A 17-year-old girl comes to the emergency department because of numbness around her mouth and uncontrolled twitching of the mouth for the past 30 minutes. Her symptoms began while she was at a concert. Her temperature is 37°C (98.6°F), pulse is 69/min, and respirations are 28/min. When the blood pressure cuff is inflated, painful contractions of the hand muscles occur. Arterial blood gas shows a pH of 7.53, pO2 of 100 mm Hg, and a pCO2 of 29 mm Hg. Which of the following additional findings is most likely in this patient?
Q32
A 24-year-old male is brought in by ambulance to the emergency department after he was found unresponsive at home for an unknown length of time. Upon arrival, he is found to be severely altered and unable to answer questions about his medical history. Based on clinical suspicion, a panel of basic blood tests are obtained including an arterial blood gas, which shows a pH of 7.32, a pCO2 of 70, and a bicarbonate level of 30 mEq/L. Which of the following is most likely the primary disturbance leading to the values found in the ABG?
Q33
A 60-year-old woman is brought to the emergency department by her husband because of worsening shortness of breath over the past 2 days. Last week, she had a sore throat and a low-grade fever. She has coughed up white sputum each morning for the past 2 years. She has hypertension and type 2 diabetes mellitus. She has smoked 2 packs of cigarettes daily for 35 years. Current medications include metformin and lisinopril. On examination, she occasionally has to catch her breath between sentences. Her temperature is 38.1°C (100.6°F), pulse is 85/min, respirations are 16/min, and blood pressure is 140/70 mm Hg. Expiratory wheezes with a prolonged expiratory phase are heard over both lung fields. Arterial blood gas analysis on room air shows:
pH 7.33
PCO2 53 mm Hg
PO2 68 mm Hg
An x-ray of the chest shows hyperinflation of bilateral lung fields and flattening of the diaphragm. Which of the following additional findings is most likely in this patient?
Q34
A 56-year-old woman comes to the physician because of a 2-year-history of intermittent upper abdominal pain that occurs a few hours after meals and occasionally wakes her up in the middle of the night. She reports that the pain is relieved with food intake. Physical examination shows no abnormalities. Endoscopy shows a 0.5 x 0.5 cm ulcer on the posterior wall of the duodenal bulb. A biopsy specimen obtained from the edge of the ulcer shows hyperplasia of submucosal glandular structures. Hyperplasia of these cells most likely results in an increase of which of the following?
Q35
A 58-year-old man presents to the emergency department with a chief complaint of ringing in his ears that started several hours previously that has progressed to confusion. The patient denies any history of medical problems except for bilateral knee arthritis. He was recently seen by an orthopedic surgeon to evaluate his bilateral knee arthritis but has opted to not undergo knee replacement and prefers medical management. His wife noted that prior to them going on a hike today, he seemed confused and not himself. They decided to stay home, and roughly 14 hours later, he was no longer making any sense. Physical exam is notable for a confused man. The patient's vitals are being performed and his labs are being drawn. Which of the following is most likely to be seen on blood gas analysis?
Q36
A 47-year-old man with a history of alcoholism undergoes an upper endoscopy, which reveals a superficial mucosal tear in the distal esophagus. Laboratory results show a metabolic alkalosis. What is the most likely mechanism of the acid/base disturbance in this patient?
Acid-base balance US Medical PG Practice Questions and MCQs
Question 31: A 17-year-old girl comes to the emergency department because of numbness around her mouth and uncontrolled twitching of the mouth for the past 30 minutes. Her symptoms began while she was at a concert. Her temperature is 37°C (98.6°F), pulse is 69/min, and respirations are 28/min. When the blood pressure cuff is inflated, painful contractions of the hand muscles occur. Arterial blood gas shows a pH of 7.53, pO2 of 100 mm Hg, and a pCO2 of 29 mm Hg. Which of the following additional findings is most likely in this patient?
