Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 551: Ventilation-perfusion ratio is maximum at which part of the lung?

A. Apex of the lung (Correct Answer)
B. Base of the lung
C. Posterior lobe of the lung
D. Middle of the lung

Explanation: **Explanation:** The Ventilation-Perfusion ratio (V/Q) is the ratio of the amount of air reaching the alveoli (V) to the amount of blood reaching the alveoli (Q). In a standing individual, gravity exerts a significant influence on both ventilation and blood flow, but the effect is much more pronounced on **perfusion**. 1. **Why the Apex is Correct:** Both ventilation and perfusion decrease as you move from the base to the apex. However, perfusion (Q) decreases much more sharply than ventilation (V) due to the low hydrostatic pressure in the pulmonary arteries at the top of the lung. Because the denominator (Q) decreases more than the numerator (V), the **V/Q ratio is highest at the apex** (approximately 3.0). 2. **Why the Base is Incorrect:** At the base of the lung, gravity increases both ventilation and perfusion. However, the increase in perfusion is disproportionately higher. This results in a **lower V/Q ratio** (approximately 0.6), making the base the most common site for "shunt-like" effects. 3. **Why Other Options are Incorrect:** The middle and posterior lobes follow the same gravitational gradient. In the upright position, the V/Q ratio progressively decreases from the superior (apex) to the inferior (base) segments. **High-Yield Facts for NEET-PG:** * **V/Q = 1:** The "ideal" ratio, usually found around the level of the 3rd rib. * **Apex (High V/Q):** Higher $P_AO_2$ (132 mmHg) and lower $P_ACO_2$ (28 mmHg). This high oxygen tension explains why **Mycobacterium tuberculosis** preferentially colonizes the apices. * **Base (Low V/Q):** Lower $P_AO_2$ and higher $P_ACO_2$. * **West Zones:** The apex corresponds to Zone 1 (potential dead space), while the base corresponds to Zone 3 (continuous flow).

Question 552: Which of the following conditions is characterized by thickening of the respiratory membrane?

A. Bronchiectasis
B. Pneumonia (Correct Answer)
C. Good-pasture syndrome
D. Henoch-Schönlein purpura (HSP)

Explanation: ### **Explanation** The **respiratory membrane** (or alveolar-capillary membrane) is the structure through which gas exchange occurs via simple diffusion. According to **Fick’s Law of Diffusion**, the rate of gas transfer is inversely proportional to the thickness of the membrane. Any condition that increases this thickness impairs oxygenation. **Why Pneumonia is Correct:** In **Pneumonia**, the alveoli become filled with inflammatory exudate, fluid, and white blood cells. This inflammatory process leads to the accumulation of fluid in the interstitial space and the alveolar lumen, effectively **thickening the respiratory membrane**. This increased distance for diffusion results in hypoxemia. Other conditions that thicken the membrane include pulmonary edema and interstitial lung diseases (e.g., asbestosis, tuberculosis). **Analysis of Incorrect Options:** * **Bronchiectasis:** This is a chronic obstructive airway disease characterized by permanent, abnormal dilation of the **bronchi and bronchioles** due to the destruction of elastic and muscular components of the bronchial wall. It primarily affects the conducting zone, not the respiratory membrane. * **Goodpasture Syndrome:** While this involves the lungs, it is characterized by anti-GBM antibodies attacking the alveolar basement membrane, leading to **intra-alveolar hemorrhage**. It is typically categorized as a pulmonary-renal syndrome rather than a primary "thickening" pathology like fibrosis or exudative pneumonia. * **Henoch-Schönlein Purpura (HSP):** This is a systemic IgA-mediated small-vessel vasculitis. While it can rarely cause pulmonary hemorrhage, it does not characteristically involve the thickening of the respiratory membrane. ### **High-Yield NEET-PG Pearls** * **Fick’s Law:** Diffusion $\propto \frac{\text{Surface Area} \times \text{Concentration Gradient} \times \text{Solubility}}{\text{Thickness} \times \sqrt{\text{Molecular Weight}}}$. * **Surface Area:** Decreased in **Emphysema** (destruction of alveolar walls). * **Diffusion Capacity ($DL_{CO}$):** The gold standard test to measure the integrity of the respiratory membrane. It is decreased in pneumonia, pulmonary edema, and interstitial fibrosis.

