Respiratory System — MCQs

Respiratory System Indian Medical PG Practice Questions and MCQs

Question 341: Which of the following types of hypoxia presents with normal arterial oxygen concentration?

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

Explanation: ### Explanation The correct answer is **Anemic Hypoxia**. **1. Why Anemic Hypoxia is Correct:** In anemic hypoxia, the **partial pressure of oxygen ($PaO_2$) is normal**, but the total oxygen-carrying capacity of the blood is reduced due to a decrease in functional hemoglobin (Hb) levels or altered Hb binding (e.g., Carbon Monoxide poisoning). Since $PaO_2$ (dissolved oxygen) remains normal, the **arterial oxygen concentration ($CaO_2$)**—which is primarily determined by Hb concentration and saturation—is reduced. However, in the context of NEET-PG questions, "normal arterial oxygen" often refers specifically to the **Partial Pressure ($PaO_2$)**, which remains unaffected because the lungs and gas exchange mechanisms are intact. **2. Analysis of Incorrect Options:** * **Stagnant Hypoxia:** Occurs due to reduced blood flow (e.g., heart failure or shock). $PaO_2$ and $CaO_2$ are initially normal, but the tissue oxygen delivery is compromised. * **Histotoxic Hypoxia:** Occurs when tissues cannot utilize oxygen (e.g., Cyanide poisoning). Arterial $PaO_2$ and $CaO_2$ are normal, but the **venous oxygen concentration is elevated** because oxygen is not unloaded. * **Hypoxic Hypoxia:** Characterized by **low arterial $PaO_2$**. This is caused by low environmental $O_2$ (high altitude), hypoventilation, or V/Q mismatch. **3. High-Yield Clinical Pearls for NEET-PG:** * **CO Poisoning:** A classic cause of anemic hypoxia where $PaO_2$ is normal, but $SaO_2$ is falsely normal on pulse oximetry (as it can't distinguish carboxyhemoglobin). * **Cyanosis:** Is typically **absent** in anemic hypoxia because there isn't enough total hemoglobin to produce the required 5g/dL of deoxygenated Hb. * **Arterio-venous (A-V) $O_2$ difference:** Is increased in stagnant hypoxia but decreased in histotoxic hypoxia.

Question 342: Which of the following is NOT a metabolic function of the lung?

A. Synthesis of surfactant
B. Inactivation of bradykinin
C. Formation of angiotensin II
D. Inactivation of vasopressin (Correct Answer)

Explanation: **Explanation:** The lungs are not merely organs of gas exchange; they possess significant metabolic and endocrine functions, primarily involving the processing of substances circulating in the blood. **Why Option D is Correct:** **Vasopressin (Antidiuretic Hormone/ADH)** is synthesized in the hypothalamus and released by the posterior pituitary. Its primary metabolism and inactivation occur in the **liver and kidneys**, not the lungs. Therefore, the lung does not play a role in the clearance of vasopressin. **Why the other options are incorrect:** * **A. Synthesis of Surfactant:** Type II pneumocytes in the lung alveoli synthesize and secrete surfactant (mainly dipalmitoylphosphatidylcholine), which reduces surface tension and prevents alveolar collapse. * **B. Inactivation of Bradykinin:** The pulmonary endothelium contains **Angiotensin-Converting Enzyme (ACE)**, which is responsible for the hydrolysis and inactivation of up to 80% of circulating bradykinin. * **C. Formation of Angiotensin II:** The lungs are the primary site for the conversion of Angiotensin I to the potent vasoconstrictor **Angiotensin II**, catalyzed by ACE located on the luminal surface of pulmonary capillary endothelial cells. **High-Yield Clinical Pearls for NEET-PG:** * **Substances inactivated by the lungs:** Bradykinin, Serotonin (5-HT), Prostaglandins (E and F series), and Noradrenaline (partial). * **Substances NOT affected by the lungs:** Epinephrine, Dopamine, Oxytocin, and Vasopressin. * **ACE Inhibitors:** Drugs like Enalapril work in the pulmonary vasculature, leading to an accumulation of bradykinin, which is the physiological basis for the common side effect of a **dry cough**.

