A 32-year-old woman is admitted to the emergency department for 36 hours of intense left-sided back pain that extends into her left groin. She reports that the pain started a day after a charitable 5 km (3.1 mi) marathon. The past medical history is relevant for multiple complaints of eye dryness and dry mouth. Physical examination is unremarkable, except for intense left-sided costovertebral pain. The results from laboratory tests are shown.
Laboratory test Result
Serum Na+ 137
Serum Cl- 110
Serum K+ 3.0
Serum creatinine (SCr) 0.82
Arterial blood gas Result
pH 7.28
pO2 98 mm Hg
pCO2 28.5 mm Hg
SaO2% 98%
HCO3- 15 mm Hg
Which of the following explains this patient’s condition?
Q12
A 3-month-old girl is brought to the physician because of poor feeding, irritability and vomiting for 2 weeks. She was born at 36 weeks' gestation and pregnancy was uncomplicated. She is at 5th percentile for length and at 3rd percentile for weight. Her temperature is 36.8°C (98.2°F), pulse is 112/min and respirations are 49/min. Physical and neurologic examinations show no other abnormalities. Laboratory studies show:
Serum
Na+ 138 mEq/L
K+ 3.1 mEq/L
Cl- 115 mEq/L
Ammonia 23 μmol/L (N <50 μmol/L)
Urine
pH 6.9
Blood negative
Glucose negative
Protein negative
Arterial blood gas analysis on room air shows:
pH 7.28
pO2 96 mm Hg
HCO3- 12 mEq/L
Which of the following is the most likely cause of these findings?
Q13
A 37-year-old G1P0 woman presents to her primary care physician for a routine checkup. She has a history of diabetes and hypertension but has otherwise been healthy with no change in her health status since the last visit. She is expecting her first child 8 weeks from now. She also enrolled in a study about pregnancy where serial metabolic panels and arterial blood gases are obtained. Partial results from these studies are shown below:
Serum:
Na+: 141 mEq/L
Cl-: 108 mEq/L
pH: 7.47
pCO2: 30 mmHg
HCO3-: 21 mEq/L
Which of the following disease processes would most likely present with a similar panel of metabolic results?
Q14
A 62-year-old woman is brought to the emergency department because of the sudden onset of severe left eye pain, blurred vision, nausea, and vomiting. She has had an upper respiratory tract infection for the past 2 days and has been taking phenylephrine to control symptoms. Examination shows a rock-hard, injected left globe and a fixed, mid-dilated pupil on the left. Gonioscopy shows that the iris meets the cornea at an angle of 10° (N = 20–45°). Systemic pharmacotherapy is initiated. Which of the following is most likely to occur in this patient?
Q15
During a study on gastrointestinal hormones, a volunteer is administered the hormone secreted by S cells. Which of the following changes most likely represent the effect of this hormone on gastric and duodenal secretions?
$$$ Gastric H+ %%% Duodenal HCO3- %%% Duodenal Cl- $$$
Q16
A person is exercising strenuously on a treadmill for 1 hour. An arterial blood gas measurement is then taken. Which of the following are the most likely values?
Q17
Two hours after undergoing laparoscopic roux-en-Y gastric bypass surgery, a 44-year-old man complains of pain in the site of surgery and nausea. He has vomited twice in the past hour. He has hypertension, type 2 diabetes mellitus, and hypercholesterolemia. Current medications include insulin, atorvastatin, hydrochlorothiazide, acetaminophen, and prophylactic subcutaneous heparin. He drinks two to three beers daily and occasionally more on weekends. He is 177 cm (5 ft 10 in) tall and weighs 130 kg (286 lb); BMI is 41.5 kg/m2. His temperature is 37.3°C (99.1°F), pulse is 103/min, and blood pressure is 122/82 mm Hg. Examination shows five laparoscopic incisions with no erythema or discharge. The abdomen is soft and non-distended. There is slight diffuse tenderness to palpation. Bowel sounds are reduced. Laboratory studies show:
Hematocrit 45%
Serum
Na+ 136 mEq/L
K+ 3.5 mEq/L
Cl- 98 mEq/L
Urea nitrogen 31 mg/dL
Glucose 88 mg/dL
Creatinine 1.1 mg/dL
Arterial blood gas analysis on room air shows:
pH 7.28
pCO2 32 mm Hg
pO2 74 mm Hg
HCO3- 14.4 mEq/L
Which of the following is the most likely cause for the acid-base status of this patient?