Question 553: Which of the following is true about residual volume?

A. Part of the expiratory reserve volume
B. Part of the vital capacity
C. Cannot be measured directly with a spirometer (Correct Answer)
D. Represents the resting volume of the lungs

Explanation: **Explanation:** **Residual Volume (RV)** is the volume of air remaining in the lungs after a maximal forced expiration. **Why Option C is correct:** Spirometry measures the volume of air that can be moved into or out of the lungs. Since RV can never be exhaled, it cannot be measured using a simple spirometer. To determine RV, indirect methods such as **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography** (the most accurate) must be used. **Analysis of Incorrect Options:** * **Option A:** RV is the air left *after* the Expiratory Reserve Volume (ERV) has been exhaled. They are distinct components of the Functional Residual Capacity (FRC = ERV + RV). * **Option B:** Vital Capacity (VC) is the maximum volume of air a person can exhale after a maximum inhalation. It consists of TV + IRV + ERV. Since RV cannot be exhaled, it is **not** part of the Vital Capacity. * **Option D:** The "resting volume" of the lungs (the volume at the end of a normal quiet expiration) is the **Functional Residual Capacity (FRC)**, not RV. **High-Yield Clinical Pearls for NEET-PG:** * **Lung Capacities involving RV:** Any capacity that includes RV cannot be measured by spirometry. This includes **FRC** and **Total Lung Capacity (TLC)**. * **Obstructive vs. Restrictive:** RV is typically **increased** in obstructive diseases (like emphysema due to air trapping) and **decreased** in restrictive lung diseases (like pulmonary fibrosis). * **Closing Volume:** This is the volume at which small airways in the lower lobes begin to close; it is equal to RV plus a portion of the ERV.

Question 554: The alveolar-arterial O2 tension gradient increases in which of the following conditions?

A. Hypoventilation (Correct Answer)
B. Right to left shunt
C. Diffusion defect
D. Ventilation-perfusion abnormality

Explanation: The **Alveolar-arterial (A-a) gradient** is a measure of the difference between the oxygen concentration in the alveoli ($P_AO_2$) and the arterial blood ($P_aO_2$). It is a critical tool for localizing the cause of hypoxemia. ### **Why Hypoventilation is the Correct Answer** In **Hypoventilation** (e.g., opioid overdose, neuromuscular weakness), the lungs are structurally normal, but the "pump" fails. This leads to a buildup of $CO_2$ and a reciprocal drop in alveolar $O_2$. Because the lungs are healthy, the oxygen that *is* present in the alveoli diffuses perfectly into the blood. Therefore, both $P_AO_2$ and $P_aO_2$ decrease proportionately, keeping the **A-a gradient normal**. *Note: The question as provided marks Hypoventilation as correct; however, physiologically, Hypoventilation and High Altitude are the two classic causes of hypoxemia with a **Normal A-a gradient**. If the goal is to identify where the gradient **increases**, options B, C, and D are the standard physiological causes.* ### **Analysis of Other Options (Causes of Increased A-a Gradient)** * **Right-to-left shunt (B):** Deoxygenated blood bypasses the ventilated alveoli and mixes with oxygenated blood. This significantly lowers $P_aO_2$ while $P_AO_2$ remains normal, **increasing** the gradient. * **Diffusion defect (C):** (e.g., Pulmonary Fibrosis) The thickened alveolar-capillary membrane hinders $O_2$ equilibrium, **increasing** the gradient. * **V/Q Mismatch (D):** (e.g., Pulmonary Embolism, Pneumonia) This is the most common cause of an **increased** A-a gradient in clinical practice. ### **High-Yield Clinical Pearls for NEET-PG** 1. **Normal A-a Gradient Formula:** $(Age / 4) + 4$. A value $>15–20$ mmHg is generally considered abnormal in a young adult. 2. **The "Normal Gradient" Rule:** If a patient is hypoxemic but the A-a gradient is **normal**, the cause is either **Hypoventilation** or **Low Inspired $O_2$ (High Altitude)**. 3. **Response to 100% $O_2$:** Hypoxemia due to a **Shunt** is the only cause that does **not** correct significantly with supplemental oxygen.

Question 555: What is the residual volume of the lung?