Question 343: The oxyhaemoglobin dissociation curve is shifted to the right in which of the following conditions?

A. Acidosis
B. Increased 2,3-DPG level
C. Hyperthermia
D. All the above (Correct Answer)

Explanation: The **Oxyhaemoglobin Dissociation Curve (ODC)** represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of haemoglobin. A **"Shift to the Right"** indicates a decreased affinity of haemoglobin for oxygen, meaning oxygen is released more easily to the tissues. ### **Explanation of the Correct Answer** The correct answer is **D (All the above)** because all three conditions decrease haemoglobin's affinity for oxygen: 1. **Acidosis (Option A):** An increase in $H^+$ concentration (decreased pH) stabilizes the "Tense" (T) state of haemoglobin, promoting oxygen unloading. This is known as the **Bohr Effect**. 2. **Increased 2,3-DPG (Option B):** 2,3-Diphosphoglycerate is a byproduct of glycolysis in RBCs. It binds to the beta chains of deoxyhaemoglobin, stabilizing the T-state and pushing the curve to the right. 3. **Hyperthermia (Option C):** Increased temperature weakens the bond between oxygen and haemoglobin, facilitating the release of oxygen to metabolically active (warm) tissues. ### **Clinical Pearls & High-Yield Facts** To remember the factors shifting the curve to the **Right**, use the mnemonic **"CADET, face Right!"**: * **C** – $CO_2$ (Hypercapnia) * **A** – Acidosis ($H^+$) * **D** – 2,3-DPG increase * **E** – Exercise * **T** – Temperature (Fever) **Key NEET-PG Points:** * **$P_{50}$ Value:** This is the $PO_2$ at which haemoglobin is 50% saturated (Normal $\approx$ 26-27 mmHg). A **Right shift increases the $P_{50}$**, while a Left shift decreases it. * **Left Shift Factors:** Hypothermia, Alkalosis, decreased 2,3-DPG, and **Fetal Haemoglobin (HbF)**. HbF has a higher affinity for $O_2$ to facilitate oxygen transfer from mother to fetus. * **Carbon Monoxide (CO):** Causes a **Left shift** and changes the curve to a **hyperbolic** shape, preventing oxygen release to tissues.

Question 344: Which of the following is used to assess breathing pattern?

A. Spirometry (Correct Answer)
B. Barometer
C. All
D. None

Explanation: **Explanation:** **Spirometry** is the gold standard diagnostic tool used to assess breathing patterns and lung function. It measures the volume of air an individual can inhale or exhale as a function of time. By analyzing the **Flow-Volume loops** and specific parameters like **FEV1** (Forced Expiratory Volume in 1 second) and **FVC** (Forced Vital Capacity), clinicians can differentiate between obstructive patterns (e.g., Asthma, COPD) and restrictive patterns (e.g., Pulmonary Fibrosis). **Analysis of Options:** * **Spirometry (Correct):** It directly records the rate and depth of breathing, allowing for the assessment of tidal volume, vital capacity, and expiratory flow rates. * **Barometer (Incorrect):** This instrument is used to measure **atmospheric pressure**, not physiological lung parameters. While atmospheric pressure influences gas exchange, it does not assess an individual's breathing pattern. * **Options C & D:** Since Spirometry is a definitive tool for this purpose, these options are incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Static vs. Dynamic:** Spirometry can measure most lung volumes and capacities, but it **cannot** measure **Residual Volume (RV)**, **Functional Residual Capacity (FRC)**, or **Total Lung Capacity (TLC)**. These require helium dilution or body plethysmography. * **Obstructive Pattern:** Characterized by a decreased FEV1/FVC ratio (<0.7). * **Restrictive Pattern:** Characterized by a normal or increased FEV1/FVC ratio, but a decrease in both FEV1 and FVC. * **Pneumotachograph:** A specific type of spirometer that measures airflow by detecting pressure differences across a fine mesh.

