Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 301: Peripheral chemoreceptors are maximally stimulated by which of the following?

A. Acidosis
B. Hypercapnia
C. Carbon dioxide
D. Cyanide (Correct Answer)

Explanation: **Explanation:** The peripheral chemoreceptors (located in the **carotid and aortic bodies**) are primarily sensitive to a decrease in arterial $PO_2$ (hypoxia), an increase in arterial $PCO_2$ (hypercapnia), and a decrease in arterial pH (acidosis). However, the question asks for what **maximally** stimulates them. **Why Cyanide is the Correct Answer:** Cyanide is a potent stimulant of peripheral chemoreceptors because it acts as a **metabolic poison**. It inhibits **cytochrome oxidase**, the terminal enzyme of the mitochondrial electron transport chain. This prevents the glomus cells (Type I cells) in the carotid bodies from utilizing oxygen for ATP production. By mimicking a state of "intracellular hypoxia" or "histotoxic hypoxia," cyanide triggers an intense, maximal discharge of the chemoreceptors, leading to a dramatic increase in ventilation (hyperpnea). **Analysis of Incorrect Options:** * **A & B (Acidosis and Hypercapnia):** While both stimulate peripheral chemoreceptors, their effect is significantly less potent than hypoxia or metabolic poisons. Furthermore, hypercapnia and acidosis exert their primary respiratory drive via **central chemoreceptors** (located in the medulla). * **C (Carbon Dioxide):** Similar to hypercapnia, $CO_2$ is a potent stimulus, but it acts mainly centrally. Peripheral chemoreceptors account for only about 20% of the response to $CO_2$. **High-Yield Facts for NEET-PG:** * **Glomus Cells (Type I):** These are the actual oxygen sensors. They contain dopamine-filled vesicles. * **Threshold for Hypoxia:** Peripheral chemoreceptors are only significantly activated when arterial $PO_2$ falls below **60 mmHg**. * **Innervation:** Carotid bodies are supplied by the **Hering’s nerve** (branch of Glossopharyngeal nerve, CN IX), while aortic bodies are supplied by the **Vagus nerve** (CN X). * **Unique Feature:** The carotid bodies have the **highest blood flow per unit weight** in the body (approx. 2000 ml/100g/min), allowing them to sense arterial blood gases accurately.

Question 302: Pulmonary surfactant is secreted by which cells?

A. Type I pneumocytes
B. Type II pneumocytes (Correct Answer)
C. Clara cells
D. Bronchial epithelial cells

Explanation: **Explanation:** **Correct Answer: B. Type II pneumocytes** Pulmonary surfactant is a surface-active lipoprotein complex (primarily composed of dipalmitoylphosphatidylcholine - DPPC). It is synthesized, stored in **lamellar bodies**, and secreted by **Type II pneumocytes** (granular pneumocytes). These cells are cuboidal and cover approximately 5% of the alveolar surface area, though they outnumber Type I cells. The primary function of surfactant is to **reduce surface tension** at the air-liquid interface, preventing alveolar collapse (atelectasis) during expiration and increasing lung compliance. **Analysis of Incorrect Options:** * **Type I pneumocytes:** These are thin, squamous cells covering 95% of the alveolar surface. Their primary role is facilitating **gas exchange** due to their minimal thickness. They do not have secretory functions. * **Clara cells (Club cells):** Found in the terminal bronchioles, these cells secrete a component of surfactant (surfactant proteins A, B, and D) and help in detoxification, but they are not the primary source of pulmonary surfactant. * **Bronchial epithelial cells:** These include ciliated and goblet cells responsible for mucus production and the mucociliary escalator, not surfactant production. **High-Yield Clinical Pearls for NEET-PG:** * **Lecithin/Sphingomyelin (L/S) Ratio:** A ratio >2:1 in amniotic fluid indicates fetal lung maturity. * **Neonatal Respiratory Distress Syndrome (NRDS):** Caused by a deficiency of surfactant in premature infants (born before 34 weeks). * **Glucocorticoids:** Stimulate the maturation of Type II pneumocytes and are administered to mothers in preterm labor to prevent NRDS. * **Law of Laplace:** $P = 2T/r$. Surfactant reduces $T$ (surface tension), preventing small alveoli from emptying into larger ones.