A. Additional volume of air that can be inspired forcefully after the end of normal inspiration
B. Volume of air breathed out of lungs in a single normal quiet respiration
C. Additional volume of air that can be expired forcefully after normal expiration
D. Volume of air remaining in the lungs even after forced expiration (Correct Answer)

Explanation: **Explanation:** **Residual Volume (RV)** is defined as the volume of air remaining in the lungs even after a maximal, forceful expiration. This volume exists because the lungs are held open by the negative intrapleural pressure and the thoracic cage, preventing the alveoli from collapsing completely. **Analysis of Options:** * **Option D (Correct):** RV is the "leftover" air that cannot be expelled. It averages approximately **1200 mL** in a healthy adult male. Its primary physiological role is to allow for continuous gas exchange between breaths and to prevent atelectasis (lung collapse). * **Option A (Incorrect):** This describes **Inspiratory Reserve Volume (IRV)**, which is the extra volume that can be inspired over and above the normal tidal volume. * **Option B (Incorrect):** This describes **Tidal Volume (TV)**, which is the volume of air inspired or expired during a single normal, quiet breath (approx. 500 mL). * **Option C (Incorrect):** This describes **Expiratory Reserve Volume (ERV)**, which is the additional air that can be forcefully exhaled after a normal tidal expiration. **NEET-PG High-Yield Pearls:** 1. **Measurement:** RV **cannot** be measured by simple spirometry because it never leaves the lungs. It is measured using **Helium Dilution**, **Nitrogen Washout**, or **Body Plethysmography**. 2. **Clinical Significance:** RV is significantly **increased in obstructive lung diseases** (e.g., Emphysema, Asthma) due to air trapping, leading to hyperinflation. 3. **Formula:** Functional Residual Capacity (FRC) = ERV + RV. 4. **Vital Capacity (VC):** Since RV cannot be exhaled, it is not part of the Vital Capacity (VC = TV + IRV + ERV).

Question 556: The pre-Bötzinger complex is a part of which respiratory center?

A. Ventral Respiratory Group (VRG) (Correct Answer)
B. Dorsal Respiratory Group (DRG)
C. Both VRG and DRG
D. Neither VRG nor DRG

Explanation: **Explanation:** The **pre-Bötzinger complex (pre-BötC)** is a small cluster of interneurons located in the ventrolateral medulla. It is considered the **pacemaker of respiration**, responsible for generating the basic respiratory rhythm. **1. Why Option A is Correct:** The pre-Bötzinger complex is anatomically and functionally a part of the **Ventral Respiratory Group (VRG)**. The VRG is a column of neurons in the medulla divided into three parts: the rostral VRG (containing the pre-BötC), the intermediate VRG, and the caudal VRG. While the VRG as a whole is primarily active during forceful breathing, the pre-BötC sub-region is essential for setting the basal rhythm of quiet breathing. **2. Why Incorrect Options are Wrong:** * **Option B (DRG):** The Dorsal Respiratory Group is located in the nucleus tractus solitarius (NTS). It primarily consists of inspiratory neurons and acts as the integration center for sensory input from the glossopharyngeal and vagus nerves. It does not contain the pacemaker cells. * **Option C & D:** Since the pre-BötC has a specific anatomical localization within the VRG, these options are incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Pacemaker Activity:** The pre-BötC contains G-protein coupled receptors; opioids (acting on $\mu$-receptors) inhibit these neurons, which is the mechanism behind **opioid-induced respiratory depression**. * **DRG vs. VRG:** Remember "D" for **D**orsal and **D**iaphragm (quiet inspiration), and "V" for **V**entral and **V**igorous (forced breathing). * **Pontine Centers:** The **Pneumotaxic center** (upper pons) limits inspiration (the "off-switch"), while the **Apneustic center** (lower pons) prolongs inspiration.

Question 557: A 30-year-old woman with gestational diabetes presents with true labor pains at 30 weeks of gestation. The woman is given glucocorticoid therapy. What is the purpose of prenatal steroid therapy?

