Acid-base balance, electrolytes and blood gases

29 Kerala MSc MLT Entrance practice questions on Acid-base balance, electrolytes and blood gases, part of Biochemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   On the second day after major abdominal surgery, a 60-year-old man has received 7 litres of 0.9% sodium chloride. Results: Na+ 143, K+ 4.0, Cl- 119, HCO3- 16 mmol/L; pH 7.32, pCO2 32 mm Hg; lactate 1.1 mmol/L (below 2.0); glucose and creatinine normal. The most likely cause of his acidosis is:

   1. ADistal renal tubular acidosis
   2. BLactic acidosis from poor perfusion
   3. CDiabetic ketoacidosis
   4. DHyperchloraemic acidosis from saline

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   Answer: D · Hyperchloraemic acidosis from saline

   Anion gap = 143 - (119 + 16) = 8 mmol/L, so this is a normal anion gap (hyperchloraemic) metabolic acidosis. Large volumes of 0.9% saline, which has 154 mmol/L chloride and no bicarbonate, raise plasma chloride and dilute bicarbonate. The pCO2 of 32 mm Hg fits the expected compensation (1.5 x 16 + 8 = 32). Lactic acidosis and ketoacidosis raise the anion gap, and distal RTA is a chronic disorder without this history.

   • A: Distal RTA is chronic; an acute post-operative saline load explains the picture.
   • B: Lactate is normal and the anion gap is not raised.
   • C: Glucose is normal and the anion gap is not raised.

2. Question 2 · Easy

   How is the bicarbonate result usually obtained on a blood gas analyser?

   1. ADerived from the measured pO2
   2. BMeasured by an enzymatic method
   3. CMeasured directly by a bicarbonate electrode
   4. DCalculated from the measured pH and pCO2

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   Answer: D · Calculated from the measured pH and pCO2

   Blood gas analysers measure pH, pCO2 and pO2 with electrodes. Bicarbonate is then calculated from the Henderson-Hasselbalch equation: pH = 6.1 + log [HCO3- / (0.03 x pCO2)]. Chemistry analysers measure total CO2 directly, usually by an enzymatic method, which runs about 1-2 mmol/L higher.

   • A: pO2 does not enter the bicarbonate calculation.
   • B: Enzymatic total CO2 is measured on chemistry analysers, not blood gas analysers.
   • C: No bicarbonate electrode is used; bicarbonate is calculated.

3. Question 3 · Hard

   A 28-year-old with type 1 diabetes has vomited repeatedly for 3 days. Results: Na+ 136, K+ 4.8, Cl- 84, HCO3- 18 mmol/L; pH 7.35, pCO2 34 mm Hg; urine ketones 3+. Taking the normal anion gap as 12 and bicarbonate as 24 mmol/L, which best describes the acid-base state?

   1. APure high anion gap metabolic acidosis
   2. BHigh anion gap acidosis with metabolic alkalosis
   3. CHigh anion gap acidosis with respiratory acidosis
   4. DNormal anion gap metabolic acidosis

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   Answer: B · High anion gap acidosis with metabolic alkalosis

   Anion gap = 136 - (84 + 18) = 34 mmol/L, a rise of 22 from normal, from ketoacids. Bicarbonate has fallen by only 6 mmol/L, giving a delta ratio of 22/6, about 3.7. A ratio above 2 means bicarbonate is higher than the acidosis alone would leave it, so a metabolic alkalosis from vomiting coexists. Winter's formula predicts pCO2 of 1.5 x 18 + 8 = 35 ± 2 mm Hg, so respiratory compensation is appropriate.

   • A: A pure acidosis would have a delta ratio of about 1-2, not 3.7.
   • C: The pCO2 of 34 mm Hg matches the expected compensation, so there is no respiratory acidosis.
   • D: The anion gap is 34 mmol/L, which is high.

4. Question 4 · Easy

   What is the sodium electrode of an ion-selective electrode analyser usually made of?

   1. ASilver coated with silver chloride
   2. BA quaternary ammonium exchanger
   3. CSodium-selective glass
   4. DValinomycin in a PVC membrane

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   Answer: C · Sodium-selective glass

   The sodium electrode uses a glass membrane of special composition (for example lithium aluminium silicate) that responds selectively to sodium ions. The potential across it, measured against a reference electrode, varies with the logarithm of sodium activity, as the Nernst equation describes. Potassium electrodes use valinomycin and chloride electrodes use an anion exchanger.

   • A: Silver/silver chloride serves as an internal or reference electrode.
   • B: A quaternary ammonium exchanger is used in the chloride electrode.
   • D: Valinomycin is the ionophore of the potassium electrode.

