Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 281: When the respiratory muscles are relaxed, the lungs are at which of the following volumes?

A. Functional residual capacity (FRC) (Correct Answer)
B. Expiratory reserve volume (ERV)
C. Residual volume (RV)
D. Inspiratory reserve volume (IRV)

Explanation: ### Explanation **1. Why Functional Residual Capacity (FRC) is correct:** Functional Residual Capacity (FRC) is defined as the volume of air remaining in the lungs at the end of a normal, quiet expiration (tidal breath). At this point, the respiratory muscles are in a state of **relaxation**. Physiologically, FRC represents the **Equilibrium Point** of the respiratory system. It is the volume where two opposing elastic recoil forces are equal and opposite: * **Lungs:** Tend to collapse inward due to elastic recoil. * **Chest Wall:** Tends to spring outward. Because these forces balance each other out (Net Pressure = 0), no muscle effort is required to maintain this volume. **2. Why other options are incorrect:** * **Expiratory Reserve Volume (ERV):** This is the extra volume that can be expired *after* a normal tidal expiration. Reaching this volume requires active contraction of expiratory muscles (e.g., abdominal muscles). * **Residual Volume (RV):** This is the air remaining after a maximal forced expiration. It cannot be reached by relaxation; it requires maximal muscle effort to squeeze the air out. * **Inspiratory Reserve Volume (IRV):** This is the volume inhaled beyond a normal tidal breath. It requires active contraction of inspiratory muscles (e.g., diaphragm and external intercostals). **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Measurement:** FRC cannot be measured by simple spirometry (because it contains RV). It is measured via **Helium Dilution** or **Body Plethysmography**. * **Clinical Significance:** FRC acts as a "buffer" for gas exchange, preventing large fluctuations in O2 and CO2 levels during the breathing cycle. * **Positioning:** FRC **decreases** when moving from a standing to a supine position due to the upward pressure of abdominal contents on the diaphragm. * **Pathology:** FRC is **increased** in obstructive lung diseases (e.g., Emphysema) due to air trapping and **decreased** in restrictive lung diseases (e.g., Fibrosis).

Question 282: Which of the following pulmonary function changes are typically seen in an untreated patient with acute bronchial asthma?

A. Increased peak expiratory flow
B. Decreased total lung capacity (TLC)
C. Decreased forced vital capacity (FVC) (Correct Answer)
D. Increased residual volume (RV)

Explanation: **Explanation:** Acute bronchial asthma is a classic **obstructive lung disease** characterized by reversible airway narrowing due to bronchospasm, mucosal edema, and mucus plugging. **Why FVC is Decreased (The Correct Answer):** In acute asthma, the primary pathology is increased airway resistance. During a forced expiration, the narrowed airways collapse prematurely (dynamic compression). This leads to **air trapping**, where air remains stuck in the lungs and cannot be exhaled. Because the patient cannot empty their lungs completely, the total volume of air they can forcibly exhale after a deep breath—the **Forced Vital Capacity (FVC)**—is significantly reduced. **Analysis of Incorrect Options:** * **A. Increased Peak Expiratory Flow (PEF):** Incorrect. PEF measures the maximum speed of expiration. In asthma, airway obstruction significantly **decreases** PEF. This is a key bedside tool for monitoring severity. * **B. Decreased Total Lung Capacity (TLC):** Incorrect. In obstructive diseases, TLC is typically **normal or increased** (hyperinflation) due to air trapping, unlike restrictive diseases where TLC decreases. * **D. Increased Residual Volume (RV):** While RV **does increase** in acute asthma due to air trapping, the question asks for the most typical change among the options provided. In many standard physiological assessments and NEET-PG patterns, the reduction in FVC and FEV1 are the hallmark findings used to define the obstructive defect. *(Note: If this were a "Multiple Correct" format, D would also be physiologically true, but C is the classic functional parameter used to describe the flow-volume loop changes).* **High-Yield Clinical Pearls for NEET-PG:** * **Hallmark of Obstruction:** Decreased FEV1, Decreased FVC, and a **Decreased FEV1/FVC ratio (<0.7)**. * **Reversibility:** A hallmark of asthma is an improvement in FEV1 by **>12% and >200ml** after bronchodilator inhalation. * **Flow-Volume Loop:** Shows a characteristic **"scooped-out"** appearance during the expiratory phase.

