Acid-Base Balance — MCQs

Acid-Base Balance Indian Medical PG Practice Questions and MCQs

Question 41: Respiratory acidosis is caused by all except?

A. Chronic bronchitis
B. COPD
C. Pulmonary hypertension (Correct Answer)
D. Interstitial lung disease

Explanation: ### Explanation **Core Concept:** Respiratory acidosis is characterized by an increase in arterial $PCO_2$ ($>45$ mmHg) due to **alveolar hypoventilation**. For respiratory acidosis to occur, there must be a failure in the exchange of gases (specifically $CO_2$ elimination) or a failure in the respiratory pump. **Why Pulmonary Hypertension is the Correct Answer:** In **Pulmonary Hypertension (Option C)**, the primary pathology is an increase in pulmonary arterial pressure. While it affects gas exchange in advanced stages, the initial and most common compensatory response to the resulting hypoxia is **hyperventilation**. This leads to increased $CO_2$ washout, typically causing **respiratory alkalosis**, not acidosis. **Analysis of Incorrect Options:** * **Chronic Bronchitis (Option A) & COPD (Option B):** These are classic obstructive airway diseases. They cause air trapping, increased physiological dead space, and ventilation-perfusion ($V/Q$) mismatch. This leads to inadequate $CO_2$ clearance, resulting in chronic respiratory acidosis. * **Interstitial Lung Disease (Option D):** While early ILD may present with respiratory alkalosis due to tachypnea, **end-stage** restrictive lung diseases lead to a significant decrease in lung compliance and a severe reduction in diffusion capacity. This eventually results in respiratory pump failure and $CO_2$ retention (respiratory acidosis). **NEET-PG High-Yield Pearls:** * **The "Blue Bloater":** Chronic Bronchitis patients are classic examples of chronic respiratory acidosis with metabolic compensation (elevated $HCO_3^-$). * **Acute vs. Chronic:** In acute respiratory acidosis (e.g., opioid overdose), $HCO_3^-$ rises by **1 mEq/L** for every 10 mmHg rise in $PCO_2$. In chronic cases (e.g., COPD), it rises by **3.5–4 mEq/L** for every 10 mmHg rise. * **Common Causes:** Always look for CNS depression (opioids), neuromuscular disorders (Guillain-Barré), or severe chest wall deformities (Kyphoscoliosis) as triggers for respiratory acidosis.

Question 42: In metabolic acidosis, what happens to the partial pressure of carbon dioxide (PCO2)?

A. Increase
B. Decrease (Correct Answer)
C. Remain constant
D. None of the above

Explanation: **Explanation:** In **metabolic acidosis**, the primary pathology is a decrease in plasma bicarbonate ($HCO_3^-$) or an increase in non-volatile acids, leading to a drop in arterial pH. To maintain homeostasis, the body initiates **respiratory compensation**. 1. **Mechanism (Why B is correct):** The decrease in pH stimulates **peripheral chemoreceptors** (located in the carotid and aortic bodies). These receptors signal the medullary respiratory centers to increase the rate and depth of ventilation (classically known as **Kussmaul breathing**). This hyperventilation "washes out" carbon dioxide ($CO_2$), thereby decreasing the arterial $PCO_2$. By lowering the $PCO_2$, the body attempts to restore the $HCO_3^-/PCO_2$ ratio toward normal, bringing the pH back toward 7.4. 2. **Why other options are wrong:** * **Option A (Increase):** An increase in $PCO_2$ would further lower the pH, exacerbating the acidosis. This occurs in respiratory acidosis, not as a compensatory mechanism for metabolic acidosis. * **Option C (Remain constant):** If $PCO_2$ remained constant, the pH would remain dangerously low. The respiratory system is a rapid-acting buffer that responds within minutes to pH changes. **High-Yield Clinical Pearls for NEET-PG:** * **Winters’ Formula:** To calculate the "expected" $PCO_2$ in metabolic acidosis: $Expected\ PCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2$. If the measured $PCO_2$ is higher than expected, a concomitant respiratory acidosis is present. * **Kussmaul Breathing:** Deep, labored breathing characteristic of severe metabolic acidosis (e.g., Diabetic Ketoacidosis). * **Limit of Compensation:** Respiratory compensation can never fully return the pH to 7.4; it only moves it toward normal.

Question 43: A 70-year-old man with a history of CHF presents with increased shortness of breath and leg swelling. ABG shows: pH 7.24, pCO2 = 60 mmHg, pO2 = 52 mmHg, HCO3 = 27 mEq/L. Interpret the acid-base status.

