Acid-base balance Practice Questions

Q: A 41-year-old man presents to urgent care with a 1-week history of severe diarrhea. He says that he has been having watery stools every 2-3 hours. The stools do not contain blood and do not float. On presentation, he is observed to have significant facial flushing, and laboratory tests reveal the following: Serum: Na+: 137 mEq/L K+: 2.7 mEq/L Cl-: 113 mEq/L HCO3-: 14 mEq/L A computed tomography scan reveals a small intra-abdominal mass. Staining of this mass would most likely reveal production of which of the following?

Vasoactive intestinal peptide. ***Vasoactive intestinal peptide*** - The patient's presentation with **severe watery diarrhea**, **hypokalemia**, **metabolic acidosis** (low HCO3-), and **facial flushing** in the presence of an **intra-abdominal mass** is highly indicative of a **VIPoma**. - VIPomas are neuroendocrine tumors that secrete large amounts of **vasoactive intestinal peptide (VIP)**, which stimulates intestinal fluid secretion and inhibits absorption, leading to characteristic symptoms. *Insulin* - **Insulinomas** typically cause symptoms related to **hypoglycemia** (e.g., sweating, palpitations, confusion), which are not described in this patient. - While insulinomas can be associated with an intra-abdominal mass, the patient's symptoms are inconsistent with excessive insulin production. *Glucagon* - **Glucagonomas** classically present with a syndrome including **necrolytic migratory erythema**, **diabetes mellitus**, and weight loss, along with diarrhea in some cases. - The distinct skin rash and hyperglycemia are absent in this patient's presentation. *Somatostatin* - **Somatostatinomas** are often associated with a triad of **diabetes mellitus**, **cholelithiasis**, and **steatorrhea**, and sometimes hypochlorhydria. - The patient's symptoms of severe watery diarrhea and flushing are not typical of somatostatin overproduction. *Gastrin* - **Gastrinomas** (Zollinger-Ellison syndrome) cause severe **peptic ulcer disease** due to excessive gastric acid secretion, often leading to abdominal pain and chronic diarrhea. - While diarrhea can occur, the prominent hypokalemia, metabolic acidosis, and flushing point away from a gastrinoma as the primary diagnosis.

Q: A 67-year-old woman presents to the Emergency Department complaining of weakness and fatigue. She says she caught a “stomach bug” and has not been able to eat anything without vomiting for three days. Past medical history is significant for hyperlipidemia. She takes atorvastatin and a multivitamin daily, except for the last two days due to nausea. Today her heart rate is 106/min, respiratory rate is 16/min, temperature is 37.6°C (99.7°F) and blood pressure of 110/70 mm Hg. On physical examination, her oral mucosa is dry and she looks pale and uncomfortable. She is admitted for care and administered ondansetron. An intravenous infusion of normal saline is also initiated. An arterial blood gas is collected. Which of the following results is expected to be seen in this patient?

pH: 7.48, pCO2: 44 mm Hg, HCO3-: 29 mEq/L. ***pH: 7.48, pCO2: 44 mm Hg, HCO3-: 29 mEq/L*** - The patient's prolonged vomiting leads to a loss of **gastric acid (HCl)**, resulting in an increase in serum bicarbonate (**metabolic alkalosis**), which is reflected by the elevated pH (7.48) and HCO3- (29 mEq/L). - The pCO2 (44 mm Hg) is slightly elevated above normal (40 mm Hg), indicating the expected **respiratory compensation** for metabolic alkalosis (hypoventilation to retain CO2 and normalize pH). *pH: 7.49, pCO2: 33 mm Hg, HCO3-: 18 mEq/L* - This option indicates a **respiratory alkalosis** with partial metabolic compensation, characterized by a high pH, low pCO2, and low HCO3-. - The patient's condition of prolonged vomiting would not lead to an acidic (low HCO3-) or hyperventilatory (low pCO2) state. *pH: 7.31, pCO2: 62 mm Hg, HCO3-: 27 mEq/L* - This result suggests **respiratory acidosis** with partial metabolic compensation, indicated by a low pH, high pCO2, and slightly elevated HCO3-. - Vomiting primarily causes metabolic alkalosis due to loss of acid, not respiratory acidosis. *pH: 7.30, pCO2: 36 mm Hg, HCO3-: 17 mEq/L* - This option points to **metabolic acidosis** with a low pH and low HCO3-, and a near-normal pCO2, indicating minimal respiratory compensation. - While dehydration can occur, the primary acid-base disturbance from prolonged vomiting is a loss of acid, leading to alkalosis, not acidosis. *pH: 7.36, pCO2: 42 mm Hg, HCO3-: 22 mEq/L* - These values fall within the **normal range**, implying no significant acid-base disturbance. - The patient's symptoms of prolonged vomiting and dehydration would inevitably lead to an acid-base imbalance.

Q: A 22-year-old woman presents to the physician due to lightheadedness. Earlier in the day, she had her first job interview since graduating from college 3 months ago. While waiting outside the interviewer’s office, she began to feel nervous and started breathing really fast. She then felt as if she was going to faint. She excused herself from the interview, and requested a friend to drive her to the clinic. Which of the following is responsible for her symptoms?