Q18
A 25-year-old woman with an extensive psychiatric history is suspected of having metabolic acidosis after ingesting a large amount of aspirin in a suicide attempt. Labs are drawn and the values from the ABG are found to be: PCO2: 25, and HCO3: 15, but the pH value is smeared on the print-out and illegible. The medical student is given the task of calculating the pH using the pCO2 and HCO3 concentrations. He recalls from his first-year physiology course that the pKa of relevance for the bicarbonate buffering system is approximately 6.1. Which of the following is the correct formula the student should use, using the given values from the incomplete ABG?
Q19
A 66-year-old man is brought to the emergency department by his daughter because of 3 days of fever, chills, cough, and shortness of breath. The cough is productive of yellow sputum. His symptoms have not improved with rest and guaifenesin. His past medical history is significant for hypertension, for which he takes hydrochlorothiazide. He has a 30-pack-year history of smoking. His temperature is 38.9 C (102.0 F), blood pressure 88/56 mm Hg, and heart rate 105/min. Following resuscitation with normal saline, his blood pressure improves to 110/70 mm Hg. His arterial blood gas is as follows:
Blood pH 7.52, PaO2 74 mm Hg, PaCO2 28 mm Hg, and HCO3- 21 mEq/L.
Which of the following acid-base disturbances best characterizes this patient's condition?
Q20
A 57-year-old male presents to his primary care physician with upper abdominal pain. He reports a 3-month history of mild epigastric pain that improves with meals. He has lost 15 pounds since his symptoms started. His past medical history is notable for gynecomastia in the setting of a prolactinoma for which he underwent surgical resection over 10 years prior. He has a 15-pack-year smoking history, a history of heroin abuse, and is on methadone. His family history is notable for parathyroid adenoma in his father. His temperature is 98.8°F (37.1°C), blood pressure is 125/80 mmHg, pulse is 78/min, and respirations are 18/min. This patient’s symptoms are most likely due to elevations in a substance with which of the following functions?
Acid-base balance US Medical PG Practice Questions and MCQs
Question 11: A 32-year-old woman is admitted to the emergency department for 36 hours of intense left-sided back pain that extends into her left groin. She reports that the pain started a day after a charitable 5 km (3.1 mi) marathon. The past medical history is relevant for multiple complaints of eye dryness and dry mouth. Physical examination is unremarkable, except for intense left-sided costovertebral pain. The results from laboratory tests are shown.
Laboratory test Result
Serum Na+ 137
Serum Cl- 110
Serum K+ 3.0
Serum creatinine (SCr) 0.82
Arterial blood gas Result
pH 7.28
pO2 98 mm Hg
pCO2 28.5 mm Hg
SaO2% 98%
HCO3- 15 mm Hg
Which of the following explains this patient’s condition?
A. Carbonic acid accumulation
B. Decreased bicarbonate renal absorption
C. Decreased renal excretion of hydrogen ions (H+) (Correct Answer)
D. Decreased synthesis of ammonia (NH3)
E. Decreased excretion of nonvolatile acids
Explanation: ***Decreased renal excretion of hydrogen ions (H+)***
- The patient presents with **metabolic acidosis** (pH 7.28, HCO3- 15 mEq/L) with **respiratory compensation** (pCO2 28.5 mm Hg). The anion gap is **normal** (Na+ - (Cl- + HCO3-) = 137 - (110 + 15) = **12 mEq/L**), indicating a **non-anion gap metabolic acidosis**.
- The history of **dry eyes and dry mouth** strongly suggests **Sjögren syndrome**, which is commonly associated with **Type 1 (distal) renal tubular acidosis**.
- In **Type 1 RTA**, the distal tubule alpha-intercalated cells cannot adequately secrete H+ ions, leading to metabolic acidosis with **inability to acidify urine** (urine pH > 5.5). Associated findings include **hypokalemia** (K+ 3.0), **nephrolithiasis** (calcium phosphate stones due to alkaline urine), and hypercalciuria.
- The left-sided flank pain radiating to the groin is consistent with **nephrolithiasis**, a common complication of Type 1 RTA.
