USMLE Board Review Questions:   Acid Base

revised 1/30/2012


 A 23-year-old woman with gastroenteritis experiences nausea and vomiting.  Aterial blood gas analysis is done 1 hour after the onset of symptoms.   Which of the following sets of blood gases is most likely.   Correct Answer  explanation of answer

A  pH 7.30; PCO2 50; HCO3- 24

B pH 7.28; PCO2 40; HCO3- 18

C pH 7.56; PCO2 40; HCO3- 35

D pH 7.51; PCO2 45; HCO3- 35




 A 35-year-old man with type 1 diabetes is admitted to the emergency depaqrtment after being found unconscious and unresponsive at home.  His breath has a "fruity" odor.  His wife told the EMTs that his diabetes had been "out of control" lately and that he has no other medical problems.  His breathing is deep and rapid.  An arterial blood sample is taken for analysis.  Which of the following sets of arterial blood gases is most likely.   Correct Answer  explanation of answer

A  pH 7.00; PCO2 50; HCO3- 12

B pH 7.22; PCO2 30; HCO3- 12

C pH 7.56; PCO2 40; HCO3- 35

D pH 7.51; PCO2 45; HCO3- 35



 A patient in a rural hospital in India was diagnosed with "metabolic alkalosis" but the chart did not include and information on the history or physical exam.  There was also no lab data.  Which of the following is a potential cause of metabolic alkalosis?  .   Correct Answer  explanation of answer

A  Cholera with prolonged diarrhea

B hyperaldosteronism (Conn's syndrome)

C ingestion of ethylene glycol (antifreeze)

D renal failure


E salicylate (aspirin) toxicity


F treatment with a carbonic anhydrase inhibitor (acetazolamide)




In the diagram shown, point X indicates the acid-base status of a healthy person at sea level.  Which of the following is the most likely cause of the condition indicated by point Y?

  .   Correct Answer  explanation of answer

Adaptation to high altitude

B Chronic obstructive pulmonary disease

C Diarrhea

Ingestion of a strong acid


Severe prolonged vomiting





















  In the diagram shown, point X indicates the acid-base status of a healthy person at sea level. Point Y indicates data from a patient's arterial blood gas analysis.  Urine chemistry revealed a pH of 4.5.  Which of the following is the most likely cause of the condition indicated by point Y?


  .   Correct Answer  explanation of answer

Adaptation to high altitude

B Chronic obstructive pulmonary disease

C Diabetic ketoacidosis

Infusion of sodium lactate 


Severe prolonged vomiting







 A 43-year-old female with a history of severe diarrhea was admitted to the ER after fainting at the supermarket.  Physical exam revealed decreased sking turgor.  Arterial blood gas analysis revealed pH = 7.21; PCO2 = 26 mmHg; [HCO3-] = 10 mEq/L; K+ = 2.2 mEq/L with normal anion gap .   The correct diagnosis of her condition is:  

      Correct Answer  explanation of answer

A combined metabolic and respiratory acidosis

B metabolic acidosis secondary to hyperkalemia

C metabolic acidosis with respiratory compensation

D respiratory alkalosis with renal compensation


E uncompensated metabolic acidosis




 The patient in question 6 has hypokalemia with a plasma K+ of 2.2 (normal = 4 mEq/L).  The most likely explanation of the hypokalemia is:   Correct Answer  explanation of answer

A cell lysis

B decreased vasopressin (ADH) levels

C exchange of intracellular K+ for extracellular K+

D increased aldosterone levels


E shift of K+ due to hyperosmolarity









 A 46-year-old female patient with epilepsy has a grand mal seizure while in the hospital.  Laboratory values obtained shortly after the seizure included


      arterial blood

         PO2 = 95 mm Hg

         PCO2 = 32 mm Hg

         pH = 7.2



         Na+ = 144 mEq/L

         K+ = 6 mEq/L

         Cl- = 100 mEq/L

         HCO3- = 12 mEq/L


      Which of the following best describes the acid base disturbance


   Correct Answer  explanation of answer

A metabolic acidosis with high anion gap

B metabolic acidosis with normal anion gap

C metabolic acidosis with low anion gap

D respiratory acidosis with high anion gap


E respiratory acidosis with normal anion gap














 A 67-year-old retired NFL player is taken to the emergency room after collapsing in the supermarket.  Paramedics applied CPR

when he was found to have no pulse.  On admission, arterial blood gases were:

pH = 7.0


PCO2 = 60 mm Hg

PO2 = 65 mm Hg

HCO3- = 14 mEq/L


Which of the following best describes the acid base disturbance


   Correct Answer  explanation of answer

A metabolic acidosis

B metabolic alkalosis

C mixed acidosis

D mixed alkalosis


E respiratory acidosis


F respiratory alkalosis 































1. Choice D is the best answer.  1. Vomiting causes loss of stomach acid leading to metabolic alkalosis.  The rise in pH will inhibit the peripheral chemoreceptor for pH located in the carotid bodies leading to hypoventilation (increased PCO2), which is compensatory.   Choices A and B are both acidosis so do not fit with vomiting.  Choice C is alkalosis but includes a normal PCO2 which is not expected given that the peripheral chemoreceptors are rapid responders and 1 hour has elapsed (unless the stem indicated that the patient was on a ventilator where PCO2 is controlled).  Physiological responses to vomiting are summarized in the figure below. The loss of hydrogen ion is accompanied by the loss of chloride leading to a reduction of extracellular fluid volume leading to decreased blood volume and decreased renal perfusion pressure.  This stimulates secretion of renin from the juxtaglomerular cells of afferent arterioles.  Renin is an enzyme that converts angiotensinogen to angiotensin I which is converted by angiotensin converting enzyme (ACE), mostly in the lungs, to angiotensin II which stimulates synthesis of aldosterone in the adrenal cortex.  Aldosterone stimulates hydrogen secretion in distal tubules exacerbating the metabolic alkalosis and to enhanced potassium excretion (also exacerbating metabolic alkalos at the cell level).  Angiotensis II also stimulates Na-H exchange in the proximal convoluted tubule which leads to increased bicarbonate reabsorption augmenting the metabolic alkalosis.