5. Question 5 · Medium

   Arterial blood on room air from a man with long-standing COPD: pH 7.34, pCO2 62 mm Hg, HCO3- 32 mmol/L (reference pH 7.35-7.45, pCO2 35-45, HCO3- 22-26). This shows:

   1. AAcute respiratory acidosis without compensation
   2. BRespiratory acidosis with metabolic acidosis
   3. CChronic respiratory acidosis with renal compensation
   4. DMetabolic alkalosis with respiratory compensation

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   Answer: C · Chronic respiratory acidosis with renal compensation

   The high pCO2 with an acid pH shows a primary respiratory acidosis. Bicarbonate is raised to 32 mmol/L, which is the renal compensation expected after several days (about 3.5 mmol/L for each 10 mm Hg rise in pCO2), so pH is almost normal. In acute respiratory acidosis bicarbonate rises only about 1 mmol/L per 10 mm Hg and the pH would be near 7.23. A primary metabolic alkalosis would give an alkaline pH.

   • A: Acute retention raises bicarbonate only slightly and gives a much lower pH.
   • B: Metabolic acidosis would lower bicarbonate, not raise it.
   • D: A primary metabolic alkalosis would give a pH above 7.45.

6. Question 6 · Medium

   A man with pyloric obstruction has vomited for a week: pH 7.54, HCO3- 38 mmol/L, K+ 2.6 mmol/L, Cl- 82 mmol/L. His urine pH is 5.5. Why is the urine acid despite the alkalosis?

   1. AThe kidney cannot reabsorb bicarbonate in alkalosis
   2. BVolume and potassium depletion drive renal hydrogen ion secretion
   3. CRespiratory compensation adds carbonic acid to urine
   4. DThe stomach passes acid directly into the urine

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7. Question 7 · Medium

   An arterial blood gas report shows a base excess of -12 mmol/L. This value indicates:

   1. AA metabolic acidosis component
   2. BA metabolic alkalosis component
   3. CFull respiratory compensation
   4. DA respiratory acidosis component

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8. Question 8 · Easy

   The Clark electrode of a blood gas analyser measures pO2 by which principle?

   1. APotentiometry
   2. BAmperometry
   3. CConductometry
   4. DCoulometry

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9. Question 9 · Hard

   A 25-year-old woman has recurrent renal stones. Results: Na+ 138, K+ 2.9, Cl- 114, HCO3- 14 mmol/L; arterial pH 7.29; urine pH 6.8 on a fresh early-morning sample; urine anion gap +30 mmol/L. The most likely diagnosis is:

   1. AChronic diarrhoea with bicarbonate loss
   2. BDistal (type 1) renal tubular acidosis
   3. CProximal (type 2) renal tubular acidosis
   4. DDiabetic ketoacidosis with hyperchloraemia

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10. Question 10 · Medium

    A clotted blood sample for electrolytes was left unseparated in the refrigerator overnight before analysis. The result most likely to be falsely raised is:

    1. ASodium
    2. BPotassium
    3. CChloride
    4. DTotal protein

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11. Question 11 · Medium

    A drowsy man has a raised-anion-gap metabolic acidosis and an osmolal gap of 35 mOsm/kg. Which poisoning best fits these findings?

    1. AMethanol
    2. BDiazepam
    3. CParacetamol
    4. DOrganophosphate

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12. Question 12 · Medium

    An arterial sample reaches the laboratory with a large air bubble that was not expelled. The likely change in the results is:

    1. ApO2 and pCO2 both rise, with pH falling
    2. BpO2 falls and pCO2 rises toward 60 mm Hg
    3. CpO2 is unchanged and only the pH rises
    4. DpO2 moves toward 150 mm Hg and pCO2 falls

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13. Question 13 · Medium

    A 55-year-old with uncontrolled diabetes has a plasma glucose of 600 mg/dL and a measured sodium of 128 mmol/L. Using a correction of 1.6 mmol/L for each 100 mg/dL of glucose above 100 mg/dL, the corrected sodium is:

    1. A138 mmol/L
    2. B120 mmol/L
    3. C130 mmol/L
    4. D136 mmol/L

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14. Question 14 · Medium

    A 40-year-old woman with resistant hypertension has K+ 2.8 mmol/L, HCO3- 33 mmol/L and a high urine potassium. Which disorder best explains this pattern?

