Q1. The anion gap is calculated as: A. Na⁺ − Cl⁻ B. Na⁺ − (Cl⁻ + HCO₃⁻) C. (Na⁺ + K⁺) − Cl⁻ D. Cl⁻ − HCO₃⁻ ANSWER: B — Na⁺ − (Cl⁻ + HCO₃⁻). Why: it estimates unmeasured anions. Normal is 8–12 mmol/L. Some laboratories include potassium, which shifts the normal range slightly, but B is the standard form. LEARNING POINT: the gap separates added acid from lost bicarbonate — the single most useful split in metabolic acidosis.

Q2. Renal compensation for a respiratory acidosis takes: A. Seconds B. Minutes C. Hours to days D. Weeks to months ANSWER: C — hours to days. Why: the kidney adjusts bicarbonate handling slowly, unlike the lung, which alters pCO₂ within minutes. LEARNING POINT: because it is slow, a raised bicarbonate in respiratory acidosis tells you the process is CHRONIC.

Q3. pH 7.31, pCO₂ 2.6 kPa, HCO₃⁻ 10, Na⁺ 140, Cl⁻ 100. The interpretation is: A. Normal-gap metabolic acidosis B. Raised-gap metabolic acidosis with respiratory compensation C. Respiratory alkalosis D. Metabolic alkalosis ANSWER: B. Why: acidaemic; HCO₃⁻ low and moving with the pH → primary metabolic acidosis. pCO₂ low in the same direction → compensation. Gap = 140 − (100 + 10) = 30, raised. A: the gap is raised, not normal. C: pCO₂ is low, but as compensation, not the primary problem — the pH is ACIDIC. D: the pH excludes it. LEARNING POINT: calculate the gap before you name the acidosis.

Q4. A patient with severe asthma has been hyperventilating for an hour. The pCO₂ has risen from 2.9 to 5.2 kPa. This indicates: A. Clinical improvement B. Impending respiratory failure from exhaustion C. A laboratory error D. Successful bronchodilator therapy ANSWER: B — impending respiratory failure. Why: a patient in severe asthma should be hyperventilating, so a LOW pCO₂ is expected. A rising pCO₂ into the normal range means they can no longer sustain the work of breathing. A, D: the classic misreading this question targets. LEARNING POINT: in acute severe asthma the "normal" number is the emergency. Escalate.

Q5. pH 7.40, pCO₂ 2.8 kPa, HCO₃⁻ 13, Na⁺ 141, Cl⁻ 105. The most accurate statement is: A. This gas is normal B. Single metabolic acidosis, fully compensated C. Two primary disorders are present D. Single respiratory alkalosis ANSWER: C — two primary disorders. Why: compensation never restores the pH exactly to normal, so a pH of 7.40 with grossly abnormal values means opposing processes. Gap = 141 − (105 + 13) = 23, raised → raised-gap metabolic acidosis, plus a primary respiratory alkalosis. A: dangerously wrong; both values are far outside range. B: full normalisation is not something compensation does. LEARNING POINT: a normal pH in a sick patient should increase your suspicion, not lower it.

Q6. A patient in DKA has received 4 litres of 0.9% saline. Ketones have cleared but the acidosis persists: pH 7.29, HCO₃⁻ 17, Na⁺ 139, Cl⁻ 114. The cause is: A. Ongoing ketoacidosis needing more insulin B. Hyperchloraemic acidosis from the saline C. Lactic acidosis from hypoperfusion D. Renal tubular acidosis ANSWER: B — hyperchloraemic acidosis from the saline. Why: the anion gap = 139 − (114 + 17) = 8, which is NORMAL. The raised-gap ketoacidosis has resolved; what remains is a normal-gap acidosis from the chloride load. A: a closed gap shows the ketoacidosis is already treated — more insulin risks hypoglycaemia. C: lactate would give a RAISED gap. D: no supporting evidence, and the timing points to fluid. LEARNING POINT: recalculate the gap before escalating treatment for a "persistent" acidosis.

Q7. pH 7.37, pCO₂ 8.4 kPa, HCO₃⁻ 34. The best interpretation is: A. Acute respiratory acidosis B. Chronic respiratory acidosis with renal compensation C. Primary metabolic alkalosis D. Mixed metabolic and respiratory alkalosis ANSWER: B — chronic respiratory acidosis. Why: pCO₂ is high with the pH at the low end of normal, so the acidosis is primary. The markedly raised bicarbonate is renal compensation — which takes DAYS, so the process is chronic. This is the classic long-standing COPD picture. A: an ACUTE rise would give a much lower pH and a normal bicarbonate, since the kidney has had no time. C: the pH sits on the acidic side. LEARNING POINT: the bicarbonate dates the disorder. Use it to tell acute from chronic.