A. Metabolic acidosis
B. Metabolic alkalosis
C. Respiratory acidosis (Correct Answer)
D. Respiratory alkalosis

Explanation: ### **Explanation** **1. Why Respiratory Acidosis is Correct:** The interpretation of any acid-base disorder follows a systematic three-step approach: * **Step 1 (pH):** The pH is **7.24** (Normal: 7.35–7.45), indicating **acidemia**. * **Step 2 (Primary Cause):** Look at the $pCO_2$ and $HCO_3^-$. The $pCO_2$ is **60 mmHg** (Normal: 40 mmHg). An elevated $pCO_2$ (hypercapnia) causes a drop in pH, identifying the primary disturbance as **Respiratory Acidosis**. * **Step 3 (Compensation):** The $HCO_3^-$ is **27 mEq/L** (Normal: 24 mEq/L). This slight elevation suggests the kidneys have begun to compensate by retaining bicarbonate, but the pH remains low, indicating an uncompensated or partially compensated state. In this clinical context (CHF/Pulmonary edema), impaired gas exchange leads to $CO_2$ retention. **2. Why Other Options are Wrong:** * **Metabolic Acidosis:** This would present with a low pH but a **low $HCO_3^-$** (primary deficit) and a compensatory decrease in $pCO_2$. * **Metabolic Alkalosis:** This would present with a **high pH** (>7.45) and an elevated $HCO_3^-$. * **Respiratory Alkalosis:** This would present with a **high pH** (>7.45) and a low $pCO_2$ (hypocapnia), typically seen in hyperventilation. **3. NEET-PG High-Yield Pearls:** * **The "Direction" Rule:** In respiratory disorders, pH and $pCO_2$ move in **opposite** directions. In metabolic disorders, pH and $HCO_3^-$ move in the **same** direction (ROME: Respiratory Opposite, Metabolic Equal). * **Acute vs. Chronic:** For every 10 mmHg rise in $pCO_2$: * **Acute:** $HCO_3^-$ rises by **1** mEq/L. * **Chronic:** $HCO_3^-$ rises by **3.5–4** mEq/L. * In this case, the $HCO_3^-$ rise (from 24 to 27) matches the acute formula ($20 \text{ mmHg rise} \times 1 = 2 \text{ to } 3 \text{ mEq/L}$), suggesting an **Acute Respiratory Acidosis**.

Question 44: A patient has the following arterial blood gas values: PaO2 is 85 mmHg, PaCO2 is 50 mmHg, pH is 7.2, and HCO3 is 32 mEq/L. What is the acid-base disorder?

A. Respiratory acidosis with compensatory metabolic alkalosis (Correct Answer)
B. Respiratory acidosis with compensatory metabolic acidosis
C. Metabolic acidosis
D. Metabolic alkalosis

Explanation: ### **Explanation** To solve any acid-base question, follow a systematic three-step approach: 1. **Check the pH:** The normal pH range is 7.35–7.45. A pH of **7.2** indicates **acidemia**. 2. **Identify the Primary Cause:** Look at the $PaCO_2$ and $HCO_3^-$. * The $PaCO_2$ is **50 mmHg** (Normal: 40 mmHg). High $CO_2$ (an acid) matches the acidic pH. This confirms **Respiratory Acidosis**. * The $HCO_3^-$ is **32 mEq/L** (Normal: 24 mEq/L). High bicarbonate (a base) does *not* match the acidic pH; therefore, it is the compensatory mechanism. 3. **Determine Compensation:** The kidneys retain $HCO_3^-$ to buffer the excess $H^+$ ions caused by $CO_2$ retention. Since the $HCO_3^-$ is elevated and the pH is moving toward normal (but not yet there), this is **Respiratory Acidosis with compensatory Metabolic Alkalosis**. --- ### **Why Incorrect Options are Wrong:** * **Option B:** Metabolic acidosis would involve a *low* $HCO_3^-$, not a high one. * **Option C:** In primary metabolic acidosis, the pH would be low, but the primary driver would be a low $HCO_3^-$, with a compensatory drop in $PaCO_2$. * **Option D:** Metabolic alkalosis would present with an alkaline pH (>7.45) and high $HCO_3^-$. --- ### **High-Yield NEET-PG Pearls:** * **Acute vs. Chronic:** In **Acute** Respiratory Acidosis, $HCO_3^-$ rises by **1 mEq/L** for every 10 mmHg rise in $PaCO_2$. In **Chronic** cases (like COPD), it rises by **3.5–4 mEq/L** per 10 mmHg rise. * **The "Rule of Match":** If the pH and $CO_2$ move in opposite directions, the primary problem is Respiratory. If pH and $HCO_3^-$ move in the same direction, it is Metabolic. * **Compensation** never over-corrects the pH; it only brings it closer to the normal range.

