Acid-Base Balance Practice Questions

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

Decrease. **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.

Q: A 2-year-old child is being evaluated for persistent metabolic acidosis. Blood tests show Na+ 140 mEq/L, K+ 3 mEq/L, Ca2+ 8 mg/L, Mg+2 mEq/L, phosphate 3 mEq/L, pH 7.22, bicarbonate 16 mEq/L, and chloride 112 mEq/L. What is the plasma anion gap?

15. ### Explanation **1. Understanding the Correct Answer (Option B: 15)** The **Plasma Anion Gap (AG)** is a calculated value used to differentiate causes of metabolic acidosis. It represents the "unmeasured anions" in the plasma (such as albumin, phosphate, and organic acids). The standard formula for calculating the Anion Gap is: **AG = [Na⁺] – ([Cl⁻] + [HCO₃⁻])** Plugging in the values from the question: * Na⁺ = 140 mEq/L * Cl⁻ = 112 mEq/L * HCO₃⁻ = 16 mEq/L * **AG = 140 – (112 + 16) = 140 – 128 = 12 mEq/L** *Note on the Answer Key:* While the calculated value is **12**, in clinical practice and NEET-PG questions, the normal range is typically **8–12 mEq/L**. However, some textbooks include Potassium in the formula: **(Na⁺ + K⁺) – (Cl⁻ + HCO₃⁻)**. Using this formula: **(140 + 3) – (112 + 16) = 143 – 128 = 15.** Given the options provided, **15** is the intended correct answer based on the inclusion of Potassium. **2. Why Other Options are Incorrect** * **Option A (9):** This would represent a low-normal anion gap, often seen in hypoalbuminemia, which is not supported by the clinical picture of acidosis. * **Options C (22) and D (25):** These represent a **High Anion Gap Metabolic Acidosis (HAGMA)**, seen in conditions like DKA, Lactic Acidosis, or Salicylate poisoning. The calculation here does not reach these values. **3. Clinical Pearls for NEET-PG** * **Normal Anion Gap Metabolic Acidosis (NAGMA):** Also called hyperchloremic acidosis. Common causes include Diarrhea and Renal Tubular Acidosis (RTA). * **High Anion Gap Metabolic Acidosis (HAGMA):** Remember the mnemonic **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Albumin Correction:** For every 1 g/dL decrease in serum albumin below 4 g/dL, the "normal" AG decreases by approximately 2.5 mEq/L. Always check albumin levels in a patient with a low AG.

Q: 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.

Respiratory acidosis. ### **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**.

Q: 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?

Respiratory acidosis with compensatory metabolic alkalosis. ### **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.

Q: 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?

Respiratory acidosis. ### 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 $pCO_2$ ( 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.

Q: Respiratory alkalosis occurs in which of the following conditions?

Excessive ventilation. **Explanation:** **Correct Option: A. Excessive ventilation** Respiratory alkalosis is characterized by a primary decrease in partial pressure of arterial carbon dioxide ($PaCO_2 7.45$). **Excessive ventilation** (hyperventilation) causes the lungs to "wash out" $CO_2$ faster than the body produces it. Since $CO_2$ acts as a volatile acid (forming $H_2CO_3$), its depletion leads to a rise in pH, resulting in respiratory alkalosis. Common triggers include anxiety, high altitude, and pulmonary embolism. **Incorrect Options:** * **B. Pyloric stenosis:** Persistent vomiting in pyloric stenosis leads to the loss of gastric hydrochloric acid ($HCl$), resulting in **Metabolic Alkalosis** (specifically hypochloremic, hypokalemic metabolic alkalosis). * **C. Diabetic ketoacidosis (DKA):** The accumulation of ketone bodies (acetoacetate and $\beta$-hydroxybutyrate) leads to **Metabolic Acidosis** with an elevated anion gap. The body compensates via Kussmaul breathing (hyperventilation) to lower $CO_2$, but the primary pathology is acidic. * **D. Primary hyperaldosteronism (Conn’s Syndrome):** Excess aldosterone promotes $H^+$ secretion in the distal renal tubules, leading to **Metabolic Alkalosis**, typically associated with hypertension and hypokalemia. **High-Yield Pearls for NEET-PG:** * **Compensation:** In acute respiratory alkalosis, for every 10 mmHg drop in $PaCO_2$, $HCO_3^-$ drops by 2 mEq/L. In chronic cases, it drops by 4-5 mEq/L. * **Ionized Calcium:** Alkalosis increases the binding of calcium to albumin. This reduces ionized calcium levels, leading to **tetany and carpopedal spasm** despite normal total serum calcium. * **Salicylate Poisoning:** Classically presents as a mixed acid-base disorder: early **Respiratory Alkalosis** (direct stimulation of the respiratory center) followed by **High Anion Gap Metabolic Acidosis**.