*Carbonic acid accumulation*
- **Carbonic acid accumulation** indicates **respiratory acidosis** with elevated pCO2, which is not present here.
- The patient has a **low pCO2 (28.5 mm Hg)**, representing appropriate **respiratory compensation** for the primary metabolic acidosis.
*Decreased bicarbonate renal absorption*
- **Decreased bicarbonate renal absorption** characterizes **Type 2 (proximal) RTA**.
- While Type 2 RTA also causes non-anion gap metabolic acidosis, it is **not typically associated with Sjögren syndrome** and would present with different features (glycosuria, aminoaciduria, phosphaturia as part of Fanconi syndrome).
- Type 2 RTA can acidify urine to pH < 5.5 when serum HCO3- is low, unlike Type 1 RTA.
*Decreased synthesis of ammonia (NH3)*
- **Decreased ammonia synthesis** is characteristic of **Type 4 RTA** or severe chronic kidney disease.
- Type 4 RTA presents with **hyperkalemia** (due to hypoaldosteronism), not the hypokalemia seen in this patient.
- The normal serum creatinine (0.82 mg/dL) rules out significant renal failure.
*Decreased excretion of nonvolatile acids*
- **Decreased excretion of nonvolatile acids** would cause **elevated anion gap metabolic acidosis** (e.g., lactic acidosis, ketoacidosis, or advanced renal failure with accumulation of organic acids).
- This patient has a **normal anion gap (12 mEq/L)** and **normal renal function** (creatinine 0.82 mg/dL), making this mechanism unlikely.
- The clinical context of Sjögren syndrome with dry eyes/mouth points specifically to distal RTA.
Question 12: A 3-month-old girl is brought to the physician because of poor feeding, irritability and vomiting for 2 weeks. She was born at 36 weeks' gestation and pregnancy was uncomplicated. She is at 5th percentile for length and at 3rd percentile for weight. Her temperature is 36.8°C (98.2°F), pulse is 112/min and respirations are 49/min. Physical and neurologic examinations show no other abnormalities. Laboratory studies show:
Serum
Na+ 138 mEq/L
K+ 3.1 mEq/L
Cl- 115 mEq/L
Ammonia 23 μmol/L (N <50 μmol/L)
Urine
pH 6.9
Blood negative
Glucose negative
Protein negative
Arterial blood gas analysis on room air shows:
pH 7.28
pO2 96 mm Hg
HCO3- 12 mEq/L
Which of the following is the most likely cause of these findings?
A. Deficiency of ornithine transcarbamylase
B. Impaired metabolism of branched-chain amino acids
C. Inability of the distal tubule to secrete H+ (Correct Answer)
D. Impaired CFTR gene function
E. Deficiency of 21-hydroxylase
Explanation: ***Inability of the distal tubule to secrete H+***
- The patient presents with **non-anion gap metabolic acidosis** (pH 7.28, HCO3- 12 mEq/L, anion gap = 138 - (115 + 12) = 11), **hypokalemia** (K+ 3.1 mEq/L), and **inappropriately alkaline urine** (pH 6.9). These findings are classic for **distal (Type 1) renal tubular acidosis (RTA)**, where the distal tubules cannot excrete H+ adequately, leading to systemic acidosis.
- The symptoms of **poor feeding, irritability, vomiting**, and **poor growth** are common manifestations of chronic acidosis in infants.
*Deficiency of ornithine transcarbamylase*
- This urea cycle disorder would lead to **hyperammonemia** (ammonia >50 μmol/L), which is not present here (ammonia 23 μmol/L).
- While it can cause metabolic acidosis due to accumulation of organic acids in severe cases, the primary biochemical derangement is hyperammonemia, and the findings of an alkaline urine pH with metabolic acidosis are not typical.
*Impaired metabolism of branched-chain amino acids*
- This condition, known as **Maple Syrup Urine Disease**, primarily presents with a characteristic sweet odor to the urine and neurological deterioration due to the accumulation of branched-chain amino acids and their ketoacids.
- It would typically cause a significant **anion gap metabolic acidosis** and not an alkaline urine pH.
*Impaired CFTR gene function*
- This describes **Cystic Fibrosis**, which primarily affects exocrine glands, leading to symptoms like **malabsorption**, recurrent pulmonary infections, and meconium ileus in neonates.