Question 45: A patient with a head injury has the following blood examination results: pH = 7.2, pCO2 = 65 mmHg, HCO3 = 30 mEq/L. What is the acid-base disturbance?

A. Respiratory acidosis (Correct Answer)
B. Respiratory alkalosis
C. Metabolic acidosis
D. Metabolic alkalosis

Explanation: ### Explanation To solve acid-base problems, follow a systematic three-step approach: **1. Analyze the pH:** The normal arterial pH is 7.35–7.45. A pH of **7.2** indicates **acidosis**. **2. Identify the Primary Cause:** * **Respiratory:** Look at $pCO_2$ (Normal: 35–45 mmHg). Here, $pCO_2$ is **65 mmHg** (elevated). High $CO_2$ acts as an acid, which correlates with the low pH. This confirms **Respiratory Acidosis**. * **Metabolic:** Look at $HCO_3^-$ (Normal: 22–26 mEq/L). Here, $HCO_3^-$ is **30 mEq/L**. Since high bicarbonate is alkaline, it does not explain the acidic pH; rather, it indicates a compensatory response. **3. Clinical Correlation:** In a head injury, depression of the medullary respiratory centers leads to hypoventilation, $CO_2$ retention, and subsequent respiratory acidosis. --- ### Why the other options are incorrect: * **B. Respiratory Alkalosis:** This would present with a high pH (>7.45) and a low $pCO_2$ (<35 mmHg), typically seen in hyperventilation. * **C. Metabolic Acidosis:** This would feature a low pH but a **low** $HCO_3^-$ (<22 mEq/L). The $pCO_2$ would usually be low as the lungs try to blow off acid (Kussmaul breathing). * **D. Metabolic Alkalosis:** This would present with a high pH (>7.45) and a high $HCO_3^-$ (>26 mEq/L). --- ### High-Yield NEET-PG Pearls: * **Compensation Rule:** The body never "over-compensates." If the pH is <7.4, the primary process is acidosis. * **Acute vs. Chronic:** In acute respiratory acidosis (like sudden trauma), the $HCO_3^-$ rises by only 1 mEq/L for every 10 mmHg rise in $pCO_2$. In this case, the $HCO_3^-$ of 30 suggests a partially compensated state. * **Common Causes:** Head injury, opioid overdose, and COPD are classic triggers for respiratory acidosis in exam vignettes.

Question 46: An arterial blood gas report shows the following values: pH: 7.00, PaO2: 60 mm Hg, PaCO2: 80 mm Hg, HCO3: 28 mEq/L. What is the acid-base disorder?

A. Respiratory acidosis with normoxemia
B. Metabolic acidosis with normoxemia
C. Metabolic acidosis with hypoxemia
D. Respiratory acidosis with hypoxemia (Correct Answer)

Explanation: To solve acid-base problems for NEET-PG, follow a systematic step-by-step approach: ### 1. Analysis of the Correct Answer (D) * **pH (7.00):** The normal pH range is 7.35–7.45. A pH of 7.00 indicates a severe **acidemia**. * **PaCO2 (80 mm Hg):** The normal range is 35–45 mm Hg. An elevated PaCO2 (>45) indicates that the primary cause of the acidosis is respiratory (retention of $CO_2$). * **HCO3 (28 mEq/L):** The normal range is 22–26 mEq/L. The slight elevation suggests a partial renal compensation, but it is insufficient to normalize the pH. * **PaO2 (60 mm Hg):** Normal PaO2 is 80–100 mm Hg. A value of 60 mm Hg indicates **hypoxemia**. **Conclusion:** The combination of low pH, high PaCO2, and low PaO2 confirms **Respiratory Acidosis with Hypoxemia**. ### 2. Why Other Options are Incorrect * **A & B (Normoxemia):** These are incorrect because the PaO2 is 60 mm Hg, which is significantly below the normal threshold (80 mm Hg). * **B & C (Metabolic Acidosis):** In metabolic acidosis, the primary change is a **low HCO3** (<22 mEq/L) and a compensatory **low PaCO2**. Here, the PaCO2 is high, pointing directly to a respiratory origin. ### 3. Clinical Pearls for NEET-PG * **Golden Rule:** If pH and PaCO2 move in **opposite** directions, the primary disorder is **Respiratory**. If they move in the **same** direction, it is **Metabolic** (ROME mnemonic: Respiratory Opposite, Metabolic Equal). * **Acute vs. Chronic:** In acute respiratory acidosis, for every 10 mmHg rise in PaCO2, HCO3 rises by 1 mEq/L. In chronic cases, it rises by 3.5–4 mEq/L. * **Hypoxemia Definition:** PaO2 < 80 mmHg is hypoxemia; PaO2 < 60 mmHg is often defined as Type 1 or Type 2 respiratory failure depending on the PaCO2 level.