Decreased arterial pCO2. ***Decreased arterial pCO2*** - The patient's **lightheadedness** and sensation of fainting, coupled with **rapid breathing** during a stressful situation, are classic signs of **hyperventilation syndrome**. - Rapid breathing leads to excessive elimination of carbon dioxide, causing **respiratory alkalosis** and a decrease in arterial pCO2. - Low pCO2 causes **cerebral vasoconstriction**, reducing cerebral blood flow and leading to **lightheadedness and presyncope**. *Increased arterial pO2* - While hyperventilation can slightly increase arterial pO2, this increase generally has **minimal physiological impact** and does not cause lightheadedness or syncope. - The primary physiological consequence in this scenario is due to the **alteration of CO2 levels**, not O2. *Decreased arterial pH* - This symptom describes **acidosis**, which would typically result from conditions like hypoventilation (leading to CO2 retention) or metabolic disturbances. - The patient's rapid breathing indicates **respiratory alkalosis**, which involves an **increased arterial pH**, not decreased. *Increased plasma lactic acid* - While stress can induce some **anaerobic metabolism**, leading to lactic acid production, the primary and most immediate cause of symptoms in acute hyperventilation is not lactic acidosis. - **Lactic acidosis** is associated with more severe metabolic disturbances or sustained intense physical exertion, not acute anxiety-related hyperventilation. *Vagus nerve stimulation* - **Vagal stimulation** can lead to symptoms like lightheadedness and fainting by causing **bradycardia** and **vasodilation** (vasovagal syncope). - However, this is typically associated with specific triggers and leads to a direct cardiovascular response rather than the **rapid breathing** pattern seen in this patient, which points to a respiratory cause.

Question 1

A 44-year-old Caucasian male complains of carpopedal spasms, peri-oral numbness, and paresthesias of the hands and feet. His wife also mentions that he had a seizure not too long ago. His past surgical history is significant for total thyroidectomy due to papillary thyroid carcinoma. They then realized all of the symptoms occurred after the surgery. Which of the following would be present in this patient?

Accepted Answer

Chvostek sign, QT prolongation, decreased PTH, decreased serum calcium, increased serum phosphate

Answer

Chvostek sign, QT prolongation, increased PTH, decreased serum calcium, decreased serum phosphate

Answer

Chvostek sign, QT shortening, increased PTH, increased serum calcium, increased serum phosphate

Answer

Chvostek sign, QT shortening, decreased PTH, decreased serum calcium, increased serum phosphate

Answer

Chvostek sign, QT prolongation, decreased PTH, increased serum calcium, decreased serum phosphate

Question 2

A 41-year-old man presents to urgent care with a 1-week history of severe diarrhea. He says that he has been having watery stools every 2-3 hours. The stools do not contain blood and do not float. On presentation, he is observed to have significant facial flushing, and laboratory tests reveal the following:

Serum:
Na+: 137 mEq/L
K+: 2.7 mEq/L
Cl-: 113 mEq/L
HCO3-: 14 mEq/L

A computed tomography scan reveals a small intra-abdominal mass. Staining of this mass would most likely reveal production of which of the following?

Accepted Answer

Vasoactive intestinal peptide

Answer

Insulin

Answer

Glucagon

Answer

Somatostatin

Answer

Gastrin

Question 3

A 67-year-old woman presents to the Emergency Department complaining of weakness and fatigue. She says she caught a “stomach bug” and has not been able to eat anything without vomiting for three days. Past medical history is significant for hyperlipidemia. She takes atorvastatin and a multivitamin daily, except for the last two days due to nausea. Today her heart rate is 106/min, respiratory rate is 16/min, temperature is 37.6°C (99.7°F) and blood pressure of 110/70 mm Hg. On physical examination, her oral mucosa is dry and she looks pale and uncomfortable. She is admitted for care and administered ondansetron. An intravenous infusion of normal saline is also initiated. An arterial blood gas is collected. Which of the following results is expected to be seen in this patient?

Accepted Answer

pH: 7.48, pCO2: 44 mm Hg, HCO3-: 29 mEq/L

Answer

pH: 7.49, pCO2: 33 mm Hg, HCO3-: 18 mEq/L

Answer

pH: 7.31, pCO2: 62 mm Hg, HCO3-: 27 mEq/L

Answer

pH: 7.30, pCO2: 36 mm Hg, HCO3-: 17 mEq/L

Answer

pH: 7.36, pCO2: 42 mm Hg, HCO3-: 22 mEq/L

Question 4

A 22-year-old woman presents to the physician due to lightheadedness. Earlier in the day, she had her first job interview since graduating from college 3 months ago. While waiting outside the interviewer’s office, she began to feel nervous and started breathing really fast. She then felt as if she was going to faint. She excused herself from the interview, and requested a friend to drive her to the clinic. Which of the following is responsible for her symptoms?

Accepted Answer

Decreased arterial pCO2

Answer

Increased arterial pO2

Answer

Decreased arterial pH

Answer

Increased plasma lactic acid

Answer

Vagus nerve stimulation

Question 5

A 54-year-old man presents with 3 days of non-bloody and non-bilious emesis every time he eats or drinks. He has become progressively weaker and the emesis has not improved. He denies diarrhea, fever, or chills and thinks his symptoms may be related to a recent event that involved sampling many different foods. His temperature is 97.5°F (36.4°C), blood pressure is 133/82 mmHg, pulse is 105/min, respirations are 15/min, and oxygen saturation is 98% on room air. Physical exam is notable for a weak appearing man with dry mucous membranes. His abdomen is nontender. Which of the following laboratory changes would most likely be seen in this patient?

Accepted Answer

Metabolic alkalosis and hypokalemia

Answer

Metabolic alkalosis and hyperkalemia

Answer

Non-anion gap metabolic acidosis and hypokalemia

Answer

Respiratory acidosis and hyperkalemia

Answer

Anion gap metabolic acidosis and hypokalemia

Acid-base balance — MCQs

Acid-base balance — MCQs

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