Question 345: What is the shape of the myoglobin dissociation curve?

A. Almost linear
B. Parabola
C. Rectangular hyperbola (Correct Answer)
D. Sigmoid

Explanation: **Explanation:** The shape of the dissociation curve is determined by the oxygen-binding affinity and the number of binding sites on the protein. **Why Rectangular Hyperbola is Correct:** Myoglobin is a monomeric protein containing only **one heme group** (one iron atom). Because it has only one binding site, it does not exhibit "cooperative binding." It follows simple Michaelis-Menten kinetics, where oxygen binding occurs independently. This results in a **rectangular hyperbola** shape. Myoglobin has a very high affinity for oxygen (P50 ≈ 2–3 mmHg), allowing it to remain saturated at low partial pressures and only release oxygen when levels in the muscle drop significantly during intense exercise. **Why Other Options are Incorrect:** * **Sigmoid:** This is the shape of the **Hemoglobin** dissociation curve. Hemoglobin is a tetramer (four subunits). The binding of the first oxygen molecule increases the affinity for subsequent ones (positive cooperativity), creating the S-shaped curve. * **Almost Linear:** No physiological respiratory pigment follows a linear curve, as binding sites are finite and must reach saturation. * **Parabola:** While a hyperbola and parabola are both conic sections, the mathematical relationship of gas-binding kinetics specifically describes a hyperbolic function. **High-Yield Facts for NEET-PG:** 1. **P50 Values:** Hemoglobin P50 is ~26.7 mmHg; Myoglobin P50 is ~2–3 mmHg. A lower P50 signifies a higher affinity. 2. **Function:** Myoglobin acts as an **oxygen storage** unit in skeletal and cardiac muscle, whereas Hemoglobin acts as an **oxygen transporter**. 3. **Left Shift:** The myoglobin curve is located far to the left of the hemoglobin curve, meaning myoglobin will take up oxygen from hemoglobin at the tissue level. 4. **Bohr Effect:** Unlike hemoglobin, myoglobin is **not** affected by pH, $CO_2$, or 2,3-BPG.

Question 346: Which reflex is considered the first respiratory reflex that aids in the initial breath of a newborn?

A. Hering-Breuer's reflex
B. J-reflex
C. Head's Paradox reflex (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Head’s Paradoxical Reflex** is the correct answer because it describes a phenomenon where lung inflation triggers a further *increase* in inspiratory effort, rather than inhibiting it. While most stretch receptors in the lungs trigger a negative feedback loop to stop inspiration, the receptors involved in Head’s reflex provide **positive feedback**. This is physiologically crucial during the **first breath of a newborn**, as it helps in the initial expansion of the collapsed alveoli (atelectatic lungs) and promotes the distribution of surfactant. **Analysis of Incorrect Options:** * **Hering-Breuer Inflation Reflex:** This is a protective mechanism where lung inflation triggers the termination of inspiration to prevent over-distension. It is a negative feedback mechanism and is generally inactive during quiet breathing in adults, becoming active only at high tidal volumes. * **J-reflex (Juxtacapillary reflex):** Triggered by the stimulation of J-receptors in the alveolar wall (usually due to pulmonary edema or congestion), this reflex leads to rapid shallow breathing (tachypnea), apnea, bradycardia, and hypotension. It is not involved in the initiation of the first breath. **Clinical Pearls for NEET-PG:** * **Receptors:** Head’s reflex is mediated by **rapidly adapting stretch receptors** (RARs) in the airways. * **Afferent Pathway:** The sensory signals for these respiratory reflexes travel via the **Vagus nerve (CN X)**. * **High-Yield Fact:** Head’s reflex is also responsible for the **"gasp"** or deep inspiration seen during periodic sighing in adults, which helps reopen collapsed small airways.

Question 347: What is the partial pressure of oxygen at 760 mmHg atmospheric pressure?