Q: 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?

Respiratory acidosis with hypoxemia. 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.

Q: Metabolic alkalosis is caused by:

Repeated vomiting. **Explanation:** **Metabolic alkalosis** is characterized by a primary increase in serum bicarbonate ($HCO_3^-$) and an increase in blood pH (>7.45). **1. Why Repeated Vomiting is Correct:** Gastric juice is highly acidic, containing high concentrations of hydrochloric acid ($HCl$). During **repeated vomiting** (or gastric suctioning), there is a massive loss of hydrogen ions ($H^+$) and chloride ions ($Cl^-$). As $H^+$ is lost, the parietal cells of the stomach generate more $HCO_3^-$ which enters the bloodstream (the "alkaline tide"). Furthermore, the resulting volume depletion triggers the Renin-Angiotensin-Aldosterone System (RAAS), leading to further $H^+$ excretion in the kidneys to conserve sodium, maintaining the alkalotic state. **2. Why Incorrect Options are Wrong:** * **Diarrhea:** Intestinal secretions are rich in bicarbonate. Loss of these fluids leads to a net loss of base, resulting in **Normal Anion Gap Metabolic Acidosis**. * **Diabetic Ketosis (DKA):** This condition involves the overproduction of organic acids (acetoacetate and $\beta$-hydroxybutyrate). The accumulation of these fixed acids consumes bicarbonate, leading to a **High Anion Gap Metabolic Acidosis**. **High-Yield Clinical Pearls for NEET-PG:** * **Saline Responsiveness:** Vomiting-induced alkalosis is "Saline Responsive" (Urinary $Cl^-$ < 10 mmol/L) because chloride replacement helps the kidneys excrete excess bicarbonate. * **Hypokalemia:** Metabolic alkalosis is almost always associated with hypokalemia, as $K^+$ shifts intracellularly in exchange for $H^+$ to buffer the serum pH. * **Paradoxical Aciduria:** In severe dehydration due to vomiting, the kidney prioritizes sodium reabsorption over pH balance, excreting $H^+$ instead of $Na^+$, leading to acidic urine despite systemic alkalosis.

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

Hypokalemia. **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 1

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

Accepted Answer

Decrease

Answer

Increase

Answer

Remain constant

Answer

None of the above

Question 2

A 2-year-old child is being evaluated for persistent metabolic acidosis. Blood tests show Na+ 140 mEq/L, K+ 3 mEq/L, Ca2+ 8 mg/L, Mg+2 mEq/L, phosphate 3 mEq/L, pH 7.22, bicarbonate 16 mEq/L, and chloride 112 mEq/L. What is the plasma anion gap?

Accepted Answer

15

Answer

9

Answer

22

Answer

25

Question 3

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.

Accepted Answer

Respiratory acidosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Answer

Respiratory alkalosis

Question 4

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?

Accepted Answer

Respiratory acidosis with compensatory metabolic alkalosis

Answer

Respiratory acidosis with compensatory metabolic acidosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Question 5

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?

Accepted Answer

Respiratory acidosis

Answer

Respiratory alkalosis

Answer

Metabolic acidosis

Answer

Metabolic alkalosis

Question 6

Respiratory alkalosis occurs in which of the following conditions?

Accepted Answer

Excessive ventilation

Answer

Pyloric stenosis

Answer

Diabetic ketoacidosis

Answer

Primary hyperaldosteronism

Question 7

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?

Accepted Answer

Respiratory acidosis with hypoxemia

Answer

Respiratory acidosis with normoxemia

Answer

Metabolic acidosis with normoxemia

Answer

Metabolic acidosis with hypoxemia

Question 8

Metabolic alkalosis is caused by:

Accepted Answer

Repeated vomiting

Answer

Diarrhea

Answer

Diabetic ketosis

Answer

All of the above

Question 9

Which of the following is not a cause of metabolic alkalosis?

Accepted Answer

Both vomiting and renal failure

Answer

Vomiting

Answer

Fever

Answer

Renal failure

Question 10

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

Accepted Answer

Hypokalemia

Answer

Metabolic acidosis

Answer

Hyperchloremia

Answer

Decreased bicarbonates

Acid-Base Balance — MCQs

Acid-Base Balance — MCQs

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