Question 303: Compliance of the lung is measured by?

A. Elasticity (Correct Answer)
B. Amount of air
C. Blood flow
D. Presence of fluid

Explanation: **Explanation:** **Compliance** is defined as the change in lung volume per unit change in transpulmonary pressure ($C = \Delta V / \Delta P$). In simpler terms, it represents the "distensibility" or "stretchability" of the lungs. 1. **Why Elasticity is Correct:** Compliance is the mathematical inverse of **elastic recoil** (Elasticity). While elasticity is the tendency of the lung to rebound to its original size after being stretched, compliance measures how easily the lung expands. A lung with high elastic recoil (like in pulmonary fibrosis) has low compliance, whereas a lung with low elastic recoil (like in emphysema) has high compliance. Therefore, compliance is fundamentally a measure of the lung's elastic properties. 2. **Why Other Options are Incorrect:** * **Amount of air:** While volume changes are used to calculate compliance, the "amount of air" (Static volumes) does not define the property of compliance itself. * **Blood flow:** This relates to perfusion ($Q$), which affects gas exchange but does not determine the mechanical stretchability of the lung tissue. * **Presence of fluid:** While pulmonary edema *decreases* compliance by making the lungs stiffer, compliance is not a measurement *of* fluid; it is a measurement of tissue and surface tension mechanics. **High-Yield Clinical Pearls for NEET-PG:** * **Increased Compliance:** Seen in **Emphysema** (due to loss of elastic fibers) and with **Aging**. * **Decreased Compliance:** Seen in **Pulmonary Fibrosis** (stiff lungs), **Pulmonary Edema**, and **NRDS** (lack of surfactant increases surface tension). * **Surfactant:** Increases compliance by reducing surface tension, preventing alveolar collapse. * **Total Compliance:** The combined compliance of the lung and chest wall is less than the compliance of either alone (calculated as $1/C_{total} = 1/C_{lung} + 1/C_{chest}$).

Question 304: What is the normal ratio of forced expiratory volume in the first second (FEV1) to the forced vital capacity (FVC) in an adult male?

A. 95%
B. 80% (Correct Answer)
C. 65%
D. 50%

Explanation: **Explanation:** The **FEV1/FVC ratio** (also known as the Tiffeneau-Pinelli index) is a critical parameter in spirometry used to differentiate between obstructive and restrictive lung diseases. In a healthy adult, approximately **80%** of the total air that can be forcibly exhaled (FVC) is expelled within the very first second (FEV1). **Why 80% is correct:** In healthy lungs with normal airway resistance and elastic recoil, the majority of the vital capacity is cleared rapidly. A ratio of 0.75 to 0.85 (averaging 80%) is considered the physiological norm. **Analysis of Incorrect Options:** * **95% (Option A):** This is abnormally high. While seen in some cases of severe restrictive lung disease (where FVC drops more than FEV1), it is not the standard physiological average. * **65% (Option C):** This indicates mild **obstructive** lung disease (e.g., Asthma or COPD). A ratio below 70% is the diagnostic threshold for airflow limitation. * **50% (Option D):** This represents significant airway obstruction. Such low values are typical of moderate-to-severe COPD. **High-Yield Clinical Pearls for NEET-PG:** 1. **Obstructive Disease (Asthma/COPD):** Both FEV1 and FVC decrease, but **FEV1 decreases significantly more**, leading to a **decreased ratio (<70%)**. 2. **Restrictive Disease (Fibrosis):** Both FEV1 and FVC decrease proportionately, or FVC decreases more. Consequently, the **ratio remains normal or is increased**. 3. **Flow-Volume Loops:** In obstructive disease, the loop shows a "scooped-out" appearance; in restrictive disease, the loop is tall, narrow, and shifted to the right.

Question 305: Which gas is used to measure the diffusing capacity of the lungs?