Question 283: The diffusion capacity of the lung is decreased in all of the following conditions except?

A. Interstitial lung disease
B. Goodpasture's syndrome (Correct Answer)
C. Pneumocystis jirovecii pneumonia
D. Primary pulmonary hypertension

Explanation: **Explanation:** The **Diffusion Capacity of the Lung for Carbon Monoxide (DLCO)** measures the ability of the lungs to transfer gas from the inhaled air to the red blood cells in the pulmonary capillaries. It depends on the surface area available for exchange, the thickness of the alveolar-capillary membrane, and the hemoglobin volume. **Why Goodpasture’s Syndrome is the Correct Answer:** In **Goodpasture’s Syndrome**, there is acute pulmonary hemorrhage. The presence of free hemoglobin (RBCs) within the alveoli binds to the carbon monoxide used during the DLCO test. This leads to an **increase in DLCO** rather than a decrease. This is a classic "exception" frequently tested in exams. **Analysis of Incorrect Options:** * **Interstitial Lung Disease (ILD):** DLCO is **decreased** due to the thickening and fibrosis of the alveolar-capillary membrane, which increases the diffusion distance. * **Pneumocystis jirovecii Pneumonia (PJP):** This infection causes significant inflammation and "foamy" exudates in the alveoli, increasing the barrier thickness and **decreasing** DLCO. In fact, a low DLCO is a highly sensitive screening marker for PJP in HIV patients. * **Primary Pulmonary Hypertension:** DLCO is **decreased** because of structural changes in the pulmonary vasculature (obliteration of capillaries), which reduces the effective surface area and capillary blood volume available for gas exchange. **High-Yield Clinical Pearls for NEET-PG:** * **Increased DLCO:** Seen in Pulmonary hemorrhage (Goodpasture’s), Polycythemia, Left-to-right shunts, and Exercise. * **Decreased DLCO:** Seen in Emphysema (loss of surface area), ILD (thickened membrane), Anemia (low Hb), and Pulmonary Embolism. * **Asthma vs. COPD:** DLCO is usually **normal or increased in Asthma**, but **decreased in Emphysema**. This is a key physiological differentiator.

Question 284: Shift of the oxygen dissociation curve to the right occurs in which of the following conditions?

A. Acidosis (Correct Answer)
B. 2,3-DPG
C. Increased temperature
D. Increased Pco2

Explanation: **Explanation:** The Oxygen Dissociation Curve (ODC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($SaO_2$). A **shift to the right** indicates a decreased affinity of hemoglobin for oxygen, meaning oxygen is more easily released to the tissues. **Why Acidosis is the Correct Answer:** Acidosis (decreased pH) causes a rightward shift through the **Bohr Effect**. When $H^+$ ions increase, they bind to specific amino acid residues in hemoglobin, stabilizing the "Tense" (T) state. This conformational change reduces hemoglobin's affinity for oxygen, facilitating its unloading in metabolically active tissues where lactic acid or $CO_2$ levels are high. **Analysis of Options:** While the question asks for "which condition," it is important to note that **technically, all four options (A, B, C, and D) cause a rightward shift.** In the context of NEET-PG, if this appears as a single-choice question, **Acidosis** or **Increased $PCO_2$** are often prioritized as primary physiological drivers. However, if this were a "multiple correct" style or if "All of the above" were an option, it would be more accurate. * **B, C, and D:** Increased 2,3-DPG, increased temperature, and increased $PCO_2$ all stabilize the T-state of hemoglobin and shift the curve to the right. **High-Yield NEET-PG Pearls:** * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG (2,3-DPG) increase, **E**xercise, **T**emperature increase. * **Left Shift:** Occurs in Alkalosis, decreased temp, decreased 2,3-DPG, and presence of **HbF (Fetal Hemoglobin)** or **Carbon Monoxide (CO)**. * **P50 Value:** The $PO_2$ at which Hb is 50% saturated. A right shift **increases** the P50 (normal is ~26.7 mmHg).