Question 47: Metabolic changes associated with excessive vomiting include which of the following?

A. Metabolic acidosis
B. Hyperchloremia
C. Hypokalemia (Correct Answer)
D. Decreased bicarbonates

Explanation: **Explanation:** Excessive vomiting leads to a classic metabolic pattern known as **Hypochloremic Hypokalemic Metabolic Alkalosis**. **1. Why Hypokalemia is correct:** Potassium loss occurs through two main mechanisms during vomiting: * **Direct Loss:** Gastric juice contains potassium, which is lost during emesis. * **Renal Compensation (Paradoxical):** As the body loses H+ and Cl-, it develops metabolic alkalosis. To conserve H+ ions, the kidneys exchange them for K+ in the distal tubule. Furthermore, the resulting dehydration activates the **Renin-Angiotensin-Aldosterone System (RAAS)**. Aldosterone acts on the principal cells to reabsorb Na+ and water at the expense of secreting K+ into the urine, significantly worsening the hypokalemia. **2. Why other options are incorrect:** * **Metabolic Acidosis (A) & Decreased Bicarbonates (D):** Vomiting causes a loss of hydrochloric acid (HCl) from the stomach. The loss of H+ ions leads to **Metabolic Alkalosis**, characterized by an **increase** in serum bicarbonate levels. * **Hyperchloremia (B):** Gastric juice is rich in chloride. Loss of gastric contents leads to **Hypochloremia**. **Clinical Pearls for NEET-PG:** * **Paradoxical Aciduria:** In severe vomiting, despite systemic alkalosis, the urine becomes acidic. This happens because the body prioritizes Na+ reabsorption (due to volume depletion) over H+ excretion. * **Contraction Alkalosis:** Loss of chloride-rich, bicarbonate-poor fluid leads to a relative increase in bicarbonate concentration in the remaining extracellular fluid. * **Treatment of choice:** Isotonic Saline (0.9% NaCl) + Potassium supplementation. Saline corrects the volume deficit and provides chloride, which allows the kidneys to stop wasting potassium and bicarbonate.

Question 48: Which is the slowest acting buffer system in the body?

A. Phosphate buffer system
B. Respiratory system
C. Bicarbonate buffer system
D. Renal system (Correct Answer)

Explanation: ### Explanation The body maintains acid-base homeostasis through three primary lines of defense, which differ significantly in their speed of onset and duration of action. **1. Why the Renal System is the Correct Answer:** The **Renal system** is the third line of defense and is the **slowest acting** buffer system. While it is the most powerful and permanent regulatory mechanism, it takes **several hours to 3–5 days** to become fully effective. It regulates pH by excreting hydrogen ions ($H^+$), reabsorbing bicarbonate ($HCO_3^-$), and generating new bicarbonate via the ammonia and phosphate buffer systems in the tubules. **2. Analysis of Incorrect Options:** * **Phosphate Buffer System (Option A):** This is a chemical buffer found primarily in the intracellular fluid and renal tubular fluid. Like all chemical buffers, it acts **instantaneously** (within seconds), making it one of the fastest, not slowest. * **Respiratory System (Option B):** This is the second line of defense. It acts within **minutes** (1–15 minutes) by adjusting the rate of alveolar ventilation to eliminate or retain $CO_2$. It is intermediate in speed. * **Bicarbonate Buffer System (Option C):** This is the most important extracellular chemical buffer. As a chemical buffer, it reacts **immediately** (seconds) to neutralize pH changes. **3. High-Yield Facts for NEET-PG:** * **Speed Hierarchy:** Chemical Buffers (Seconds) > Respiratory (Minutes) > Renal (Days). * **Power Hierarchy:** Renal > Respiratory > Chemical. * **First Line of Defense:** Chemical buffers (Bicarbonate, Phosphate, Protein). * **Primary Intracellular Buffer:** Proteins (e.g., Hemoglobin) and Phosphates. * **Primary Extracellular Buffer:** Bicarbonate system. * **Renal Compensation:** The kidneys are the only system that can actually eliminate fixed acids (like lactic acid or phosphoric acid) from the body.