A. 76
B. 160 (Correct Answer)
C. 120
D. 130

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The partial pressure of oxygen ($PO_2$) in dry atmospheric air is determined by **Dalton’s Law**, which states that the total pressure of a gas mixture is the sum of the partial pressures of its individual components. * **Atmospheric Pressure ($P_{atm}$):** 760 mmHg at sea level. * **Fraction of Oxygen ($FiO_2$):** Oxygen constitutes approximately 21% of the atmosphere. * **Calculation:** $PO_2 = P_{atm} \times FiO_2$ * $760 \text{ mmHg} \times 0.21 = \mathbf{159.6 \text{ mmHg}}$ (rounded to **160 mmHg**). **2. Why the Incorrect Options are Wrong:** * **Option A (76):** This represents 10% of the atmospheric pressure, which does not correspond to any physiological gas concentration at sea level. * **Option C (120):** This is closer to the $PO_2$ of **humidified tracheal air** (approx. 149 mmHg) or the mean arterial pressure, but it is too low for dry atmospheric air. * **Option D (130):** This value does not correlate with standard respiratory physiological measurements at sea level. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Humidified Air:** As air enters the respiratory tract, it is saturated with water vapor ($PH_2O = 47 \text{ mmHg}$ at 37°C). The $PO_2$ in the trachea drops: $(760 - 47) \times 0.21 \approx \mathbf{150 \text{ mmHg}}$. * **Alveolar Air ($PAO_2$):** Due to the continuous uptake of $O_2$ and the addition of $CO_2$, the $PO_2$ in the alveoli is approximately **100–104 mmHg**. * **Fractional Concentration:** Note that while $PO_2$ decreases at high altitudes (due to lower $P_{atm}$), the **percentage** of oxygen (21%) remains constant. * **Alveolar Gas Equation:** $PAO_2 = FiO_2(P_{atm} - PH_2O) - (PaCO_2 / R)$. This is a frequent source of numerical questions in NEET-PG.

Question 348: What is vital capacity?

A. Tidal volume + Expiratory Reserve Volume
B. Tidal volume + Inspiratory Reserve Volume
C. Inspiratory Reserve Volume + Expiratory Reserve Volume
D. Tidal volume + Inspiratory Reserve Volume + Expiratory Reserve Volume (Correct Answer)

Explanation: ### Explanation **Vital Capacity (VC)** is the maximum volume of air a person can expel from the lungs after a maximum inhalation. It represents the total "usable" gas exchange capacity of the lungs, excluding the air that always remains in the passages (Residual Volume). **1. Why Option D is Correct:** The formula for Vital Capacity is **VC = TV + IRV + ERV**. * **Tidal Volume (TV):** Normal quiet breathing (~500 mL). * **Inspiratory Reserve Volume (IRV):** Extra air inhaled deeply (~3000 mL). * **Expiratory Reserve Volume (ERV):** Extra air exhaled forcefully (~1100 mL). Summing these three gives the total volume of air that can be voluntarily moved into or out of the lungs. **2. Why Other Options are Incorrect:** * **Option A (TV + ERV):** This represents the **Expiratory Capacity (EC)**, which is the total air one can exhale starting from a normal inspiratory level. * **Option B (TV + IRV):** This is the **Inspiratory Capacity (IC)**, the maximum air one can breathe in starting from a normal expiratory level. * **Option C (IRV + ERV):** This combination does not represent a standard clinical lung capacity as it omits the Tidal Volume, which is essential for any full breath cycle. **3. NEET-PG High-Yield Pearls:** * **VC vs. TLC:** Total Lung Capacity (TLC) = VC + Residual Volume (RV). Remember: **RV cannot be measured by simple spirometry.** * **Clinical Significance:** VC is decreased in **Restrictive Lung Diseases** (e.g., Pulmonary Fibrosis) due to reduced lung compliance, but remains relatively normal in early obstructive diseases. * **Timed Vital Capacity:** The most important clinical derivative is **FEV1** (Forced Expiratory Volume in 1 second), used to differentiate between obstructive and restrictive patterns. * **Average Value:** In a healthy adult male, VC is approximately **4.6 to 4.8 Liters**.

