Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 121: Which of the following statements is true regarding asthma?

A. Increased FRC and reduced residual volume
B. Increased FRC and increased residual volume (Correct Answer)
C. Reduced FRC and reduced residual volume
D. Reduced FRC and increased residual volume

Explanation: **Explanation:** Asthma is a chronic inflammatory **obstructive lung disease** characterized by reversible airway narrowing. The pathophysiology revolves around **air trapping** and **hyperinflation**. **1. Why Option B is Correct:** During an asthma attack, bronchoconstriction and mucus plugging lead to premature closure of the small airways during expiration. This prevents the lungs from emptying completely, a phenomenon known as **air trapping**. * **Residual Volume (RV):** The volume of air remaining in the lungs after maximal expiration increases because air is trapped behind closed airways. * **Functional Residual Capacity (FRC):** Since FRC is the sum of RV and Expiratory Reserve Volume (ERV), an increase in RV leads to a compensatory increase in FRC. The patient breathes at higher lung volumes to keep the airways open via increased radial traction. **2. Why Other Options are Incorrect:** * **Options A & C:** Residual Volume (RV) is **never reduced** in obstructive diseases like asthma or COPD; it is typically increased. A reduced RV is characteristic of restrictive lung diseases (e.g., pulmonary fibrosis). * **Options C & D:** FRC is **increased** in asthma due to hyperinflation. A reduced FRC occurs in conditions that cause lung collapse or stiffness (e.g., ARDS, obesity, or interstitial lung disease). **High-Yield Clinical Pearls for NEET-PG:** * **PFT Pattern:** Decreased FEV1, decreased FEV1/FVC ratio (<70%), and increased Total Lung Capacity (TLC). * **Reversibility:** A hallmark of asthma is a >12% and >200ml improvement in FEV1 after bronchodilator administration. * **Flow-Volume Loop:** Shows a characteristic **"scooped-out"** appearance during the expiratory phase. * **Status Asthmaticus:** A normal or rising PaCO2 in a severe attack is an ominous sign of impending respiratory failure (muscle fatigue).

Question 122: Which of the following is preferred for the diagnosis of obstructive airway disease?

A. Vital capacity
B. Timed vital capacity (Correct Answer)
C. Tidal volume
D. Blood gas analysis

Explanation: **Explanation:** The diagnosis of obstructive airway diseases (like Asthma and COPD) relies on assessing the **rate of airflow** rather than just the volume of air. **Why Timed Vital Capacity is correct:** Timed Vital Capacity, specifically **FEV1** (Forced Expiratory Volume in 1 second), measures the volume of air exhaled during the first second of a forced expiration. In obstructive diseases, the airway resistance is increased, leading to a prolonged expiratory phase and a significant reduction in FEV1. While the total Forced Vital Capacity (FVC) may remain normal or slightly decreased, the **FEV1/FVC ratio** (Tiffeneau Index) drops below 70%, making it the gold standard for identifying obstruction. **Why other options are incorrect:** * **Vital Capacity (VC):** This measures the maximum volume of air moved in or out. It is primarily reduced in **restrictive** lung diseases (like Pulmonary Fibrosis). In early obstruction, VC may be normal. * **Tidal Volume (TV):** This is the volume of air inspired or expired during normal quiet breathing (~500ml). It is non-specific and does not provide information about airway resistance or lung capacity. * **Blood Gas Analysis (ABG):** While useful for assessing the severity of an acute exacerbation (hypoxia/hypercapnia), it is not a diagnostic tool for the underlying airway disease itself. **High-Yield Clinical Pearls for NEET-PG:** * **Obstructive Pattern:** ↓FEV1, ↓FEV1/FVC ratio (<0.7), ↑Residual Volume (due to air trapping). * **Restrictive Pattern:** ↓FVC, **Normal or ↑FEV1/FVC ratio**, ↓Total Lung Capacity. * **MVV (Maximum Voluntary Ventilation):** Also significantly reduced in obstructive diseases but is more strenuous for the patient than FEV1.

Question 123: Which of the following is NOT true regarding restrictive lung disease?