A. Carbon monoxide (Correct Answer)
B. Carbon dioxide
C. Nitrogen
D. Helium

Explanation: **Explanation:** The diffusing capacity of the lung (DLCO) measures the ability of the lungs to transfer gas from the inhaled air to the red blood cells in the pulmonary capillaries. **Why Carbon Monoxide (CO) is the Correct Answer:** Carbon monoxide is the gas of choice because it is **diffusion-limited**. CO has an extremely high affinity for hemoglobin (approximately 210–240 times that of oxygen). When a small, non-toxic amount is inhaled, it binds almost instantaneously to hemoglobin, keeping the partial pressure of CO in the plasma near zero. This creates a stable and maximal partial pressure gradient between the alveoli and the capillary, ensuring that the rate of transfer depends solely on the properties of the alveolar-capillary membrane (thickness and surface area). **Analysis of Incorrect Options:** * **B. Carbon Dioxide (CO₂):** CO₂ diffuses 20 times faster than oxygen; its transfer is limited by perfusion (blood flow), not diffusion, making it unsuitable for measuring diffusing capacity. * **C. Nitrogen (N₂):** Nitrogen is an inert gas that does not bind to hemoglobin and reaches equilibrium rapidly, making it ineffective for measuring membrane diffusion. * **D. Helium (He):** Helium is used in pulmonary function tests to measure **Functional Residual Capacity (FRC)** and residual volume via the helium dilution method, but it does not cross the alveolar-capillary membrane. **High-Yield Clinical Pearls for NEET-PG:** * **DLCO is increased in:** Polycythemia, Alveolar hemorrhage (e.g., Goodpasture syndrome), and early stages of Congestive Heart Failure (due to increased pulmonary capillary blood volume). * **DLCO is decreased in:** Emphysema (loss of surface area), Interstitial Lung Disease/Fibrosis (increased membrane thickness), and Anemia. * **Formula:** $DLCO = \text{Rate of CO uptake} / (\text{Alveolar } P_{CO} - \text{Capillary } P_{CO})$. Since capillary $P_{CO}$ is negligible, it simplifies the calculation.

Question 306: The provided graph illustrates the relationship between lung volume and intrapleural pressure during inspiration and expiration. What is the most likely explanation for the observed pattern?

A. The difference between active and passive work performed during respiration.
B. The difference in surfactant activity during inspiration and expiration. (Correct Answer)
C. The variation in airway resistance between inspiration and expiration.
D. The fluctuations in intrapleural pressure.

Explanation: ***The difference in surfactant activity during inspiration and expiration*** - During inspiration, **surfactant molecules** become more dispersed as alveolar surface area increases, reducing their effectiveness and requiring higher pressure for lung expansion. - During expiration, **surfactant concentration** increases as alveolar surface area decreases, enhancing surface tension reduction and allowing the lungs to deflate at lower pressures, creating the **hysteresis loop**. *The difference between active and passive work performed during respiration* - This concept relates to **energy expenditure** during breathing phases but does not explain why the pressure-volume curves follow different paths. - **Hysteresis** is a physical property of lung tissue and surfactant, not a result of work differences between respiratory phases. *The variation in airway resistance between inspiration and expiration* - **Airway resistance** affects airflow rates but does not significantly influence the **static pressure-volume relationship** shown in the graph. - The hysteresis pattern occurs even during **slow, quasi-static** breathing where airway resistance effects are minimal. *The fluctuations in intrapleural pressure* - **Intrapleural pressure** changes are the driving force for ventilation but do not explain why inspiration and expiration follow **different pressure-volume curves**. - The graph shows the relationship between lung volume and pressure, not the cause of pressure fluctuations themselves.

Question 307: A 4-year-old asthmatic child presents with respiratory distress, characterized by unresponsiveness, rapid shallow breathing, and pulsus paradoxus. Arterial blood gas analysis reveals respiratory acidosis. What is the next most appropriate step in managing this patient?

