COMSAE PHASE 1 ACID-BASE &
ELECTROLYTE PHYSIOLOGY PRACTICE
EXAM WITH ACTUAL QUESTIONS AND
VERIFIED ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
Question 1
A 24-year-old woman presents with severe diarrhea for 3 days. She has
dry mucous membranes, orthostatic hypotension, and tachycardia.
Laboratory studies show:
Na⁺ = 140 mEq/L
Cl⁻ = 112 mEq/L
HCO₃⁻ = 14 mEq/L
K⁺ = 3.1 mEq/L
Arterial blood gas analysis demonstrates a pH of 7.28 and a PCO₂ of 29
mm Hg. Which of the following best describes her acid-base disorder?
A. High-anion-gap metabolic acidosis with respiratory compensation
B. Normal-anion-gap metabolic acidosis with respiratory compensation
C. Metabolic alkalosis with respiratory compensation
D. Respiratory alkalosis with metabolic compensation
Answer: B. Normal-anion-gap metabolic acidosis with respiratory
compensation
Rationale: Diarrhea causes gastrointestinal loss of bicarbonate,
producing a metabolic acidosis. The anion gap is calculated as Na⁺ −
(Cl⁻ + HCO₃⁻) = 140 − (112 + 14) = 14 mEq/L, which is near the upper
1
,end of the traditional normal range and, considering typical laboratory
variation, is most consistent with a hyperchloremic/non-anion-gap
metabolic acidosis. The reduced PCO₂ reflects compensatory
hyperventilation. In bicarbonate-loss states, chloride increases
proportionally to maintain electroneutrality, so the anion gap remains
relatively normal.
Question 2
A 67-year-old man with chronic obstructive pulmonary disease has the
following arterial blood gas values:
pH = 7.36
PCO₂ = 60 mm Hg
HCO₃⁻ = 33 mEq/L
Which acid-base disorder is most likely?
A. Acute respiratory acidosis
B. Chronic respiratory acidosis
C. Acute metabolic alkalosis
D. Chronic metabolic acidosis
Answer: B. Chronic respiratory acidosis
Rationale: Chronic CO₂ retention stimulates renal compensation,
increasing HCO₃⁻ reabsorption and generation. In chronic respiratory
acidosis, bicarbonate increases by approximately 3.5–4 mEq/L for
every 10-mm Hg increase in PCO₂ above 40 mm Hg. Here, PCO₂ is 20
mm Hg above normal, so HCO₃⁻ would be expected to rise to
approximately 31–32 mEq/L, closely matching the measured 33
mEq/L. The near-normal pH indicates substantial renal
compensation.
2
,Question 3
A patient with diabetic ketoacidosis has:
Na⁺ = 138 mEq/L
Cl⁻ = 100 mEq/L
HCO₃⁻ = 10 mEq/L
What is the patient's anion gap?
A. 18 mEq/L
B. 24 mEq/L
C. 28 mEq/L
D. 38 mEq/L
Answer: C. 28 mEq/L
Rationale: The anion gap is calculated as Na⁺ − (Cl⁻ + HCO₃⁻).
Therefore, 138 − (100 + 10) = 28 mEq/L. This represents a markedly
elevated anion gap due to accumulation of unmeasured anions,
particularly β-hydroxybutyrate and acetoacetate, in diabetic
ketoacidosis.
Question 4
A 19-year-old woman is brought to the emergency department after
ingesting a large amount of aspirin. She has tinnitus, nausea, and rapid
breathing. ABG results show:
pH = 7.46
PCO₂ = 22 mm Hg
HCO₃⁻ = 15 mEq/L
Which acid-base abnormality is most likely present?
3
, A. Isolated respiratory alkalosis
B. Isolated metabolic acidosis
C. Mixed respiratory alkalosis and metabolic acidosis
D. Mixed respiratory acidosis and metabolic alkalosis
Answer: C. Mixed respiratory alkalosis and metabolic acidosis
Rationale: Salicylate toxicity classically produces a mixed acid-base
disorder. Early toxicity directly stimulates the medullary respiratory
center, causing respiratory alkalosis and reduced PCO₂.
Simultaneously, salicylates impair oxidative phosphorylation and
increase organic acid production, producing a high-anion-gap
metabolic acidosis. The combination of low PCO₂ and low HCO₃⁻ with
a relatively alkalemic pH is characteristic.
Question 5
A hospitalized patient has the following laboratory values:
Na⁺ = 142 mEq/L
Cl⁻ = 104 mEq/L
HCO₃⁻ = 12 mEq/L
Which additional finding would most strongly suggest a high-anion-gap
metabolic acidosis caused by lactic acidosis?