A. Residual volume is increased in diaphragmatic paralysis.
B. Diffusion lung capacity for carbon monoxide is decreased in interstitial lung disease.
C. Residual volume is decreased in interstitial lung disease.
D. FEV1% is decreased in interstitial lung disease. (Correct Answer)

Explanation: **Explanation:** In **Restrictive Lung Diseases (RLD)**, the hallmark is a reduction in lung volumes due to either parenchymal stiffness (e.g., Interstitial Lung Disease) or extrapulmonary constraints (e.g., chest wall deformities, diaphragmatic paralysis). **Why Option D is the Correct Answer (The "False" Statement):** In RLD, both Forced Expiratory Volume in 1 second (FEV1) and Forced Vital Capacity (FVC) decrease. However, because the lung tissue is stiff (increased elastic recoil), the airways are pulled open wider (radial traction), allowing air to exit rapidly. Consequently, **FVC decreases more than FEV1**, leading to a **normal or increased FEV1/FVC ratio (FEV1%)**. A decreased FEV1% is characteristic of *obstructive* lung diseases like asthma or COPD. **Analysis of Other Options:** * **Option A (True):** In **extrapulmonary** restriction like diaphragmatic paralysis, the lungs cannot be fully emptied because the respiratory pump is weak, leading to an **increased Residual Volume (RV)**. * **Option B (True):** In **Interstitial Lung Disease (ILD)**, the alveolar-capillary membrane thickens (fibrosis), which increases the diffusion distance, thereby **decreasing the DLCO**. * **Option C (True):** In **intrapulmonary** restriction (ILD), the increased elastic recoil of the fibrotic lung parenchyma pulls the lungs inward, resulting in a **decreased RV** and Total Lung Capacity (TLC). **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** A decrease in **Total Lung Capacity (TLC)** is the definitive marker for restriction. * **Flow-Volume Loop:** RLD shows a **"Witch’s Hat"** appearance (narrow, tall loop shifted to the right). * **DLCO Differentiation:** DLCO is **decreased** in intrinsic RLD (fibrosis) but typically **normal** in extrinsic RLD (obesity, kyphoscoliosis, neuromuscular weakness).

Question 124: What is the primary function of the carotid body?

A. To measure changes in PO2 in arterial blood (Correct Answer)
B. To measure PO2 in venous blood
C. To measure changes in CO2 in arterial blood
D. To measure changes in CO2 in venous blood

Explanation: The **carotid bodies** are the primary peripheral chemoreceptors responsible for monitoring the chemical composition of arterial blood. ### **Why Option A is Correct** The carotid bodies are located at the bifurcation of the common carotid arteries. Their primary function is to detect a **decrease in the partial pressure of oxygen (PO2)** in arterial blood. When arterial PO2 drops below 60 mmHg, glomus cells (Type I cells) in the carotid body depolarize, sending signals via the **glossopharyngeal nerve (CN IX)** to the respiratory centers in the medulla to increase ventilation. While they also respond to increases in PCO2 and decreases in pH, their most critical and unique role is sensing **hypoxia**. ### **Why Other Options are Incorrect** * **Options B & D:** Chemoreceptors do not monitor venous blood. Venous blood parameters reflect tissue metabolism rather than the efficiency of gas exchange. Arterial blood provides the necessary information regarding how well the lungs are oxygenating the blood. * **Option C:** While carotid bodies are sensitive to arterial CO2, this is not their *primary* function. The **central chemoreceptors** in the medulla are the main sensors for arterial PCO2 (via changes in CSF pH) and are responsible for approximately 70-80% of the ventilatory response to CO2. ### **High-Yield Facts for NEET-PG** * **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 dissolved PO2 rather than oxygen content bound to hemoglobin. * **Anemia/CO Poisoning:** In these conditions, arterial PO2 is normal but oxygen content is low. Therefore, the **carotid bodies are NOT stimulated**, and ventilation does not increase. * **Innervation:** Carotid body → Hering’s nerve (branch of CN IX); Aortic body → Vagus nerve (CN X).

Question 125: A man connected to a body plethysmograph for estimation of FRC inspires against a closed glottis. Which of the following statements is true?

