ACID-BASE DISORDERS - CLINICAL CASE MCQs – 30 MCQs | Kenya MBChB

30 Year 3: Respiratory System Pathology exam questions on ACID-BASE DISORDERS - CLINICAL CASE MCQs for medical students. Includes MCQs, answers, explanations an

This MCQ set contains 30 questions on ACID-BASE DISORDERS - CLINICAL CASE MCQs in the Year 3: Respiratory System Pathology unit. Each question includes the correct answer and a detailed explanation for active recall and exam preparation.

Q1: A 62-year-old man with COPD is brought in with confusion. ABG: pH 7.28, PaCO₂ 64 mmHg, HCO₃⁻ 28 mmol/L. Which is the most likely primary disorder?

  1. A. Acute respiratory acidosis
  2. B. Chronic respiratory acidosis with metabolic compensation
  3. C. Metabolic acidosis with respiratory compensation
  4. D. Mixed respiratory acidosis and metabolic acidosis
  5. E. Metabolic alkalosis

Correct answer: B – Chronic respiratory acidosis with metabolic compensation

pH is low (acidosis), PaCO₂ is markedly elevated (respiratory acidosis), and HCO₃⁻ is elevated above normal (24-26), indicating metabolic compensation. The presence of elevated bicarbonate suggests chronic respiratory acidosis with renal compensation, typical in COPD patients.

Q2: A 28-year-old woman with vomiting presents with cramps. ABG: pH 7.54, PaCO₂ 48 mmHg, HCO₃⁻ 38 mmol/L. Which is the best interpretation?

  1. A. Primary metabolic alkalosis with respiratory compensation
  2. B. Primary respiratory alkalosis with renal compensation
  3. C. Mixed metabolic alkalosis and acute respiratory acidosis
  4. D. Primary metabolic acidosis with compensatory hypoventilation
  5. E. Mixed respiratory alkalosis and metabolic acidosis

Correct answer: A – Primary metabolic alkalosis with respiratory compensation

pH is high (alkalosis), HCO₃⁻ is markedly elevated (primary metabolic alkalosis from vomiting and loss of gastric acid), and PaCO₂ is appropriately elevated (respiratory compensation through hypoventilation to retain CO₂).

Q3: A 45-year-old diabetic in DKA: ABG: pH 7.18, PaCO₂ 24 mmHg, HCO₃⁻ 9 mmol/L; Na⁺ 138, Cl⁻ 100, HCO₃⁻ 9. What is the most likely acid-base disorder?

  1. A. High anion gap metabolic acidosis only
  2. B. High anion gap metabolic acidosis + respiratory alkalosis
  3. C. Normal anion gap metabolic acidosis only
  4. D. Mixed metabolic acidosis and metabolic alkalosis
  5. E. Respiratory acidosis

Correct answer: B – High anion gap metabolic acidosis + respiratory alkalosis

AG = 138 - (100 + 9) = 29 (high). pH is low with low HCO₃⁻ (metabolic acidosis). Expected PaCO₂ by Winter's = 1.5(9) + 8 = 21.5 ± 2 (19.5-23.5). Actual PaCO₂ is 24, which is slightly higher than expected but close, suggesting possible mixed disorder with mild respiratory alkalosis component from Kussmaul breathing.

Q4: A 70-year-old man with vomiting and nasogastric suction: ABG: pH 7.50, PaCO₂ 54 mmHg, HCO₃⁻ 40 mmol/L. Which is correct?

  1. A. Metabolic alkalosis with expected respiratory compensation
  2. B. Primary respiratory alkalosis
  3. C. Mixed metabolic alkalosis and primary respiratory acidosis
  4. D. Metabolic acidosis with compensation
  5. E. Primary metabolic alkalosis with inadequate compensation

Correct answer: A – Metabolic alkalosis with expected respiratory compensation

pH is high (alkalosis), HCO₃⁻ is markedly elevated (metabolic alkalosis from vomiting/NG suction), and PaCO₂ is appropriately elevated as respiratory compensation (hypoventilation to retain CO₂ and normalize pH).