Question 285: Oxygen dissociation curve shifts to the right in all the following conditions except:

A. Diabetic ketoacidosis
B. Blood transfusion (Correct Answer)
C. High altitude
D. Anaemia

Explanation: The **Oxygen Dissociation Curve (ODC)** represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **shift to the right** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to tissues. ### Why "Blood Transfusion" is the Correct Answer: Stored blood undergoes a depletion of **2,3-Bisphosphoglycerate (2,3-BPG)** over time. 2,3-BPG is essential for stabilizing the "T" (Tense) state of hemoglobin, which promotes oxygen release. When a patient receives a transfusion of stored blood, the low levels of 2,3-BPG cause the curve to **shift to the left** (increased affinity), meaning the hemoglobin holds onto oxygen more tightly. ### Explanation of Incorrect Options (Conditions that shift the curve to the Right): * **Diabetic Ketoacidosis (DKA):** This condition involves metabolic **acidosis** (decreased pH). According to the **Bohr Effect**, an increase in $H^+$ ions stabilizes the T-state of hemoglobin, shifting the curve to the right. * **High Altitude:** Hypoxia at high altitudes stimulates an increase in **2,3-BPG production** within RBCs to compensate for lower atmospheric $PO_2$, shifting the curve to the right to enhance tissue oxygenation. * **Anaemia:** In chronic anaemia, there is a compensatory **increase in 2,3-BPG** levels to ensure that the limited amount of hemoglobin available can efficiently deliver oxygen to the tissues. ### NEET-PG High-Yield Pearls: * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase. * **P50 Value:** The $PO_2$ at which hemoglobin is 50% saturated. A **Right shift increases the P50**, while a Left shift decreases it. * **Fetal Hemoglobin (HbF):** Always causes a **Left shift** because it has a poor binding affinity for 2,3-BPG, ensuring it can take oxygen from maternal blood.

Question 286: What is the alveolar ventilation for a person with a vital capacity of 500 mL, a respiratory rate of 14/min, a tidal volume of 500 mL, and a dead space of 150 mL?

A. 2500 mL
B. 3500 mL
C. 4900 mL (Correct Answer)
D. 6000 mL

Explanation: ### Explanation **1. Understanding the Correct Answer (C: 4900 mL)** Alveolar ventilation ($\dot{V}_A$) is the volume of fresh air that reaches the gas-exchange units (alveoli) per minute. Unlike Minute Ventilation, it accounts for the **Anatomical Dead Space** ($V_D$), which is the air that remains in the conducting airways and does not participate in gas exchange. The formula for Alveolar Ventilation is: $$\dot{V}_A = (\text{Tidal Volume} - \text{Dead Space}) \times \text{Respiratory Rate}$$ **Calculation:** * Tidal Volume ($V_T$) = 500 mL * Dead Space ($V_D$) = 150 mL * Respiratory Rate ($RR$) = 14/min * $\dot{V}_A = (500 - 150) \times 14$ * $\dot{V}_A = 350 \times 14 = \mathbf{4900\ mL/min}$ **2. Analysis of Incorrect Options** * **A (2500 mL):** This is a distractor value with no physiological basis in this context. * **B (3500 mL):** This represents the total volume of air reaching the alveoli in one minute if the respiratory rate were 10/min, or simply $350 \times 10$. * **D (6000 mL):** This represents the **Minute Ventilation** ($\dot{V}_E = V_T \times RR$), which is $500 \times 12$ (using a standard rate) or $500 \times 14 = 7000$ mL. It incorrectly ignores the dead space. **3. Clinical Pearls & High-Yield Facts** * **Vital Capacity (VC):** In this question, VC is a "distractor." It is the maximum volume of air a person can exhale after maximum inhalation and is not used to calculate ventilation. * **Dead Space:** In a healthy adult, anatomical dead space is approximately **2 mL/kg** of ideal body weight (roughly 150 mL). * **Efficiency:** Increasing the **depth** of breathing (Tidal Volume) is more effective at increasing alveolar ventilation than increasing the **rate** of breathing, because the dead space is constant for every breath. * **Physiological Dead Space:** This equals Anatomical Dead Space + Alveolar Dead Space (wasted ventilation in non-perfused alveoli). In healthy individuals, they are nearly equal.