A. Increased serum potassium concentration
B. Increased serum phosphate concentration
C. Decreased serum ionized calcium concentration (Correct Answer)
D. Increased peripheral oxygen unloading from hemoglobin
E. Decreased cerebral blood flow
Explanation: ***Decreased serum ionized calcium concentration***
- The patient presents with **circumoral numbness** and **muscle twitching** (consistent with tetany), and a positive **Trousseau's sign** (painful contractions of hand muscles with blood pressure cuff inflation). These are classic signs of **hypocalcemia**.
- The **arterial blood gas** shows **respiratory alkalosis** (elevated pH 7.53, decreased pCO2 29 mm Hg). In alkalosis, more calcium binds to albumin, causing a **decrease in ionized (free) calcium** even when total calcium is normal. It is the ionized calcium that is physiologically active and responsible for neuromuscular function.
- The **decreased ionized calcium** directly causes the tetany and neuromuscular irritability observed in this patient.
*Increased serum potassium concentration*
- **Hyperkalemia** typically presents with muscle weakness, fatigue, and cardiac arrhythmias, not tetany or circumoral numbness.
- The symptoms described are not characteristic of high potassium levels.
*Increased serum phosphate concentration*
- **Hyperphosphatemia** can lead to decreased serum calcium due to the formation of calcium-phosphate complexes, but it is not the primary direct cause of the symptoms in the setting of acute respiratory alkalosis.
- Furthermore, hyperphosphatemia itself does not directly cause muscle twitching or circumoral numbness as a primary effect.
*Increased peripheral oxygen unloading from hemoglobin*
- **Increased oxygen unloading** from hemoglobin (a right shift of the oxygen dissociation curve) is typically associated with acidosis, fever, or increased 2,3-BPG.
- **Alkalosis** (as seen in this patient) causes a **left shift** of the oxygen dissociation curve, leading to **decreased oxygen unloading** in the periphery.
*Decreased cerebral blood flow*
- **Hyperventilation** causes **respiratory alkalosis**, which in turn leads to **cerebral vasoconstriction** and therefore **decreased cerebral blood flow**.
- While this is a true physiological consequence of the patient's hyperventilation, it does not directly explain the **tetany** and **circumoral numbness** (which are due to decreased ionized calcium). The neuromuscular symptoms are specifically caused by hypocalcemia, making ionized calcium the most relevant additional finding.
Question 32: A 24-year-old male is brought in by ambulance to the emergency department after he was found unresponsive at home for an unknown length of time. Upon arrival, he is found to be severely altered and unable to answer questions about his medical history. Based on clinical suspicion, a panel of basic blood tests are obtained including an arterial blood gas, which shows a pH of 7.32, a pCO2 of 70, and a bicarbonate level of 30 mEq/L. Which of the following is most likely the primary disturbance leading to the values found in the ABG?
A. Respiratory acidosis (Correct Answer)
B. Metabolic alkalosis
C. Respiratory alkalosis
D. Metabolic acidosis
E. Mixed alkalosis
Explanation: ***Respiratory acidosis***
- The **pH (7.32)** is acidic (normal 7.35-7.45), and the **pCO2 (70 mmHg)** is significantly elevated (normal 35-45 mmHg), indicating **primary respiratory acidosis** due to hypoventilation.
- The **bicarbonate (30 mEq/L)** is elevated above normal (22-26 mEq/L), indicating **partial metabolic compensation** by the kidneys retaining bicarbonate to buffer the acidosis.
- This pattern suggests **chronic respiratory acidosis** (e.g., from COPD, CNS depression, neuromuscular disease) with renal compensation.
*Metabolic alkalosis*
- This would present with **elevated pH** (>7.45) and **elevated bicarbonate** as the primary disturbance, often with compensatory elevation in pCO2.
- The patient's **pH is acidic (7.32)**, not alkalotic, ruling out metabolic alkalosis as the primary process.