A. The pressure in both the lungs and the box increases
B. The pressure in both the lungs and the box decreases
C. The pressure in the lungs decreases, but that in the box increases (Correct Answer)
D. The pressure in the lungs increases, but that in the box decreases

Explanation: ### Explanation **1. Why the correct answer is right:** Body plethysmography is based on **Boyle’s Law** ($P \times V = \text{constant}$), which states that at a constant temperature, pressure and volume are inversely proportional. When the subject attempts to **inspire against a closed glottis** (Müller’s maneuver): * **In the Lungs:** The chest wall expands, increasing the thoracic volume. According to Boyle’s Law, as volume increases, the **intrapulmonary pressure decreases** (becomes sub-atmospheric). * **In the Box:** The expansion of the subject's chest compresses the air remaining in the airtight plethysmograph. As the available volume in the box decreases, the **box pressure increases**. By measuring these reciprocal changes in pressure, the Functional Residual Capacity (FRC) can be calculated. **2. Why the incorrect options are wrong:** * **Option A & B:** These are incorrect because the lungs and the box are two separate compartments. An increase in volume in one (lungs) necessitates a decrease in volume in the other (box), leading to opposite pressure changes. * **Option D:** This describes the opposite maneuver (**expiration** against a closed glottis or Valsalva maneuver). During expiration, thoracic volume decreases (increasing lung pressure) and box volume increases (decreasing box pressure). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Gold Standard:** Body plethysmography is the most accurate method for measuring FRC because it measures **total gas volume**, including "trapped air" (e.g., in emphysema or asthma). * **Comparison:** Helium Dilution and Nitrogen Washout methods only measure **communicating gas volume** and will underestimate FRC in patients with obstructive lung diseases. * **Formula:** $FRC = \text{Thoracic Gas Volume (TGV)}$ at the end of a normal tidal expiration. * **Boyle's Law Application:** Remember: **V**olume up = **P**ressure down. This is the fundamental principle behind the mechanics of breathing.

Question 126: Respiratory burst in neutrophils occurs due to which enzyme system?

A. NADPH oxidase (Correct Answer)
B. NADP oxidase
C. FADH2 oxidase
D. FAD

Explanation: **Explanation:** **1. Why NADPH Oxidase is Correct:** Respiratory burst (or oxidative burst) is the rapid release of reactive oxygen species (ROS) from neutrophils and macrophages to destroy phagocytosed microbes. The key enzyme responsible for this process is **NADPH oxidase** (Nicotinamide adenine dinucleotide phosphate oxidase). * **Mechanism:** It transfers electrons from NADPH to molecular oxygen ($O_2$), converting it into the **Superoxide anion** ($O_2^-$). * **Reaction:** $NADPH + 2O_2 \xrightarrow{\text{NADPH Oxidase}} NADP^+ + H^+ + 2O_2^-$ * The superoxide is subsequently converted to hydrogen peroxide ($H_2O_2$) by superoxide dismutase, and finally into highly bactericidal hypochlorite ($HOCl$) by myeloperoxidase (MPO). **2. Why Other Options are Incorrect:** * **NADP oxidase:** This is a misnomer. The enzyme requires the reduced form (NADPH) as an electron donor, not the oxidized form (NADP). * **FADH2 oxidase / FAD:** While FAD is a cofactor in many redox reactions (like those in the Krebs cycle or Electron Transport Chain), it is not the primary enzyme system responsible for the phagocytic respiratory burst. **3. Clinical Pearls for NEET-PG:** * **Chronic Granulomatous Disease (CGD):** A high-yield clinical correlation. CGD is caused by a genetic **deficiency of NADPH oxidase**. Patients suffer from recurrent infections with catalase-positive organisms (e.g., *S. aureus*, *Aspergillus*) because they cannot produce their own $H_2O_2$. * **Nitroblue Tetrazolium (NBT) Test:** Used to diagnose CGD. In healthy neutrophils, NADPH oxidase reduces the yellow NBT dye to blue formazan. In CGD, the test remains negative (yellow). * **HMP Shunt:** This metabolic pathway is crucial for respiratory burst as it provides the necessary **NADPH** substrate.

Question 127: If the alveolar oxygenation is halved, what happens to the PACO2?