A. To increase blood flow to the fetal lungs
B. To increase fetal PO2
C. To shift the fetal oxyhemoglobin dissociation curve to the right
D. To increase the lecithin/sphingomyelin ratio in the amniotic fluid (Correct Answer)

Explanation: **Explanation:** The primary purpose of prenatal glucocorticoid therapy (e.g., Betamethasone or Dexamethasone) in preterm labor is to accelerate **fetal lung maturity** and prevent Respiratory Distress Syndrome (RDS). **Why Option D is Correct:** Glucocorticoids stimulate **Type II pneumocytes** in the fetal lungs to produce and release **surfactant**. Surfactant is primarily composed of phospholipids, specifically **Dipalmitoylphosphatidylcholine (Lecithin)**. As the lungs mature, the concentration of lecithin increases significantly while sphingomyelin levels remain relatively constant. Therefore, steroids increase the **Lecithin/Sphingomyelin (L/S) ratio**. An L/S ratio > 2.0 generally indicates fetal lung maturity. **Why Other Options are Incorrect:** * **Option A & B:** Steroids do not directly alter fetal pulmonary hemodynamics or immediate PO2 levels. Their role is structural and biochemical (surfactant production), which ensures the alveoli can remain open for gas exchange *after* birth. * **Option C:** The fetal oxyhemoglobin dissociation curve is shifted to the **left** compared to adults (due to Hemoglobin F's high affinity for O2). Steroids do not shift this curve to the right; such a shift would decrease O2 affinity, which is not the goal of therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Target Window:** Steroids are most effective when administered between **24 and 34 weeks** of gestation. * **Mechanism:** They induce enzymes like *cholinephosphotransferase*, which is the rate-limiting step in surfactant synthesis. * **Drug of Choice:** **Betamethasone** (12 mg IM, 2 doses 24 hours apart) is preferred over dexamethasone due to better neurological outcomes. * **Other Benefits:** Prenatal steroids also reduce the risk of **Intraventricular Hemorrhage (IVH)** and **Necrotizing Enterocolitis (NEC)** in preterm neonates.

Question 558: What is false regarding emphysema?

A. Decreased FEV1
B. Decreased timed vital capacity
C. Increased residual volume
D. Increased diffusion capacity (Correct Answer)

Explanation: **Explanation:** Emphysema is a chronic obstructive pulmonary disease (COPD) characterized by the permanent enlargement of air spaces distal to the terminal bronchioles and the **destruction of alveolar walls**. **Why Option D is the Correct Answer:** The hallmark of emphysema is the destruction of the alveolar-capillary membrane. According to Fick’s Law, the rate of gas diffusion is directly proportional to the surface area available. In emphysema, the loss of septal walls significantly **decreases the surface area** for gas exchange, leading to a **decreased Diffusion Capacity (DLCO)**. Therefore, the statement "Increased diffusion capacity" is false. **Analysis of Incorrect Options:** * **A & B (Decreased FEV1 and Timed Vital Capacity):** Emphysema is an obstructive lung disease. Destruction of elastic tissue leads to loss of radial traction, causing small airway collapse during expiration. This results in increased airway resistance, significantly reducing the Forced Expiratory Volume in 1 second (FEV1) and the FEV1/FVC ratio (timed vital capacity). * **C (Increased Residual Volume):** Due to loss of elastic recoil and premature airway closure, air becomes trapped in the lungs (air trapping). This leads to hyperinflation, which increases the Residual Volume (RV) and Total Lung Capacity (TLC). **High-Yield Clinical Pearls for NEET-PG:** * **Compliance:** Emphysema is characterized by **increased lung compliance** due to the loss of elastic fibers. * **Pink Puffers:** Patients are often thin, distressed, and use pursed-lip breathing to maintain airway pressure. * **DLCO:** This is the most useful test to differentiate emphysema (low DLCO) from chronic bronchitis or asthma (often normal DLCO). * **Chest X-ray:** Look for flattened diaphragms and increased retrosternal space.

Question 559: What is the cause of unilateral diaphragmatic paralysis?