Q5: A trauma patient: ABG: pH 7.10, PaCO₂ 35 mmHg, HCO₃⁻ 10 mmol/L. Using Winter's formula, expected PaCO₂ for this HCO₃⁻ is approximately? (choose closest)

  1. A. 23 mmHg
  2. B. 35 mmHg
  3. C. 50 mmHg
  4. D. 17 mmHg
  5. E. 41 mmHg

Correct answer: A – 23 mmHg

Winter's formula: Expected PaCO₂ = 1.5(10) + 8 = 23 ± 2 (21-25 mmHg). The actual PaCO₂ of 35 is higher than expected, suggesting inadequate respiratory compensation or a mixed disorder with respiratory acidosis component.

Q6: A patient's electrolytes: Na⁺ 140, K⁺ 4.2, Cl⁻ 102, HCO₃⁻ 8. Calculate anion gap (AG = Na - (Cl + HCO₃)). Which is the AG?

  1. A. 30 mmol/L
  2. B. 20 mmol/L
  3. C. 10 mmol/L
  4. D. 8 mmol/L
  5. E. 15 mmol/L

Correct answer: A – 30 mmol/L

AG = 140 - (102 + 8) = 140 - 110 = 30 mmol/L. This is a markedly elevated anion gap (normal 8-12), indicating high anion gap metabolic acidosis from unmeasured anions like ketones, lactate, or toxins.

Q7: A 50-year-old with chronic COPD: ABG: pH 7.36, PaCO₂ 56 mmHg, HCO₃⁻ 32 mmol/L. Best interpretation?

  1. A. Chronic respiratory acidosis with metabolic compensation
  2. B. Acute respiratory acidosis
  3. C. Primary metabolic alkalosis
  4. D. Mixed respiratory acidosis & metabolic acidosis
  5. E. Respiratory alkalosis

Correct answer: A – Chronic respiratory acidosis with metabolic compensation

pH is near normal (compensated), PaCO₂ is elevated (respiratory acidosis), and HCO₃⁻ is elevated (metabolic compensation). This represents chronic respiratory acidosis with complete renal compensation, typical in chronic COPD.

Q8: A septic patient: ABG: pH 7.25, PaCO₂ 28 mmHg, HCO₃⁻ 12 mmol/L; electrolytes: Na 140, Cl 100, HCO₃ 12. AG = 28. Delta ratio: ΔAG = (28-12) = 16, ΔHCO₃ = (24-12) = 12. What is ΔAG/ΔHCO₃ and its interpretation?

  1. A. 16/12 = 1.33 → pure HAGMA
  2. B. 16/12 = 1.33 → mixed NAGMA & HAGMA
  3. C. 8/12 = 0.67 → NAGMA + HAGMA
  4. D. 16/12 = 1.33 → HAGMA + metabolic alkalosis
  5. E. 4/12 = 0.33 → pure NAGMA

Correct answer: A – 16/12 = 1.33 → pure HAGMA

Delta ratio = 16/12 = 1.33. A ratio between 1-2 suggests pure high anion gap metabolic acidosis (HAGMA). The rise in anion gap equals the fall in bicarbonate, consistent with lactic acidosis in sepsis.

Q9: A patient with salicylate overdose: early ABG: pH 7.55, PaCO₂ 26 mmHg, HCO₃⁻ 22 mmol/L. Which best describes this?

  1. A. Primary respiratory alkalosis with early metabolic acidosis developing → mixed disorder
  2. B. Pure respiratory alkalosis
  3. C. Pure metabolic alkalosis
  4. D. Primary metabolic acidosis with respiratory compensation
  5. E. Normal ABG

Correct answer: A – Primary respiratory alkalosis with early metabolic acidosis developing → mixed disorder

Salicylates initially stimulate the respiratory center causing respiratory alkalosis (low PaCO₂, high pH). HCO₃⁻ is starting to drop (22, slightly below 24) indicating developing metabolic acidosis. This represents the early mixed disorder typical of salicylate toxicity.

Q10: ABG: pH 7.10, PaCO₂ 70 mmHg, HCO₃⁻ 22 mmol/L. What is the best interpretation?

  1. A. Mixed respiratory acidosis and metabolic acidosis
  2. B. Pure respiratory acidosis with metabolic compensation
  3. C. Pure metabolic acidosis with respiratory compensation
  4. D. Primary metabolic alkalosis with hypoventilation
  5. E. Normal

Correct answer: A – Mixed respiratory acidosis and metabolic acidosis

pH is severely low (acidosis), PaCO₂ is markedly elevated (respiratory acidosis), but HCO₃⁻ is normal/slightly low. The severe acidemia with inadequate bicarbonate elevation suggests mixed respiratory and metabolic acidosis rather than chronic respiratory acidosis with compensation.