A. Doubled instantly
B. Doubled in a few minutes
C. Halved instantly
D. Halved in a few minutes (Correct Answer)

Explanation: ### Explanation The relationship between alveolar ventilation and the partial pressure of arterial/alveolar carbon dioxide ($P_aCO_2$) is governed by the **Alveolar Ventilation Equation**: $$V_A \propto \frac{VCO_2}{P_A CO_2}$$ *(Where $V_A$ = Alveolar Ventilation and $VCO_2$ = Rate of $CO_2$ production)* **1. Why the Correct Answer is Right:** If alveolar ventilation is **doubled**, the removal of $CO_2$ from the lungs increases proportionally. Since $CO_2$ production ($VCO_2$) remains constant, the concentration of $CO_2$ in the alveoli (and subsequently the arterial blood) will be **halved**. However, this change is not instantaneous. It takes **a few minutes** for the body’s $CO_2$ stores (found in blood and tissues) to reach a new steady-state equilibrium after a change in ventilation. **2. Why the Incorrect Options are Wrong:** * **Options A & B (Doubled):** Doubling ventilation *decreases* $P_aCO_2$. $P_aCO_2$ would only double if ventilation were halved (hypoventilation). * **Option C (Halved instantly):** While the mathematical relationship is inverse, the physiological response is delayed. $CO_2$ must diffuse from the tissues and blood into the alveoli to be exhaled; therefore, the "washout" period prevents an instantaneous drop. **3. Clinical Pearls & High-Yield Facts:** * **Hyperventilation:** Defined as ventilation in excess of metabolic needs, leading to **hypocapnia** ($P_aCO_2 < 35$ mmHg) and respiratory alkalosis. * **Hypoventilation:** Defined as insufficient ventilation, leading to **hypercapnia** ($P_aCO_2 > 45$ mmHg) and respiratory acidosis. * **Dead Space:** Remember that $V_A = (\text{Tidal Volume} - \text{Dead Space}) \times \text{Respiratory Rate}$. Increasing tidal volume is more effective at lowering $P_aCO_2$ than increasing respiratory rate due to the constant nature of anatomical dead space.

Question 128: Adequate oxygen is delivered to the tissues in which type of hypoxia?

A. Hypoxic hypoxia
B. Stagnant hypoxia
C. Anemic hypoxia
D. Histotoxic hypoxia (Correct Answer)

Explanation: **Explanation:** The core concept in this question is the distinction between **oxygen delivery** ($DO_2$) and **oxygen utilization**. **Why Histotoxic Hypoxia is correct:** In histotoxic hypoxia (most commonly caused by **cyanide poisoning**), the partial pressure of oxygen in the blood ($PaO_2$), the hemoglobin concentration, and the cardiac output are all normal. Therefore, the **delivery of oxygen to the tissues is adequate**. The pathology lies at the cellular level: cyanide inhibits the **cytochrome oxidase enzyme** in the electron transport chain, preventing the mitochondria from utilizing the oxygen provided. Since oxygen is delivered but not consumed, the venous blood remains highly oxygenated, leading to a characteristic narrowing of the arterial-venous oxygen difference. **Why the other options are incorrect:** * **Hypoxic Hypoxia:** Characterized by low $PaO_2$ (e.g., high altitude, hypoventilation). Delivery is inadequate because the blood is not sufficiently loaded with oxygen in the lungs. * **Anemic Hypoxia:** Delivery is inadequate because the oxygen-carrying capacity of the blood is reduced due to low hemoglobin or carbon monoxide poisoning, despite normal $PaO_2$. * **Stagnant Hypoxia:** Delivery is inadequate because of reduced blood flow (low cardiac output or localized ischemia), even though the blood itself is well-oxygenated. **High-Yield NEET-PG Pearls:** * **Cyanosis** is absent in histotoxic hypoxia; the skin often appears **"cherry-red"** because venous blood remains saturated with oxyhemoglobin. * In histotoxic hypoxia, the **Arterial-Venous (A-V) oxygen difference** is significantly decreased (approaching zero). * **Specific Antidote for Cyanide:** Amyl nitrite/Sodium nitrite (creates methemoglobin to sequester cyanide) and Sodium thiosulfate. Hydroxocobalamin is the modern preferred agent.

Question 129: A foreign body completely obstructing the right main bronchus causes what?