A. Urgent endotracheal intubation (Correct Answer)
B. Hyperbaric oxygen therapy
C. Metered-dose inhaler with salbutamol and corticosteroids
D. Subcutaneous terbutaline with an aminophylline drip

Explanation: ### Explanation The patient is presenting with **Status Asthmaticus** with "Imminent Respiratory Failure." The clinical signs of **unresponsiveness** (altered sensorium), **rapid shallow breathing** (exhaustion), and **respiratory acidosis** on ABG indicate that the patient is no longer able to maintain adequate ventilation. **1. Why Option A is Correct:** In severe asthma, respiratory acidosis (elevated $PaCO_2$) is an ominous sign. Normally, an asthmatic patient hyperventilates, leading to respiratory alkalosis. When the patient tires, $CO_2$ begins to rise. The combination of altered mental status and acidosis indicates that the respiratory muscles are failing. **Urgent endotracheal intubation** and mechanical ventilation are mandatory to secure the airway and provide ventilatory support. **2. Why Other Options are Incorrect:** * **Option B:** Hyperbaric oxygen is used for carbon monoxide poisoning or decompression sickness; it has no role in acute asthma management. * **Option C:** While MDIs are standard for mild-to-moderate asthma, they are insufficient for a patient in respiratory failure who cannot coordinate breaths or move enough air to deliver the drug to the distal airways. * **Option D:** Terbutaline and aminophylline are second-line bronchodilators. In a patient with altered sensorium and acidosis, pharmacological bronchodilation alone is too slow and risky; the immediate priority is airway protection. **Clinical Pearls for NEET-PG:** * **The "Silent Chest":** A dangerous sign in asthma where airflow is so limited that wheezing disappears. * **ABG Progression:** Early Asthma → Respiratory Alkalosis (Low $PaCO_2$). Late/Severe Asthma → Normal $PaCO_2$ (Pseudo-normalization, a warning sign). Imminent Failure → **Respiratory Acidosis** (High $PaCO_2$). * **Pulsus Paradoxus:** Defined as a drop in systolic BP >10 mmHg during inspiration; it signifies severe air trapping and increased work of breathing.

Question 308: A 70-year-old man presents with a 16-week history of progressive dysphagia and recurrent pneumonia episodes. He also has palpable stony hard neck nodes on examination. What is the most likely diagnosis?

A. Carcinoma of the esophagus (Correct Answer)
B. Achalasia Cardia
C. Diffuse esophageal spasm
D. Zenker's diverticulum

Explanation: **Explanation:** The clinical presentation of **progressive dysphagia** in an elderly male (70 years), associated with **stony hard neck nodes** (suggestive of metastatic lymphadenopathy), is a classic "red flag" for malignancy. In this case, **Carcinoma of the Esophagus** is the most likely diagnosis. **Why the correct answer is right:** 1. **Progressive Dysphagia:** Malignant strictures typically cause dysphagia that starts with solids and progresses to liquids as the lumen narrows. 2. **Metastasis:** The presence of "stony hard" neck nodes (likely Virchow’s node or deep cervical nodes) strongly indicates metastatic spread, a hallmark of advanced esophageal cancer. 3. **Recurrent Pneumonia:** This occurs due to chronic aspiration of saliva/food or the development of a **tracheoesophageal fistula**, a common complication of esophageal malignancy. **Why other options are incorrect:** * **Achalasia Cardia:** Typically presents in younger patients with long-standing dysphagia (often paradoxical, for liquids more than solids) and lacks hard lymphadenopathy. * **Diffuse Esophageal Spasm:** Characterized by intermittent chest pain and "corkscrew esophagus" on imaging; it does not cause weight loss or lymphadenopathy. * **Zenker’s Diverticulum:** While it causes dysphagia and aspiration, the neck mass is typically soft, fluctuant, and may gurgle (Boyce sign), rather than being stony hard. **High-Yield Clinical Pearls for NEET-PG:** * **Squamous Cell CA:** Most common worldwide; associated with smoking and alcohol. * **Adenocarcinoma:** Most common in the West; associated with GERD and **Barrett’s Esophagus**. * **Investigation of Choice:** Upper GI Endoscopy (UGIE) with biopsy. * **Staging:** EUS (Endoscopic Ultrasound) is the most accurate for T and N staging.