Question 287: Which of the following compounds is elevated in bronchial asthma?

A. PGI2
B. PGH2
C. Leukotrienes (Correct Answer)
D. Thromboxane

Explanation: **Explanation:** **Bronchial Asthma** is a chronic inflammatory airway disease characterized by reversible airway obstruction, mucus hypersecretion, and bronchial hyperresponsiveness. The underlying pathophysiology involves a Type I Hypersensitivity reaction. **Why Leukotrienes are correct:** In asthma, the activation of mast cells and eosinophils leads to the metabolism of arachidonic acid via the **5-Lipoxygenase (5-LOX) pathway**. This results in the production of **Cysteinyl Leukotrienes (LTC4, LTD4, and LTE4)**, formerly known as the *Slow-Reacting Substance of Anaphylaxis (SRS-A)*. These compounds are potent bronchoconstrictors (1000x more potent than histamine), increase vascular permeability, and stimulate mucus secretion, making them central to asthma pathogenesis. **Why the other options are incorrect:** * **PGI2 (Prostacyclin):** Produced via the Cyclooxygenase (COX) pathway, it is a potent vasodilator and inhibitor of platelet aggregation. It does not play a primary role in the bronchoconstriction seen in asthma. * **PGH2:** This is an unstable intermediate in the COX pathway that is rapidly converted into various prostaglandins or thromboxanes; it is not a specific marker for asthma. * **Thromboxane (TXA2):** Primarily involved in platelet aggregation and vasoconstriction. While it has minor bronchoconstrictor effects, it is not the hallmark mediator elevated in asthma compared to leukotrienes. **High-Yield Clinical Pearls for NEET-PG:** * **Zileuton:** A drug that inhibits the 5-Lipoxygenase enzyme. * **Montelukast/Zafirlukast:** Selective antagonists of the **CysLT1 receptor**, used as maintenance therapy in asthma. * **Aspirin-Exacerbated Respiratory Disease (AERD):** Occurs because Aspirin blocks the COX pathway, shunting arachidonic acid toward the LOX pathway, leading to an overproduction of leukotrienes. * **Charcot-Leyden Crystals:** Found in the sputum of asthmatics, derived from eosinophil lysophospholipase.

Question 288: Which muscles are involved in quiet expiration?

A. Diaphragm
B. Intercostal muscles
C. Both diaphragm and intercostal muscles
D. Neither diaphragm nor intercostal muscles (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Neither diaphragm nor intercostal muscles.** **1. Underlying Medical Concept:** In respiratory physiology, **quiet expiration** (eupnea) is a purely **passive process**. It does not require active muscle contraction. Instead, it relies on the **elastic recoil** of the lungs and the chest wall. When the muscles of inspiration (diaphragm and external intercostals) relax, the intra-thoracic volume decreases, and the intra-alveolar pressure rises above atmospheric pressure, causing air to flow out of the lungs. **2. Why the other options are incorrect:** * **A & B (Diaphragm and Intercostal muscles):** These are the primary muscles of **quiet inspiration**. The diaphragm is the most important muscle, responsible for about 75% of the air movement during quiet breathing. * **C (Both):** This is incorrect because, while these muscles are active during the *inspiratory* phase, they are inhibited/relaxing during quiet expiration. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Forced Expiration:** Unlike quiet expiration, forced expiration (e.g., coughing, sneezing, or exercise) is an **active process**. The primary muscles involved are the **abdominal muscles** (rectus abdominis, obliques) and the **internal intercostal muscles**. * **Accessory Muscles of Inspiration:** These include the sternocleidomastoid and scalene muscles, which are recruited during respiratory distress. * **Compliance:** The ease with which the lungs expand is called compliance. A loss of elastic recoil (as seen in **Emphysema**) makes even quiet expiration difficult, often requiring the use of accessory muscles. * **Key Rule:** Inspiration is always active; Expiration is passive at rest but active during exertion.

Question 289: Which of the following is NOT decreased in infiltrative lung disease?