*Respiratory alkalosis*
- This would present with **elevated pH** (>7.45) and **decreased pCO2** (<35 mmHg) due to hyperventilation.
- The patient has the opposite: **acidic pH and elevated pCO2**, ruling out respiratory alkalosis.
*Metabolic acidosis*
- This would present with **decreased pH** and **decreased bicarbonate** (<22 mEq/L) as the primary disturbance.
- While the pH is low, the **bicarbonate is elevated (30 mEq/L)**, not decreased, ruling out metabolic acidosis as the primary disorder.
*Mixed alkalosis*
- A mixed alkalosis would involve simultaneous respiratory and metabolic processes causing **elevated pH**.
- The patient's **pH is acidic (7.32)**, making any form of alkalosis impossible as the primary disturbance.
Question 33: A 60-year-old woman is brought to the emergency department by her husband because of worsening shortness of breath over the past 2 days. Last week, she had a sore throat and a low-grade fever. She has coughed up white sputum each morning for the past 2 years. She has hypertension and type 2 diabetes mellitus. She has smoked 2 packs of cigarettes daily for 35 years. Current medications include metformin and lisinopril. On examination, she occasionally has to catch her breath between sentences. Her temperature is 38.1°C (100.6°F), pulse is 85/min, respirations are 16/min, and blood pressure is 140/70 mm Hg. Expiratory wheezes with a prolonged expiratory phase are heard over both lung fields. Arterial blood gas analysis on room air shows:
pH 7.33
PCO2 53 mm Hg
PO2 68 mm Hg
An x-ray of the chest shows hyperinflation of bilateral lung fields and flattening of the diaphragm. Which of the following additional findings is most likely in this patient?
A. Decreased urinary bicarbonate excretion (Correct Answer)
B. Decreased urinary chloride concentration
C. Increased serum anion gap
D. Increased urine osmolar gap
E. Increased urinary pH
Explanation: ***Decreased urinary bicarbonate excretion***
- The patient's ABG results (pH 7.33, PCO2 53 mmHg) indicate **chronic respiratory acidosis**, consistent with a **COPD exacerbation** on a background of chronic disease.
- In chronic respiratory acidosis, the kidneys compensate by **retaining bicarbonate** (increasing reabsorption) and **excreting hydrogen ions** to normalize pH.
- Therefore, urinary bicarbonate excretion is **decreased** as the kidneys conserve bicarbonate to buffer the chronic acidosis.
*Decreased urinary chloride concentration*
- This is typically seen in states of **metabolic alkalosis** (with volume contraction) or profound **volume depletion**, neither of which is the primary condition here.
- The patient has respiratory acidosis, not metabolic alkalosis.
*Increased serum anion gap*
- An increased anion gap indicates **metabolic acidosis** due to accumulation of unmeasured anions (e.g., lactate, ketones, toxins).
- The patient has **respiratory acidosis**, not metabolic acidosis with an anion gap.
- The anion gap is not directly affected by primary respiratory disorders.
*Increased urine osmolar gap*
- An increased urine osmolar gap suggests the presence of **unmeasured osmolytes** in the urine (e.g., from methanol or ethylene glycol ingestion).
- There is nothing in the patient's presentation to suggest toxic ingestion.
*Increased urinary pH*
- Increased urinary pH would occur if the kidneys were **excreting bicarbonate**, which happens in metabolic alkalosis or renal tubular acidosis.
- In chronic respiratory acidosis, the kidneys compensate by **excreting acid** (lowering urinary pH) and **retaining bicarbonate**.
- Therefore, urinary pH would be **decreased**, not increased.
Question 34: A 56-year-old woman comes to the physician because of a 2-year-history of intermittent upper abdominal pain that occurs a few hours after meals and occasionally wakes her up in the middle of the night. She reports that the pain is relieved with food intake. Physical examination shows no abnormalities. Endoscopy shows a 0.5 x 0.5 cm ulcer on the posterior wall of the duodenal bulb. A biopsy specimen obtained from the edge of the ulcer shows hyperplasia of submucosal glandular structures. Hyperplasia of these cells most likely results in an increase of which of the following?