2.  Choice B is the best answer.  The presentation is consistent with ketoacidosis (ketones are volatile acids that are eliminated via both kidneys and lungs).  The overutilization of fats for metabolism leads to ketoacidosis, a metabolic acidosis.  The low pH stimulates the carotid pH receptor leading to hyperventilation (lower PCO2) which is compensatory.  recall that according to the Henderson-Hasselbalch equation, pH = 6.1 + log [HCO3]/PCO2 x αlpha.  Compensation is always aimed at restoring the ratio HCO3/PCO2 back to a normal value, so if HCO3 decreases, PCO2 must decrease via hyperventilation to provide compensation.

Choice A is a mixed acid base disorder with both metabolic and respiratory acidosis (hyperventilation) and not consistent with the history and the pattern of breathing.  Choices C and D are both alkalosis so not consistent with the history, presentation, etc.

3. Choice B is correct.  Hyperaldosteronism increases the activity of most nephron ion transporters including sodium and hydrogen.  In the distal tubule, the effect of high aldosterone is to increase H+ transport into the lumen by H+ ATPase, leading to increased reabsorption of "new" bicarbonate.


Choice A, diarrhea, is wrong because diarrhea results in loss of bicarbonate, secreted into the small intestine. 

Choice C, antifreeze ingestion, is wrong because ethylene glycol is metabolized to glycolic acid (catalyzed by alcohol dehyrogenase) and causes metabolic acidosis.

Choice D, renal failure, is wrong because renal failure normally results in loss of ability of tubule cells, esp. distal cells, to secrete hydrogen ions resulting in metabolic acidosis (figure above illustrates this).

Choice E, aspirin toxicity, is wrong because metabolism of aspirin (acetylsalicylic acid) leads to metabolic acidosis.  Like ethylene glycol toxicity, this metabolic acidosis involves an exogenous unmeasured anion which is measured as an increased anion gap.  Aspiring toxicity is complex and age dependent.  In children the direct stimulation of the respiratory center of the brain results in respiratory alkalosis which may be coupled (dose dependent) with a (compensatory) metabolic acidosis.

Choice F, acetylzolamide, is wrong because inhibition of carbonic anhydrase in the renal tubules results in failure to reabsorb bicarbonate.  This leads to bicarbonate loss in the urine and metabolic acidosis.  Diamox is also a potent diuretic but not used for this anymore due to side effects including metabolic acidosis, hyponatremia, and hypokalemia. One current use is to counter acute mountain sickness from respiratory alkalosis. 

4.  Choice B is correct.  Chronic COPD often leads to CO2 retention (hypercapnia) with renal compensation of bicarbonate retention, leading to pH compensation (partial in this case).

5.  Choice C is correct.  If the pH had been exactly 7.4 it would have been difficult to decide whether the cause was respiratory alkalosis with renal compensation (choice A) OR metabolic acidosis with respiratory compensation (choice C).  The keys were the fact the pH was slightly below 7.4 (compensation is usually not complete and does not overshoot) AND the stem stated that urine pH was acidic.  With respiratory alkalosis, the kidneys are excreting bicarbonate and pH is normally alkaline.

6.  Choice C is correct.  Diarrhea causes loss of intestinal bicarbonate leading to metabolic acidosis.  Reduced arterial pH stimulates carotid body pH receptor leading to compensatory hyperventilation leading to reduced PCO2.  The anion gap is normal in this case because in order to maintain electrical neutrality, chloride replaces the lost bicarbonate.  There are no exogenous unmeasure anions as would be present if the anion gap were increased.  the anion gap, shown below, is calculated from measured cations minus measured anions (K is ignored in most cases).


7.  Choice D is the best answer.  The decrease in skin turgor plus the history of diarrhea indicates dehydration.


dehydration stimulates aldosterone secretion (see below).  cellular exchange is not correct for this case because acidosis causes the opposite result; i.e., H+ shifts into cells and K+ shifts out.  Hyperosmolarity would also cause water and K to come out of cells causing hyperkalemia.  exercise and cell lysis also cause hyperkalemia not hypokalemia.  Insulin and beta agonists can cause hypokalemia but are not listed as choices here.


The contraction of ECF volume leads to decreased renal perfusion pressure and activation of the renin-angiotensin-aldosterone system.  Increased levels of aldosterone causer increased Na reabsorption and increased K secretion as shown below:

stimulation of Na uptake and the Na/K pump leads to increased intracellular K and a greater diffusion gradient for K to diffuse into the lumen.  Also, increased uptake of Na leads to more negative lumen charge facilitating the diffusion of K into the lumen.

8. Choice A is the best answer.  Anion gap is calculated as measured cations - measured anions, so in this case

AG = (144 + 6) - (100 + 12) = 38 mEq/L, much higher than the normal of 10-15.  Seizures cause lactic acidosis, so lactate is the unmeasured anion creating the high anion gap.  Elevated potassium is observed after heavy exercise or, in this case, muscle seizures.

9. Choice C is the answer.  pH 7.1 indicates acidosis.  low bicarbonate indicates a metabolic origin.  normally, there would be respiratory compensation due to stimulation of ventilation by acidosis leading to lowered PCO2.  However, in this case ventilation is depressed (possibly due to general CNS depression from the severe acidosis) and PCO2 is elevated leading to a mixture of metabolic 
and respiratory acidosis.