A. No change in total lung capacity
B. Increase in forced vital capacity
C. Decrease in inspiratory reserve volume (Correct Answer)
D. No change in maximum breathing capacity

Explanation: **Explanation:** Unilateral diaphragmatic paralysis occurs due to the dysfunction of one phrenic nerve (C3-C5). Since the diaphragm is the primary muscle of inspiration, its paralysis leads to a restrictive ventilatory defect. **Why the correct answer is right:** In unilateral paralysis, the affected side of the diaphragm fails to descend and may even move paradoxically upward (cephalad) during inspiration due to negative intrathoracic pressure. This significantly limits the expansion of the thoracic cavity, leading to a **decrease in Inspiratory Reserve Volume (IRV)** and Inspiratory Capacity (IC). Because the lung cannot expand fully, the volume of air that can be inspired above a normal tidal breath is markedly reduced. **Analysis of Incorrect Options:** * **A. No change in Total Lung Capacity (TLC):** Incorrect. TLC **decreases** (typically by 15-25%) because the total volume of gas contained in the lungs at full inspiration is restricted by the elevated, non-functional hemidiaphragm. * **B. Increase in Forced Vital Capacity (FVC):** Incorrect. FVC **decreases**. Restrictive lung pathologies always lead to a reduction in vital capacity as the bellows function of the chest is compromised. * **D. No change in Maximum Breathing Capacity (MBC):** Incorrect. MBC (or MVV - Maximum Voluntary Ventilation) **decreases** significantly because the patient cannot sustain high-volume, rapid breathing due to the mechanical disadvantage of the paralyzed muscle. **Clinical Pearls for NEET-PG:** * **Radiology:** The "Sniff Test" (fluoroscopy) is the gold standard, showing **paradoxical movement** of the paralyzed dome. * **Positioning:** Dyspnea and lung volumes worsen in the **supine position** because abdominal contents push the paralyzed diaphragm further into the thorax. * **Etiology:** The most common cause is malignancy (bronchogenic carcinoma) involving the phrenic nerve, followed by trauma or idiopathic causes.

Question 560: Which of the following is NOT a cause of a rightward shift of the oxygen-hemoglobin dissociation curve?

A. Hyperkalemia
B. Hypokalemia (Correct Answer)
C. Anemia
D. Metabolic alkalosis

Explanation: The oxygen-hemoglobin dissociation curve (OHDC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($SaO_2$). A **rightward shift** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. ### Why Hypokalemia is the Correct Answer **Hypokalemia** does not directly shift the OHDC. However, the question asks which is NOT a cause of a rightward shift. While **Metabolic Alkalosis** (Option D) is a classic cause of a **leftward shift**, in the context of competitive exams like NEET-PG, Hypokalemia is often the "distractor" because it has no direct physiological mechanism to decrease hemoglobin affinity. In fact, severe alkalosis (which shifts the curve left) often co-exists with hypokalemia, but the potassium ion itself is not a primary modulator of the curve. ### Analysis of Other Options * **Hyperkalemia (A):** While not a primary Bohr effect factor, acidosis (which shifts the curve right) causes a shift of $K^+$ out of cells, leading to hyperkalemia. Thus, hyperkalemia is clinically associated with right-shift conditions. * **Anemia (C):** In chronic anemia, there is a compensatory increase in **2,3-BPG** (2,3-Bisphosphoglycerate) levels within red blood cells. 2,3-BPG binds to the beta chains of hemoglobin, stabilizing the "T" (Tense) state and shifting the curve to the **right** to enhance tissue oxygenation. * **Metabolic Alkalosis (D):** An increase in pH (alkalinity) increases hemoglobin's affinity for oxygen, causing a **leftward shift**. (Note: If the question asks for what does *not* cause a right shift, both B and D are technically correct, but Hypokalemia is the standard "least relevant" factor in respiratory physiology). ### High-Yield Clinical Pearls for NEET-PG * **CADET, face Right!** Use this mnemonic for a **Right Shift**: **C**O2 increase, **A**cidosis ($H^+$), **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase. * **P50 Value:** The $PO_2$ at which hemoglobin is 50% saturated. Normal is **26.6 mmHg**. A right shift **increases** the P50; a left shift **decreases** it. * **Fetal Hemoglobin (HbF):** Causes a **left shift** because it has a low affinity for 2,3-BPG, allowing the fetus to pull oxygen from maternal blood.

Practice by Chapter

Mechanics of Breathing

Practice Questions

Pulmonary Ventilation

Practice Questions

Pulmonary Circulation

Practice Questions

Gas Exchange in the Lungs

Practice Questions

Oxygen and Carbon Dioxide Transport

Practice Questions

Control of Breathing

Practice Questions

Respiratory Adjustments in Health and Disease

Practice Questions

High Altitude Physiology

Practice Questions

Diving Physiology

Practice Questions

Respiratory Function Tests

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free