Question 49: Metabolic acidosis is seen in all except?

A. Diabetic ketoacidosis
B. Emphysema (Correct Answer)
C. Aspirin overdose
D. Uremia

Explanation: **Explanation:** The core concept in this question is distinguishing between **Metabolic Acidosis** (primary decrease in $HCO_3^-$) and **Respiratory Acidosis** (primary increase in $PaCO_2$). **Why Emphysema is the Correct Answer:** Emphysema is a type of Chronic Obstructive Pulmonary Disease (COPD). It causes destruction of alveolar walls and air trapping, leading to **hypoventilation**. This results in the retention of Carbon Dioxide ($CO_2$), leading to **Respiratory Acidosis**, not metabolic acidosis. **Analysis of Incorrect Options:** * **Diabetic Ketoacidosis (DKA):** This is a classic cause of **High Anion Gap Metabolic Acidosis (HAGMA)**. The accumulation of ketone bodies (acetoacetate and $\beta$-hydroxybutyrate) consumes bicarbonate buffers. * **Aspirin Overdose (Salicylate Toxicity):** Salicylates directly stimulate the respiratory center (causing early respiratory alkalosis) but also interfere with mitochondrial metabolism, leading to the accumulation of organic acids (lactic acid and ketoacids). This results in a **mixed acid-base disorder**, primarily featuring **HAGMA**. * **Uremia:** Seen in chronic kidney disease, uremia leads to metabolic acidosis because the kidneys fail to excrete fixed acids (phosphates, sulfates) and have a reduced capacity to regenerate bicarbonate. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** "MUDPILES" (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Winter’s Formula:** In metabolic acidosis, expected $pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2$. If the measured $pCO_2$ is higher, there is a concurrent respiratory acidosis. * **Salicylate Toxicity:** Remember the "Double Whammy"—it is the most common cause of a mixed Respiratory Alkalosis and Metabolic Acidosis.

Question 50: Respiratory acidosis is characterized by a primary increase in carbonic acid concentration?

A. Deficit of carbonic acid
B. Excess of carbonic acid (Correct Answer)
C. Deficit of bicarbonate
D. Excess of bicarbonate

Explanation: ### Explanation **Core Concept: The Henderson-Hasselbalch Relationship** Acid-base balance is governed by the ratio of bicarbonate ($HCO_3^-$) to carbonic acid ($H_2CO_3$). In clinical practice, $H_2CO_3$ is directly proportional to the partial pressure of carbon dioxide ($PaCO_2$). Respiratory acidosis occurs when there is **alveolar hypoventilation**, leading to the retention of $CO_2$. According to the equation $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$, an accumulation of $CO_2$ shifts the equilibrium to the right, resulting in an **excess of carbonic acid** and a subsequent drop in pH. **Analysis of Options:** * **Option B (Correct):** Respiratory acidosis is defined by hypercapnia ($PaCO_2 > 45$ mmHg). This elevation in $CO_2$ increases the concentration of dissolved carbonic acid, making it the primary disturbance. * **Option A:** A **deficit of carbonic acid** (low $PaCO_2$) characterizes **Respiratory Alkalosis**, typically caused by hyperventilation. * **Option C:** A **deficit of bicarbonate** is the primary hallmark of **Metabolic Acidosis**. * **Option D:** An **excess of bicarbonate** is the primary hallmark of **Metabolic Alkalosis**. **High-Yield Clinical Pearls for NEET-PG:** * **Primary vs. Compensatory:** In respiratory acidosis, the primary change is $\uparrow PaCO_2$. The **compensatory** response is the renal retention of $HCO_3^-$ (which takes 24–72 hours to fully manifest). * **Common Causes:** COPD, opioid overdose (respiratory depression), Guillain-Barré syndrome (respiratory muscle weakness), and chest wall deformities. * **Rule of Thumb:** For every 10 mmHg rise in $PaCO_2$, the $HCO_3^-$ rises by 1 mEq/L in acute cases and 3.5–4 mEq/L in chronic cases.

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Acid-Base Chemistry

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Respiratory Regulation of Acid-Base Balance

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Renal Regulation of Acid-Base Balance

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Bicarbonate Buffer System

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Non-Bicarbonate Buffer Systems

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Respiratory Acidosis and Alkalosis

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Metabolic Acidosis and Alkalosis

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Mixed Acid-Base Disorders

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Compensatory Mechanisms

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Clinical Assessment of Acid-Base Status

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