Q11: Patient with severe diarrhea: ABG: pH 7.32, PaCO₂ 34 mmHg, HCO₃⁻ 17 mmol/L. Which is most likely?

  1. A. Normal anion gap metabolic acidosis (NAGMA) with respiratory compensation
  2. B. High anion gap metabolic acidosis
  3. C. Mixed metabolic acidosis & alkalosis
  4. D. Respiratory alkalosis
  5. E. Respiratory acidosis

Correct answer: A – Normal anion gap metabolic acidosis (NAGMA) with respiratory compensation

Diarrhea causes loss of bicarbonate-rich intestinal fluid, leading to normal anion gap metabolic acidosis (NAGMA). pH is low, HCO₃⁻ is low, and PaCO₂ is appropriately reduced as respiratory compensation.

Q12: Post-operative patient receiving large IV normal saline: ABG: pH 7.32, HCO₃⁻ 18 mmol/L, Na 140, Cl 108. What acid-base disturbance is likely?

  1. A. Normal anion gap metabolic acidosis due to hyperchloremia (dilutional)
  2. B. High anion gap metabolic acidosis
  3. C. Metabolic alkalosis
  4. D. Respiratory alkalosis
  5. E. Mixed disorder

Correct answer: A – Normal anion gap metabolic acidosis due to hyperchloremia (dilutional)

Large volumes of normal saline (0.9% NaCl) contain high chloride (154 mmol/L), causing hyperchloremic metabolic acidosis. The elevated Cl⁻ (108) and low HCO₃⁻ with normal anion gap confirms NAGMA from dilutional hyperchloremia.

Q13: A patient with known COPD presents with acute worsening and fever. ABG: pH 7.22, PaCO₂ 81 mmHg, HCO₃⁻ 34 mmol/L. Which is the best interpretation?

  1. A. Acute on chronic respiratory acidosis (acute rise on chronic)
  2. B. Pure chronic respiratory acidosis
  3. C. Metabolic alkalosis only
  4. D. Mixed respiratory alkalosis & metabolic acidosis
  5. E. Pure acute respiratory acidosis

Correct answer: A – Acute on chronic respiratory acidosis (acute rise on chronic)

Elevated baseline HCO₃⁻ (34) suggests chronic respiratory acidosis with compensation. However, pH is significantly acidotic (7.22) with very high PaCO₂ (81), indicating an acute exacerbation on top of chronic disease - acute-on-chronic respiratory acidosis.

Q14: ABG: pH 7.43, PaCO₂ 30 mmHg, HCO₃⁻ 20 mmol/L. Which is correct?

  1. A. Primary respiratory alkalosis with renal compensation
  2. B. Primary metabolic alkalosis with respiratory compensation
  3. C. Mixed respiratory alkalosis + metabolic acidosis
  4. D. Normal ABG
  5. E. Primary metabolic acidosis with compensation

Correct answer: A – Primary respiratory alkalosis with renal compensation

pH is slightly alkalotic, PaCO₂ is low (respiratory alkalosis), and HCO₃⁻ is appropriately reduced as renal compensation. The pH being on the alkalotic side suggests the primary disorder is respiratory alkalosis with compensation.

Q15: A 65-year-old with acute pulmonary edema: ABG: pH 7.25, PaCO₂ 52 mmHg, HCO₃⁻ 22 mmol/L. Most likely?

  1. A. Primary respiratory acidosis with some metabolic acidosis → mixed disorder
  2. B. Pure respiratory acidosis with metabolic compensation
  3. C. Primary metabolic acidosis with respiratory compensation
  4. D. Metabolic alkalosis
  5. E. Respiratory alkalosis

Correct answer: A – Primary respiratory acidosis with some metabolic acidosis → mixed disorder

Pulmonary edema impairs ventilation causing respiratory acidosis (high PaCO₂). The HCO₃⁻ is normal/low, which doesn't show appropriate elevation for pure respiratory acidosis. The severe acidemia suggests coexisting metabolic acidosis (possibly from hypoperfusion/lactic acidosis) - a mixed disorder.

Q16: ABG after vomiting & diuretic use: pH 7.56, PaCO₂ 54 mmHg, HCO₃⁻ 45 mmol/L. Which is the best statement?