Question 309: What is the effect of cutting the spinal cord above the medulla on respiration?

A. Breathing becomes slower and deeper
B. Apneustic breathing
C. Breathing ceases
D. Irregular and gasping (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Irregular and gasping** **Underlying Concept:** The fundamental rhythm of respiration is generated by the **Pre-Bötzinger complex** and the **Medullary Respiratory Centers** (Dorsal and Ventral Respiratory Groups). While the medulla can generate a basic rhythm, it requires input from higher centers (Pons and Cortex) and peripheral feedback to maintain a smooth, regular pattern. When the spinal cord is cut **above the medulla** (specifically at the midbrain-medullary junction or high medullary level), the connection between the **Pons** (Pneumotaxic and Apneustic centers) and the **Medulla** is severed. The isolated medulla, deprived of the "fine-tuning" influence of the pontine centers, produces a primitive, disorganized rhythm characterized by **irregular and gasping** breaths (ataxic breathing). **Why other options are incorrect:** * **A. Breathing becomes slower and deeper:** This occurs when the **Vagus nerve** is bilaterally transected (loss of Hering-Breuer reflex) or when the **Pneumotaxic center** is inhibited. * **B. Apneustic breathing:** This is characterized by prolonged inspiratory gasps. It occurs when there is a lesion in the **upper pons** (Pneumotaxic center) combined with a bilateral **Vagotomy**. * **C. Breathing ceases:** Respiration stops (Apnea) only if the lesion is **below the medulla** (at or above C3-C5), which severs the connection between the respiratory centers and the phrenic nerve (which innervates the diaphragm). **High-Yield Facts for NEET-PG:** * **Pneumotaxic Center (Upper Pons):** Acts as the "off-switch" for inspiration; limits tidal volume. * **Apneustic Center (Lower Pons):** Delays the "off-switch," prolonging inspiration. * **C3, 4, 5 keep the diaphragm alive:** A spinal cord injury at C3 or above leads to immediate respiratory arrest. * **Cheyne-Stokes Breathing:** Often seen in heart failure or cortical brain damage (uphill/downhill pattern).

Question 310: Laminar flow is dependent on which of the following factors?

A. Critical velocity (Correct Answer)
B. Viscosity
C. Constant velocity
D. Critical closing pressure

Explanation: **Explanation:** The type of airflow in the respiratory tract (laminar vs. turbulent) is determined by the **Reynolds Number (Re)**. According to the formula $Re = \frac{\rho vd}{\eta}$ (where $\rho$ is density, $v$ is velocity, $d$ is diameter, and $\eta$ is viscosity), flow remains **laminar** as long as the velocity of the gas stays below a specific threshold known as the **Critical Velocity**. 1. **Why A is correct:** Critical velocity is the maximum velocity at which airflow remains laminar. Once the gas velocity exceeds this point, the Reynolds number surpasses 2000, and the flow transitions from smooth, streamlined (laminar) layers to disorganized, eddy-forming **turbulent flow**. Therefore, the maintenance of laminar flow is directly dependent on staying below this critical velocity. 2. **Why B is incorrect:** While viscosity ($\eta$) influences the Reynolds number, laminar flow itself is primarily driven by the pressure gradient and is inversely proportional to viscosity (Poiseuille’s Law). However, the *transition* and *dependency* of flow type are defined by the velocity threshold. 3. **Why C is incorrect:** Constant velocity simply means the speed does not change; it does not dictate whether the flow is laminar or turbulent. 4. **Why D is incorrect:** Critical closing pressure refers to the pressure at which small airways or blood vessels collapse (Laplace’s Law), which is unrelated to the pattern of fluid flow. **High-Yield NEET-PG Pearls:** * **Laminar Flow:** Occurs in small peripheral airways (bronchioles) where velocity is low. * **Turbulent Flow:** Occurs in large airways (trachea) where velocity is high. * **Heliox Therapy:** In airway obstruction, replacing Nitrogen with Helium (lower density) reduces the Reynolds number, converting turbulent flow back to laminar flow, thereby decreasing the work of breathing.

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