A. Severe diarrhea
B. Prolonged tissue hypoperfusion
C. Repeated vomiting
D. Mineralocorticoid excess
Answer: B. Prolonged tissue hypoperfusion
Rationale: Tissue hypoperfusion causes inadequate oxygen delivery,
increasing anaerobic glycolysis and lactate production. Lactate is an
4
ELECTROLYTE PHYSIOLOGY PRACTICE
EXAM WITH ACTUAL QUESTIONS AND
VERIFIED ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
Question 1
A 24-year-old woman presents with severe diarrhea for 3 days. She has
dry mucous membranes, orthostatic hypotension, and tachycardia.
Laboratory studies show:
Na⁺ = 140 mEq/L
Cl⁻ = 112 mEq/L
HCO₃⁻ = 14 mEq/L
K⁺ = 3.1 mEq/L
Arterial blood gas analysis demonstrates a pH of 7.28 and a PCO₂ of 29
mm Hg. Which of the following best describes her acid-base disorder?
A. High-anion-gap metabolic acidosis with respiratory compensation
B. Normal-anion-gap metabolic acidosis with respiratory compensation
C. Metabolic alkalosis with respiratory compensation
D. Respiratory alkalosis with metabolic compensation
Answer: B. Normal-anion-gap metabolic acidosis with respiratory
compensation
Rationale: Diarrhea causes gastrointestinal loss of bicarbonate,
producing a metabolic acidosis. The anion gap is calculated as Na⁺ −
(Cl⁻ + HCO₃⁻) = 140 − (112 + 14) = 14 mEq/L, which is near the upper
1
,end of the traditional normal range and, considering typical laboratory
variation, is most consistent with a hyperchloremic/non-anion-gap
metabolic acidosis. The reduced PCO₂ reflects compensatory
hyperventilation. In bicarbonate-loss states, chloride increases
proportionally to maintain electroneutrality, so the anion gap remains
relatively normal.
Question 2
A 67-year-old man with chronic obstructive pulmonary disease has the
following arterial blood gas values:
pH = 7.36
PCO₂ = 60 mm Hg
HCO₃⁻ = 33 mEq/L
Which acid-base disorder is most likely?
A. Acute respiratory acidosis
B. Chronic respiratory acidosis
C. Acute metabolic alkalosis
D. Chronic metabolic acidosis
Answer: B. Chronic respiratory acidosis
Rationale: Chronic CO₂ retention stimulates renal compensation,
increasing HCO₃⁻ reabsorption and generation. In chronic respiratory
acidosis, bicarbonate increases by approximately 3.5–4 mEq/L for
every 10-mm Hg increase in PCO₂ above 40 mm Hg. Here, PCO₂ is 20
mm Hg above normal, so HCO₃⁻ would be expected to rise to
approximately 31–32 mEq/L, closely matching the measured 33
mEq/L. The near-normal pH indicates substantial renal
compensation.
2
,Question 3
A patient with diabetic ketoacidosis has:
Na⁺ = 138 mEq/L
Cl⁻ = 100 mEq/L
HCO₃⁻ = 10 mEq/L
What is the patient's anion gap?
A. 18 mEq/L
B. 24 mEq/L
C. 28 mEq/L
D. 38 mEq/L
Answer: C. 28 mEq/L
Rationale: The anion gap is calculated as Na⁺ − (Cl⁻ + HCO₃⁻).
Therefore, 138 − (100 + 10) = 28 mEq/L. This represents a markedly
elevated anion gap due to accumulation of unmeasured anions,
particularly β-hydroxybutyrate and acetoacetate, in diabetic
ketoacidosis.
Question 4
A 19-year-old woman is brought to the emergency department after
ingesting a large amount of aspirin. She has tinnitus, nausea, and rapid
breathing. ABG results show:
pH = 7.46
PCO₂ = 22 mm Hg
HCO₃⁻ = 15 mEq/L
Which acid-base abnormality is most likely present?
3
, A. Isolated respiratory alkalosis
B. Isolated metabolic acidosis
C. Mixed respiratory alkalosis and metabolic acidosis
D. Mixed respiratory acidosis and metabolic alkalosis
Answer: C. Mixed respiratory alkalosis and metabolic acidosis
Rationale: Salicylate toxicity classically produces a mixed acid-base
disorder. Early toxicity directly stimulates the medullary respiratory
center, causing respiratory alkalosis and reduced PCO₂.
Simultaneously, salicylates impair oxidative phosphorylation and
increase organic acid production, producing a high-anion-gap
metabolic acidosis. The combination of low PCO₂ and low HCO₃⁻ with
a relatively alkalemic pH is characteristic.
Question 5
A hospitalized patient has the following laboratory values:
Na⁺ = 142 mEq/L
Cl⁻ = 104 mEq/L
HCO₃⁻ = 12 mEq/L
Which additional finding would most strongly suggest a high-anion-gap
metabolic acidosis caused by lactic acidosis?
A. Severe diarrhea
B. Prolonged tissue hypoperfusion
C. Repeated vomiting
D. Mineralocorticoid excess
Answer: B. Prolonged tissue hypoperfusion
Rationale: Tissue hypoperfusion causes inadequate oxygen delivery,
increasing anaerobic glycolysis and lactate production. Lactate is an
4