  1. A. Metabolic alkalosis with appropriate respiratory compensation (hypoventilation)
  2. B. Primary respiratory acidosis only
  3. C. Mixed respiratory alkalosis & metabolic acidosis
  4. D. Primary metabolic acidosis
  5. E. Respiratory alkalosis

Correct answer: A – Metabolic alkalosis with appropriate respiratory compensation (hypoventilation)

pH is high (alkalosis), HCO₃⁻ is markedly elevated (metabolic alkalosis from vomiting and diuretics causing H⁺ and Cl⁻ loss), and PaCO₂ is appropriately elevated as respiratory compensation through hypoventilation.

Q17: An intoxicated patient: ABG pH 7.20, PaCO₂ 18 mmHg, HCO₃⁻ 7 mmol/L. Which is correct?

  1. A. High anion gap metabolic acidosis with respiratory compensation (likely pure metabolic acidosis)
  2. B. Primary respiratory alkalosis
  3. C. Mixed respiratory acidosis & metabolic alkalosis
  4. D. Pure metabolic alkalosis
  5. E. Normal

Correct answer: A – High anion gap metabolic acidosis with respiratory compensation (likely pure metabolic acidosis)

pH is very low (severe acidosis), HCO₃⁻ is very low (metabolic acidosis), and PaCO₂ is appropriately low (respiratory compensation via hyperventilation). Using Winter's: expected PaCO₂ = 1.5(7) + 8 = 18.5, which matches actual. This is appropriate compensation for severe metabolic acidosis, likely from toxic alcohol ingestion.

Q18: ABG: pH 7.39, PaCO₂ 48 mmHg, HCO₃⁻ 29 mmol/L. Patient asymptomatic. Interpretation?

  1. A. Chronic respiratory acidosis with metabolic compensation (compensated)
  2. B. Acute respiratory acidosis
  3. C. Metabolic alkalosis
  4. D. Mixed metabolic acidosis & respiratory acidosis
  5. E. Respiratory alkalosis

Correct answer: A – Chronic respiratory acidosis with metabolic compensation (compensated)

pH is normal (compensated), PaCO₂ is elevated (respiratory acidosis), and HCO₃⁻ is elevated (metabolic compensation). This represents fully compensated chronic respiratory acidosis, typical in asymptomatic chronic lung disease patients.

Q19: A patient: Na 138, Cl 90, HCO₃ 8, AG = 40. ΔAG = 28 (40-12), ΔHCO₃ = 16 (24-8). ΔAG/ΔHCO₃ = ? (select closest) and interpretation.

  1. A. 1.75 → HAGMA + metabolic alkalosis or chronic respiratory acidosis
  2. B. 0.5 → mixed NAGMA & HAGMA
  3. C. 2.5 → pure HAGMA
  4. D. 0.2 → pure NAGMA
  5. E. 1.0 → pure HAGMA

Correct answer: A – 1.75 → HAGMA + metabolic alkalosis or chronic respiratory acidosis

Delta ratio = 28/16 = 1.75. A ratio 1.6-2.0 suggests HAGMA with concurrent metabolic alkalosis or chronic respiratory acidosis. The rise in anion gap exceeds the fall in bicarbonate, indicating another process is maintaining bicarbonate levels.

Q20: ABG: pH 7.30, PaCO₂ 26 mmHg, HCO₃⁻ 12 mmol/L. Which best describes respiratory compensation? (expected PaCO₂ using Winter's formula for HCO₃ 12)

  1. A. Expected PaCO₂ ≈ 26 mmHg → appropriate respiratory compensation (pure metabolic acidosis)
  2. B. Expected PaCO₂ ≈ 35 mmHg → coexisting respiratory acidosis
  3. C. Expected PaCO₂ ≈ 50 mmHg → mixed disorder
  4. D. Expected PaCO₂ ≈ 17 mmHg → respiratory alkalosis present
  5. E. Expected PaCO₂ ≈ 41 mmHg → chronic respiratory acidosis present

Correct answer: A – Expected PaCO₂ ≈ 26 mmHg → appropriate respiratory compensation (pure metabolic acidosis)

Winter's formula: Expected PaCO₂ = 1.5(12) + 8 = 26 ± 2 (24-28 mmHg). The actual PaCO₂ of 26 matches expected, indicating appropriate respiratory compensation for pure metabolic acidosis without mixed disorder.