    1. APrimary hyperaldosteronism
    2. BChronic diarrhoea
    3. CAddison disease
    4. DDistal renal tubular acidosis

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15. Question 15 · Hard

    A 30-year-old woman with no symptoms has routine results: K+ 9.6 mmol/L, calcium 3.1 mg/dL, alkaline phosphatase 18 U/L (both far below the reference ranges), sodium and creatinine normal, ECG normal. The sample was drawn with several tubes in one sitting. The most likely cause and action are:

    1. AIn vitro haemolysis; repeat the analysis on the same sample
    2. BK2 EDTA contamination; recollect in the correct order of draw
    3. CAcute kidney injury; inform the doctor as a critical value
    4. DHypoparathyroidism; request a parathyroid hormone level

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16. Question 16 · Easy

    In a child being tested for cystic fibrosis, a sweat chloride of how much supports the diagnosis?

    1. A20 mmol/L or more
    2. B10-29 mmol/L
    3. C30-39 mmol/L
    4. D60 mmol/L or more

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17. Question 17 · Hard

    A 45-year-old man is admitted confused after drinking illicit liquor. Results: Na+ 140, K+ 4.0, Cl- 100, HCO3- 10 mmol/L. Using the formula Na+ - (Cl- + HCO3-), what is his anion gap?

    1. A30 mmol/L
    2. B50 mmol/L
    3. C34 mmol/L
    4. D40 mmol/L

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18. Question 18 · Hard

    A 52-year-old man has: Na+ 124 mmol/L on the main analyser (indirect ISE), Na+ 139 mmol/L on the blood gas analyser (direct ISE), measured osmolality 289 mOsm/kg, glucose 95 mg/dL, and lipaemic serum with triglycerides 2,800 mg/dL. Which explanation fits?

    1. ATranslocational hyponatraemia from an osmotically active solute
    2. BInterference from lipid with the direct ISE membrane
    3. CPseudohyponatraemia from reduced plasma water in the indirect method
    4. DTrue dilutional hyponatraemia from water retention

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19. Question 19 · Medium

    A patient with essential thrombocythaemia (platelets 14 lakhs/µL) has a serum potassium of 6.4 mmol/L, a normal ECG and normal renal function. Which test best shows whether this is a true hyperkalaemia?

    1. ARepeat serum potassium after one hour
    2. BSerum potassium by flame photometry
    3. CPotassium in heparinised plasma
    4. DUrine potassium in a 24-hour sample

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20. Question 20 · Easy

    The potassium ion-selective electrode in electrolyte analysers uses which ionophore in its PVC membrane?

    1. ACrown ether
    2. BValinomycin
    3. CGramicidin
    4. DMonensin

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21. Question 21 · Easy

    The pCO2 (Severinghaus) electrode of a blood gas analyser actually senses:

    1. AA pH change in a bicarbonate layer behind a CO2-permeable membrane
    2. BA potential across a valinomycin membrane
    3. CA current from reduction of CO2 at a platinum cathode
    4. DAbsorbance of infrared light by dissolved CO2

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22. Question 22 · Hard

    A child with severe diarrhoea has: pH 7.15, HCO3- 12 mmol/L, pCO2 36 mm Hg (reference pCO2 35-45 mm Hg). Using Winter's formula (expected pCO2 = 1.5 x HCO3- + 8 +/- 2), how is the acid-base state best classified?

    1. AMetabolic acidosis with an added respiratory alkalosis
    2. BPrimary respiratory acidosis with renal compensation
    3. CMetabolic acidosis with an added respiratory acidosis
    4. DMetabolic acidosis with full respiratory compensation

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23. Question 23 · Medium

    Metabolic alkalosis is commonly accompanied by hypokalaemia. Which statement explains this link?

    1. AK+ is lost with bicarbonate in the stool
    2. BH+ leaves cells in exchange for K+, and the kidney wastes K+
    3. CAlkalosis destroys the Na+-K+ ATPase pump
    4. DAlkalosis increases potassium binding to albumin

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24. Question 24 · Medium

    A 1 mL blood gas syringe was drawn with only 0.3 mL of blood after rinsing it with liquid heparin solution. Which results are most affected by this excess heparin?

    1. AOnly the haemoglobin is falsely high
    2. BpO2 and pH are falsely low
    3. CSodium and pH are falsely high
    4. DpCO2 and bicarbonate are falsely low

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25. Question 25 · Medium

    Hyperventilation during a panic attack causes tingling and carpopedal spasm. The laboratory basis of this tetany is:

    1. AAlkalosis shifts potassium into cells, causing tetany
    2. BAlkalosis lowers total serum calcium by increasing urinary loss
    3. CAlkalosis increases calcium binding to albumin, lowering ionised calcium
    4. DLow pCO2 blocks parathyroid hormone release

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