A. Bicarbonate secretion (Correct Answer)
B. Antigen presentation
C. Glycoprotein synthesis
D. Hydrochloric acid secretion
E. Lysozyme secretion
Explanation: ***Bicarbonate secretion***
- The symptoms of pain relieved by food intake and an ulcer in the **duodenal bulb** are classic for a **duodenal ulcer**
- Duodenal ulcers are often due to an imbalance between protective factors and aggressive factors in the duodenum; hyperplasia of submucosal glandular structures, specifically **Brunner's glands**, represents an attempt to increase **bicarbonate secretion** and protect against acid
- Brunner's glands secrete alkaline mucus rich in bicarbonate to neutralize gastric acid entering the duodenum, which is a key protective mechanism
*Antigen presentation*
- **Antigen presentation** is primarily a function of immune cells (e.g., macrophages, dendritic cells) and is not directly related to the function of Brunner's glands or the pathogenesis of duodenal ulcers
- While immune cells are present in the gastrointestinal tract, the hyperplasia described is not linked to an increase in antigen presentation
*Glycoprotein synthesis*
- While **mucosal cells** in the gastrointestinal tract do synthesize glycoproteins (e.g., mucins) for protection, the hyperplasia of **submucosal glandular structures** (Brunner's glands) is specifically associated with bicarbonate and mucin secretion, with **bicarbonate being the primary protective mechanism**
- An increase in glycoprotein synthesis, while part of mucosal defense, is not the most direct or primary consequence of Brunner's gland hyperplasia as a compensatory mechanism for duodenal ulcers
*Hydrochloric acid secretion*
- **Hydrochloric acid (HCl)** is secreted by **parietal cells** in the stomach, not by glands in the duodenum
- An increase in HCl secretion would worsen a duodenal ulcer, not be a protective response
*Lysozyme secretion*
- **Lysozyme** is an enzyme with antimicrobial properties found in various secretions (e.g., tears, saliva, phagocytes) and some glandular cells in the intestines (e.g., Paneth cells)
- While it plays a role in innate immunity, it is not the primary secretion associated with the protective function of **Brunner's glands** in response to duodenal ulcers
Question 35: A 58-year-old man presents to the emergency department with a chief complaint of ringing in his ears that started several hours previously that has progressed to confusion. The patient denies any history of medical problems except for bilateral knee arthritis. He was recently seen by an orthopedic surgeon to evaluate his bilateral knee arthritis but has opted to not undergo knee replacement and prefers medical management. His wife noted that prior to them going on a hike today, he seemed confused and not himself. They decided to stay home, and roughly 14 hours later, he was no longer making any sense. Physical exam is notable for a confused man. The patient's vitals are being performed and his labs are being drawn. Which of the following is most likely to be seen on blood gas analysis?
Explanation: ***pH: 7.30, PaCO2: 15 mmHg, HCO3-: 16 mEq/L***
- This blood gas analysis shows a **low pH** (acidemia), **low PaCO2** (hypocapnia), and **low HCO3-** (bicarbonate). This pattern is consistent with a **primary metabolic acidosis** with a **compensatory respiratory alkalosis**.
- In this clinical scenario, the patient likely has **salicylate toxicity** (aspirin poisoning). Salicylate toxicity initially causes respiratory alkalosis due to direct stimulation of the respiratory center, followed by a high anion gap metabolic acidosis as salicylates interfere with cellular metabolism. This specific ABG reflects a mixed disorder where metabolic acidosis is predominant and respiratory compensation is attempting to raise the pH. The **tinnitus** and **confusion** are classic symptoms of salicylate toxicity.