A. Decreased ventilation-perfusion ratio (Correct Answer)
B. Increased ventilation in the left lung
C. Perfusion to the right lung to double
D. Increased ventilation-perfusion ratio in the right lung

Explanation: **Explanation:** The correct answer is **A. Decreased ventilation-perfusion ratio.** **1. Why Option A is Correct:** The ventilation-perfusion ratio ($V/Q$) is the ratio of the amount of air reaching the alveoli to the amount of blood reaching the alveoli. When a foreign body completely obstructs the right main bronchus, **ventilation ($V$) to the right lung drops to zero**. However, blood flow (perfusion, $Q$) continues to the lung (though it may decrease due to hypoxic pulmonary vasoconstriction, it does not reach zero). Mathematically, when $V$ decreases while $Q$ persists, the $V/Q$ ratio decreases toward zero. This state is known as a **physiological shunt**, where deoxygenated blood bypasses ventilated alveoli and enters the systemic circulation. **2. Why the Other Options are Incorrect:** * **Option B:** While the left lung may increase its respiratory rate or tidal volume to compensate, "increased ventilation" is a compensatory mechanism, not a direct physiological consequence of the obstruction itself. * **Option C:** Perfusion to the right lung actually **decreases**, not doubles. This occurs due to **hypoxic pulmonary vasoconstriction**, a protective mechanism where pulmonary vessels constrict in poorly ventilated areas to divert blood to better-oxygenated parts of the lung. * **Option D:** An increased $V/Q$ ratio occurs when ventilation is maintained but perfusion is decreased (e.g., Pulmonary Embolism). In this case, ventilation is obstructed, leading to a decrease in the ratio. **Clinical Pearls for NEET-PG:** * **$V/Q = 0$:** Represents a **Shunt** (e.g., Foreign body, airway obstruction, atelectasis). * **$V/Q = \infty$ (Infinity):** Represents **Dead Space** (e.g., Pulmonary embolism). * **Hypoxic Pulmonary Vasoconstriction:** This is unique to the pulmonary circulation; in systemic circulation, hypoxia causes vasodilation. * **Foreign Body Location:** In a standing individual, foreign bodies most commonly lodge in the **Right Principal Bronchus** because it is wider, shorter, and more vertical than the left.

Question 130: Pulmonary micro-circulation differs from systemic circulation in having what characteristics?

A. Low resistance and high pulsatile flow
B. Low resistance and low capillary pressure (Correct Answer)
C. High resistance and low pulsatile flow
D. High resistance and high capillary pressure

Explanation: The pulmonary circulation is a unique, low-pressure system designed to facilitate gas exchange while protecting the delicate alveolar-capillary membrane. ### **Why Option B is Correct** The pulmonary circulation is characterized by **low resistance** and **low capillary pressure**. 1. **Low Resistance:** The pulmonary vessels are thin-walled, highly distensible, and contain less smooth muscle than systemic vessels. This allows the entire cardiac output to pass through the lungs with minimal effort. 2. **Low Capillary Pressure:** The mean pulmonary capillary pressure is approximately **7–10 mmHg** (compared to ~25–30 mmHg in systemic capillaries). This low pressure is vital to prevent fluid from being forced out of the capillaries into the alveoli, thereby preventing pulmonary edema. ### **Analysis of Incorrect Options** * **Options C & D (High Resistance):** These are incorrect because high resistance is a feature of the systemic circulation (needed to regulate blood flow to various organs). High resistance in the lungs would lead to Right Ventricular hypertrophy and failure (Cor Pulmonale). * **Option A (High Pulsatile Flow):** While pulmonary flow is pulsatile, the defining physiological hallmark that differentiates it from systemic circulation in a clinical/exam context is the **pressure-resistance relationship**. ### **NEET-PG High-Yield Pearls** * **Recruitment and Distension:** When cardiac output increases (e.g., during exercise), pulmonary resistance drops even further through "recruitment" (opening closed capillaries) and "distension" (widening open ones). * **Hypoxic Pulmonary Vasoconstriction (HPV):** Unlike systemic vessels which dilate in response to hypoxia, pulmonary vessels **constrict**. This shunts blood away from poorly ventilated areas to well-ventilated ones (V/Q matching). * **Starling Forces:** The low capillary hydrostatic pressure (7 mmHg) is significantly lower than the plasma colloid osmotic pressure (28 mmHg), ensuring the lungs remain "dry."

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Control of Breathing

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