Q21: A 30-year-old with repeated vomiting: ABG pH 7.58, PaCO₂ 60 mmHg, HCO₃⁻ 48 mmol/L. Which is most likely among choices?

  1. A. Metabolic alkalosis with appropriate respiratory compensation (hypoventilation)
  2. B. Mixed respiratory alkalosis & metabolic acidosis
  3. C. Primary respiratory alkalosis
  4. D. Metabolic acidosis with compensation
  5. E. Normal

Correct answer: A – Metabolic alkalosis with appropriate respiratory compensation (hypoventilation)

pH is high (alkalosis), HCO₃⁻ is markedly elevated (metabolic alkalosis from vomiting with loss of gastric HCl), and PaCO₂ is appropriately elevated (respiratory compensation through hypoventilation to retain CO₂ and normalize pH).

Q22: Post-op patient given large amount of lactated Ringer's now with hypotension: ABG pH 7.34, HCO₃⁻ 20 mmol/L, Na 135, Cl 100. Likely disturbance?

  1. A. Mild metabolic acidosis; consider lactic acidosis (HAGMA) in sepsis/hypoperfusion
  2. B. Normal anion gap metabolic acidosis from hyperchloremia
  3. C. Metabolic alkalosis
  4. D. Respiratory alkalosis
  5. E. Mixed disorder

Correct answer: A – Mild metabolic acidosis; consider lactic acidosis (HAGMA) in sepsis/hypoperfusion

AG = 135 - (100 + 20) = 15 (mildly elevated). With hypotension suggesting hypoperfusion, lactic acidosis (HAGMA) is most likely. The mild metabolic acidosis with elevated AG in the setting of shock points to tissue hypoperfusion and lactate accumulation.

Q23: ABG: pH 7.15, PaCO₂ 20 mmHg, HCO₃⁻ 6 mmol/L. Which is the most likely cause?

  1. A. Severe metabolic acidosis (e.g., DKA, toxins) with respiratory compensation
  2. B. Primary respiratory alkalosis
  3. C. Chronic respiratory acidosis
  4. D. Metabolic alkalosis
  5. E. Mixed metabolic alkalosis & respiratory acidosis

Correct answer: A – Severe metabolic acidosis (e.g., DKA, toxins) with respiratory compensation

pH is very low (severe acidosis), HCO₃⁻ is critically low (severe metabolic acidosis), and PaCO₂ is appropriately very low (respiratory compensation). Using Winter's: expected = 1.5(6) + 8 = 17 ± 2, actual is 20 (close). This represents severe metabolic acidosis with appropriate compensation, likely DKA, lactic acidosis, or toxic alcohol.

Q24: A patient's labs: Na 142, Cl 108, HCO₃ 22, AG = 12. ABG pH 7.41, PaCO₂ 38, HCO₃ 24. What does this ABG suggest?

  1. A. Essentially normal acid-base status (AG ~ normal)
  2. B. High AG metabolic acidosis with respiratory alkalosis
  3. C. Mixed metabolic alkalosis & metabolic acidosis
  4. D. Respiratory acidosis
  5. E. Metabolic alkalosis

Correct answer: A – Essentially normal acid-base status (AG ~ normal)

AG = 142 - (108 + 22) = 12 (normal). All ABG values are within normal ranges: pH 7.35-7.45, PaCO₂ 35-45, HCO₃⁻ 22-26. This represents normal acid-base status.

Q25: A 55-year-old with liver failure: ABG pH 7.29, PaCO₂ 28 mmHg, HCO₃⁻ 12 mmol/L; Na 138, Cl 96. AG = ? (choose closest) and likely disorder.

  1. A. AG ≈ 30 → HAGMA from lactic acidosis (with appropriate respiratory compensation)
  2. B. AG ≈ 20 → NAGMA
  3. C. AG ≈ 10 → normal
  4. D. AG ≈ 5 → hypo-anion gap
  5. E. AG ≈ 15 → mixed disorder

Correct answer: A – AG ≈ 30 → HAGMA from lactic acidosis (with appropriate respiratory compensation)

AG = 138 - (96 + 12) = 30 (markedly elevated). Liver failure patients can develop lactic acidosis from impaired lactate clearance and tissue hypoperfusion. The HAGMA with appropriate respiratory compensation (low PaCO₂) is consistent with lactic acidosis.

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