*pH: 7.37, PaCO2: 41 mmHg, HCO3-: 12 mEq/L*
- This blood gas shows a **normal pH**, **normal PaCO2**, and **low HCO3-**. This suggests a **compensated metabolic acidosis**, where the body has fully compensated to bring the pH back to normal.
- While the patient likely has metabolic acidosis from salicylate toxicity, full compensation to a normal pH is less characteristic of an acute, severe presentation with significant neurological symptoms.
*pH: 7.41, PaCO2: 65 mmHg, HCO3-: 34 mEq/L*
- This blood gas shows a **normal pH**, **high PaCO2**, and **high HCO3-**. This indicates a **compensated respiratory acidosis**, where the kidneys have compensated for chronic CO2 retention.
- This pattern is not consistent with salicylate toxicity, which typically causes **respiratory alkalosis** early on, and later **metabolic acidosis**.
*pH: 7.47, PaCO2: 11 mmHg, HCO3-: 24 mEq/L*
- This blood gas analysis shows a **high pH** (alkalemia), **very low PaCO2** (severe hypocapnia), and a **normal HCO3-**. This indicates a **primary respiratory alkalosis** with no significant metabolic compensation.
- While salicylate toxicity can cause respiratory alkalosis, severe confusion and the progression of symptoms suggest a more advanced stage, usually involving a metabolic acidosis component, making a pure, uncompensated respiratory alkalosis less likely.
*pH: 7.31, PaCO2: 31 mmHg, HCO3-: 15 mEq/L*
- This blood gas shows a **low pH**, **low PaCO2**, and **low HCO3-**. This also indicates a **metabolic acidosis** with **respiratory compensation**.
- However, compared to pH 7.30, PaCO2 15 mmHg, and HCO3- 16 mEq/L, this option shows slightly **less severe respiratory compensation** (PaCO2 is higher), which is less typical for the profound respiratory stimulation seen in severe salicylate poisoning. The chosen correct option demonstrates a more characteristic and maximal respiratory compensation for the degree of metabolic acidosis.
Question 36: A 47-year-old man with a history of alcoholism undergoes an upper endoscopy, which reveals a superficial mucosal tear in the distal esophagus. Laboratory results show a metabolic alkalosis. What is the most likely mechanism of the acid/base disturbance in this patient?
A. Hepatic cirrhosis
B. Hypokalemia
C. B12 deficiency
D. Anemia
E. Vomiting (Correct Answer)
Explanation: ***Vomiting***
- The superficial mucosal tear in the distal esophagus ("Mallory-Weiss tear") is strongly associated with **forceful vomiting**, which can lead to significant loss of gastric acid.
- Loss of gastric acid (HCl) through vomiting causes a **metabolic alkalosis** as hydrogen ions are excreted, and the kidneys compensate by retaining bicarbonate.
*Hepatic cirrhosis*
- While common in alcoholics, **hepatic cirrhosis** typically leads to **metabolic acidosis** due to impaired lactate metabolism and renal dysfunction, rather than alkalosis.
- It would not directly explain the acute esophageal tear or the direct cause of metabolic alkalosis seen here.
*Hypokalemia*
- **Hypokalemia** can result from vomiting and can perpetuate metabolic alkalosis, but it is a consequence or a contributing factor, not the primary mechanism of acid-base disturbance.
- The initial loss of acid through vomiting is the direct cause of the alkalosis, which then often leads to compensatory hypokalemia.
*B12 deficiency*
- **B12 deficiency** is common in alcoholics but primarily causes **macrocytic anemia** and neurological symptoms, not metabolic alkalosis or esophageal tears.
- It has no direct physiological link to acid-base balance in a way that would cause metabolic alkalosis.
*Anemia*
- **Anemia** can be caused by chronic alcoholism or blood loss from the esophageal tear, but it does not directly lead to **metabolic alkalosis**.
- While blood loss can have various systemic effects, it does not involve the loss of gastric acid that defines a vomiting-induced alkalosis.
