation, normal perfusion and low V/Q ratio

Normal ventilation, low or nonexistent perfusion and infinite V/Q ratio

Medium ventilation and perfusion, V/Q that equals 0.8

Higher ventilation and perfusion with lower V/Q ratio

Lower ventilation and perfusion, but higher V/Q ratio

A 37-year-old woman comes to the physician because of a 10-month history of excessive daytime sleepiness and fatigue. She says she has difficulty concentrating and has fallen asleep at work on numerous occasions. She also reports having frequent headaches during the day. She has no difficulty falling asleep at night, but wakes up gasping for breath at least once. She has always snored loudly and began using an oral device to decrease her snoring a year ago. She has occasional lower back pain, for which she takes tramadol tablets 1–2 times per week. She also began taking one rabeprazole tablet daily 3 weeks ago. She does not smoke. She is 175 cm (5 ft 7 in) tall and weighs 119 kg (262 lb); BMI is 38.8 kg/m2. Her vital signs are within normal limits. Physical and neurologic examinations show no other abnormalities. Arterial blood gas analysis on room air shows: pH 7.35 PCO2 51 mm Hg PO2 64 mm Hg HCO3- 29 mEq/L O2 saturation 92% An x-ray of the chest and ECG show no abnormalities. Which of the following is the most likely cause of this patient's condition?

Apneic episodes with obstructed upper airways

Chronic inflammatory airflow limitation

Thickening of alveolar membranes

Drug-induced respiratory depression

Diurnal alveolar hypoventilation

A 62-year-old man presents to the emergency department for evaluation of a 2-year history of increasing shortness of breath. He also has an occasional nonproductive cough. The symptoms get worse with exertion. The medical history is significant for hypertension and he takes chlorthalidone. He is a smoker with a 40-pack-year smoking history. On physical examination, the patient is afebrile; the vital signs include: blood pressure 125/78 mm Hg, pulse 90/min, and respiratory rate 18/min. The body mass index (BMI) is 31 kg/m2. The oxygen saturation is 94% at rest on room air. A pulmonary examination reveals decreased breath sounds bilaterally, but is otherwise normal with no wheezes or crackles. The remainder of the examination is unremarkable. A chest radiograph shows hyperinflation of both lungs with mildly increased lung markings, but no focal findings. Based on this clinical presentation, which of the following is most likely?

FEV1/FVC of 80% with an FEV1 of 82%

A 3-day-old newborn is brought to the physician because of abdominal distention, inconsolable crying, and 3 episodes of bilious vomiting since the previous evening. He was delivered at home at 40 weeks' gestation by a trained midwife. He has not passed meconium. Physical examination shows abdominal distention, a tight anal sphincter, and an explosive passage of air and feces on removal of the examining finger. Abnormal development of which of the following best explains this patient's condition?

Submucosa and muscularis externa

Muscularis mucosae and lamina propria

An investigator is studying the outcomes of a malaria outbreak in an endemic region of Africa. 500 men and 500 women with known malaria exposure are selected to participate in the study. Participants with G6PD deficiency are excluded from the study. The clinical records of the study subjects are reviewed and their peripheral blood smears are evaluated for the presence of Plasmodium trophozoites. Results show that 9% of the exposed population does not have clinical or laboratory evidence of malaria infection. Which of the following best explains the absence of infection seen in this subset of participants?

Glutamic acid substitution in the β-globin chain

Inherited mutation affecting ribosome synthesis

Inherited defect in erythrocyte membrane ankyrin protein

Defective X-linked ALA synthase gene

A 55-year-old man with a 60 pack-year smoking history is referred by his primary care physician for a pulmonary function test (PFT). A previously obtained chest x-ray is shown below. Which of the following will most likely appear in his PFT report?

Residual volume increased, total lung capacity increased

Residual volume increased, total lung capacity decreased

Residual volume normal, total lung capacity decreased

Residual volume normal, total lung capacity normal

Residual volume decreased, total lung capacity increased

Obstructive lung disease effects for USMLE Step 3: High-Yield Physiology Lessons

Lung Compliance - The Stretch Factor

Flow-volume loops: normal, obstructive, and restrictive

Compliance is the lung's ability to stretch for a given change in pressure; it is the inverse of elastance (recoil).
Formula: $C = \frac{\Delta V}{\Delta P}$
Obstructive Lung Disease (e.g., Emphysema, COPD):
- ↑↑ Compliance: Destruction of alveolar walls and elastic fibers.
- ↓ Elastic Recoil: Lungs inflate very easily but struggle to deflate.
- Leads to air trapping, ↑ Functional Residual Capacity (FRC), and ↑ Total Lung Capacity (TLC).
- The pressure-volume loop shifts up and to the left.

⭐ In emphysema, loss of radial traction (outward pulling) on small airways leads to their collapse during exhalation, trapping air distally.

Obstructive Disease - Can't Get Air Out

Primary Defect: Increased airway resistance leading to difficulty with expiration (air trapping). Lungs are easier to inflate but harder to deflate.
Pathophysiology:
- Loss of alveolar elastic recoil (e.g., emphysema) reduces radial traction that holds small airways open.
- Leads to premature airway collapse during forced expiration.
Compliance & Lung Volumes:
- Static Compliance: ↑ due to loss of elastic tissue. The pressure-volume loop shifts UP and LEFT.
- Lung Volumes: ↑ TLC, ↑ FRC, and significantly ↑ RV due to air trapping.
- PFTs: Hallmark is a ↓ FEV1/FVC ratio (< 0.7). FEV1 is disproportionately decreased.

Flow-volume loops: normal, obstructive, and restrictive

⭐ In obstructive lung disease, dynamic compliance decreases as respiratory rate increases because there is not enough time for expiration, worsening air trapping.

P-V Loop Changes - Graphic Evidence

The entire P-V loop shifts left and upward, reflecting ↑ lung compliance and ↑ lung volumes (TLC, FRC).
Inspiratory Limb: Follows a steeper slope, indicating less pressure is needed for a given volume change.
Expiratory Limb: Exhibits a characteristic "scooped-out" appearance (coving).
- This is due to dynamic airway collapse and expiratory airflow limitation, which traps air.
Hysteresis: The loop is wider, signifying ↑ resistive work of breathing, especially during expiration.

⭐ The starting point of the loop on the x-axis represents the Functional Residual Capacity (FRC). In obstructive disease, this point is shifted to a higher volume, visually confirming air trapping.

High‑Yield Points - ⚡ Biggest Takeaways

Obstructive lung diseases like COPD and asthma are defined by ↑ airway resistance, leading to air trapping.

This results in lung hyperinflation, with an ↑ Total Lung Capacity (TLC) and ↑ Residual Volume (RV).

Lung compliance is abnormally high in emphysema due to the destruction of elastic fibers and loss of recoil.

The pressure-volume curve for the lung shifts upward and to the left.

The hallmark finding is a decreased FEV1/FVC ratio (< 0.7).

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