A. Vital capacity
B. Alveolar-arterial difference in PaO2 (Correct Answer)
C. Total lung capacity
D. Lung compliance

Explanation: **Explanation:** Infiltrative lung diseases (such as Idiopathic Pulmonary Fibrosis or Sarcoidosis) are classic examples of **Restrictive Lung Diseases**. The fundamental pathology involves the deposition of fibrous tissue in the alveolar walls, making the lungs "stiff" and difficult to expand. **Why Option B is the Correct Answer:** The **Alveolar-arterial (A-a) gradient** is the difference between the partial pressure of oxygen in the alveoli ($PAO_2$) and the arterial blood ($PaO_2$). In infiltrative diseases, the thickened alveolar-capillary membrane creates a diffusion barrier. This impairs oxygen transfer, leading to a lower $PaO_2$ relative to $PAO_2$. Consequently, the **A-a gradient is INCREASED**, not decreased. **Why the other options are incorrect:** * **A & C (Vital Capacity and Total Lung Capacity):** In restrictive disease, the lungs cannot expand fully due to increased elastic recoil. This leads to a reduction in all lung volumes and capacities, including TLC, VC, and FRC. * **D (Lung Compliance):** Compliance is the change in volume per unit change in pressure ($C = \Delta V / \Delta P$). Because the lungs are stiff (fibrotic), they require much higher pressure to achieve a small change in volume. Therefore, **lung compliance is decreased**. **High-Yield Clinical Pearls for NEET-PG:** * **FEV1/FVC Ratio:** In restrictive lung disease, both FEV1 and FVC decrease, but the ratio remains **normal or is increased** (unlike obstructive disease where it decreases). * **Diffusion Capacity (DLCO):** This is characteristically **decreased** in infiltrative diseases due to the thickened interstitium. * **Radiology:** Look for "honeycombing" or "ground-glass opacities" on HRCT, which are hallmarks of advanced infiltrative disease.

Question 290: Carotid body receptors are stimulated when there is:

A. Increased O2 tension
B. Decreased O2 tension (Correct Answer)
C. Decreased CO2 tension
D. Increased heart rate

Explanation: ### Explanation The **carotid bodies** are peripheral chemoreceptors located at the bifurcation of the common carotid arteries. Their primary function is to monitor the chemical composition of arterial blood to regulate ventilation. **Why Option B is Correct:** The carotid bodies are uniquely sensitive to a **decrease in the partial pressure of arterial oxygen ($PaO_2$)**, also known as hypoxemia. When $PaO_2$ falls below approximately **60 mmHg**, specialized **Type I (Glomus) cells** are stimulated. These cells contain oxygen-sensitive potassium ($K^+$) channels that close during hypoxia, leading to cell depolarization, calcium influx, and the release of neurotransmitters (like ATP and dopamine). This triggers action potentials in the **Glossopharyngeal nerve (CN IX)**, which signals the respiratory centers in the medulla to increase the rate and depth of breathing. **Why the Other Options are Incorrect:** * **Option A:** Increased $O_2$ tension (hyperoxia) actually inhibits peripheral chemoreceptor firing, reducing the drive to breathe. * **Option C:** Decreased $CO_2$ tension (hypocapnia) leads to an increase in pH (alkalosis), which suppresses chemoreceptor activity. Carotid bodies are stimulated by **increased** $CO_2$ or **decreased** pH (acidosis). * **Option D:** Increased heart rate is a physiological response to various stimuli (like sympathetic activation) but is not a direct *trigger* for carotid body stimulation. **High-Yield NEET-PG Pearls:** * **Primary Stimulus:** The carotid bodies respond to **dissolved $O_2$ ($PaO_2$)**, not total oxygen content. Therefore, they are **not** stimulated in carbon monoxide poisoning or anemia (where $PaO_2$ remains normal). * **Innervation:** Carotid body → Hering’s nerve (branch of **CN IX**); Aortic body → **Vagus nerve (CN X)**. * **Blood Flow:** The carotid body has the **highest blood flow per unit weight** in the body (approx. 2000 mL/100g/min), allowing it to sense real-time changes in arterial blood.

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