Question 349: What is the earliest symptom of acute mountain sickness?

A. Blurring of vision
B. Fever
C. Nausea and vomiting
D. Headache (Correct Answer)

Explanation: **Explanation:** **Acute Mountain Sickness (AMS)** is a syndrome caused by the body's failure to acclimatize to high altitudes (typically above 2,500m). **Why Headache is the Correct Answer:** Headache is the **earliest, most common, and hallmark symptom** of AMS. It is typically described as bifrontal or occipital and is throbbing in nature. The underlying pathophysiology involves **hypoxia-induced cerebral vasodilation** and a mild increase in intracranial pressure. According to the Lake Louise Scoring System (the clinical standard for diagnosis), a headache is mandatory for the diagnosis of AMS. **Analysis of Incorrect Options:** * **A. Blurring of vision:** This is not a primary symptom of AMS. Visual disturbances may occur in severe cases of High-Altitude Cerebral Edema (HACE) due to papilledema, but it is not the earliest sign. * **B. Fever:** Fever is not a feature of AMS. If present at high altitude, it usually indicates an underlying infection or High-Altitude Pulmonary Edema (HAPE). * **C. Nausea and vomiting:** These are common symptoms of AMS, but they typically develop **after** the onset of the headache. **High-Yield NEET-PG Pearls:** * **Lake Louise Criteria:** Diagnosis of AMS = Recent ascent + Headache + at least one other symptom (Nausea/Vomiting, Fatigue, Dizziness, or Insomnia). * **Drug of Choice (Prophylaxis):** Acetazolamide (Carbonic anhydrase inhibitor). It works by inducing metabolic acidosis, which stimulates ventilation. * **Definitive Treatment:** Immediate descent and supplemental oxygen. * **Cheyne-Stokes Respiration:** This is the most common breathing pattern seen during sleep at high altitudes.

Question 350: A fetus born during the 6th month of intrauterine life will NOT be able to survive due to which of the following reasons?

A. Lack of subcutaneous connective tissue
B. Lack of co-ordination between the respiratory and nervous system
C. Absence or insufficient amount of surfactant (Correct Answer)
D. Lack of sufficient capillaries

Explanation: **Explanation:** The primary reason for the poor survival of a fetus born during the 6th month (approx. 24–26 weeks) is the **absence or insufficient production of surfactant**. **1. Why the correct answer is right:** Surfactant is a surface-active lipoprotein complex produced by **Type II Pneumocytes**. Its primary role is to reduce surface tension at the air-liquid interface of the alveoli, preventing them from collapsing during expiration (atelectasis). While surfactant production begins around the 20th week, it does not reach functional levels until the **28th to 32nd week**. A fetus born in the 6th month lacks enough surfactant to maintain lung expansion, leading to **Infant Respiratory Distress Syndrome (IRDS)** or Hyaline Membrane Disease. **2. Why the other options are incorrect:** * **Option A:** While a 6th-month fetus lacks subcutaneous fat (leading to poor thermoregulation), this is not the immediate life-limiting factor compared to respiratory failure. * **Option B:** The basic respiratory centers in the medulla are developed; the failure is mechanical (lung compliance) rather than neurological coordination. * **Option D:** By the **Canalicular stage** (16–26 weeks), the vascular supply increases significantly and the blood-air barrier begins to form. Capillaries are present, but they cannot exchange gas if the alveoli are collapsed. **High-Yield Clinical Pearls for NEET-PG:** * **Lecithin/Sphingomyelin (L/S) Ratio:** A ratio **>2:1** in amniotic fluid indicates mature lungs. * **Glucocorticoids (e.g., Betamethasone):** Administered to the mother in preterm labor to accelerate fetal surfactant production. * **Composition:** Surfactant is 90% lipids and 10% proteins. The most important component is **Dipalmitoylphosphatidylcholine (DPPC)**.

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