- It does not typically cause the specific electrolyte and acid-base disturbances seen here (non-anion gap metabolic acidosis with hypokalemia and alkaline urine).
*Deficiency of 21-hydroxylase*
- This enzyme deficiency causes **Congenital Adrenal Hyperplasia (CAH)**. The most common form leads to **salt-wasting crises** with hyponatremia and hyperkalemia.
- It would not typically present with hypokalemia and non-anion gap metabolic acidosis with alkaline urine pH.
Question 13: A 37-year-old G1P0 woman presents to her primary care physician for a routine checkup. She has a history of diabetes and hypertension but has otherwise been healthy with no change in her health status since the last visit. She is expecting her first child 8 weeks from now. She also enrolled in a study about pregnancy where serial metabolic panels and arterial blood gases are obtained. Partial results from these studies are shown below:
Serum:
Na+: 141 mEq/L
Cl-: 108 mEq/L
pH: 7.47
pCO2: 30 mmHg
HCO3-: 21 mEq/L
Which of the following disease processes would most likely present with a similar panel of metabolic results?
A. Diarrheal disease
B. Loop diuretic abuse
C. Living at high altitude (Correct Answer)
D. Ingestion of metformin
E. Anxiety attack
Explanation: ***Living at high altitude***
- Chronic exposure to **high altitude** leads to sustained **hypoxia**, which stimulates **hyperventilation** as a compensatory mechanism.
- This persistent hyperventilation causes a **respiratory alkalosis** (high pH, low pCO2) and a compensatory **metabolic acidosis** (low HCO3-) to normalize pH, mimicking the presented metabolic panel.
*Diarrheal disease*
- Severe **diarrhea** leads to the loss of bicarbonate from the gastrointestinal tract, causing a **non-anion gap metabolic acidosis**.
- This would present with a **low pH**, **low HCO3-**, and a **compensatory drop in pCO2**, not a respiratory alkalosis with a high pH.
*Loop diuretic abuse*
- Chronic abuse of **loop diuretics** can cause **metabolic alkalosis** due to increased renal excretion of hydrogen ions and potassium, leading to volume contraction.
- This would typically present with a **high pH**, high HCO3-, and a compensatory rise in pCO2, which is different from the given values.
*Ingestion of metformin*
- **Metformin** can cause **lactic acidosis** (a type of high anion gap metabolic acidosis), especially in patients with renal impairment.
- This would manifest as a **low pH**, **low HCO3-**, and a **compensatory decrease in pCO2**, along with an elevated anion gap, not the respiratory alkalosis seen here.
*Anxiety attack*
- An **anxiety attack** causes acute **hyperventilation**, leading to **acute respiratory alkalosis** (high pH, low pCO2).
- However, in an acute setting, there is insufficient time for significant renal compensation, so the HCO3- would remain near normal, unlike the compensated state shown in the panel.
Question 14: A 62-year-old woman is brought to the emergency department because of the sudden onset of severe left eye pain, blurred vision, nausea, and vomiting. She has had an upper respiratory tract infection for the past 2 days and has been taking phenylephrine to control symptoms. Examination shows a rock-hard, injected left globe and a fixed, mid-dilated pupil on the left. Gonioscopy shows that the iris meets the cornea at an angle of 10° (N = 20–45°). Systemic pharmacotherapy is initiated. Which of the following is most likely to occur in this patient?
A. Epithelial keratopathy
B. Xerostomia (Correct Answer)
C. Metabolic acidosis
D. Bradycardia
E. Diaphoresis
Explanation: ***Xerostomia***
- The patient is experiencing an **acute angle-closure glaucoma (AACG) attack**, precipitated by **phenylephrine**, an alpha-1 adrenergic agonist causing pupillary dilation.
- The systemic pharmacotherapy for AACG includes **acetazolamide**, a carbonic anhydrase inhibitor, to reduce aqueous humor production.
- **Xerostomia** (dry mouth) is one of the **most common and immediately noticeable side effects** of acetazolamide, making it the most likely symptom the patient will experience.
- Other common side effects include paresthesias (tingling in fingers/toes) and altered taste.
*Epithelial keratopathy*
- While ocular surface changes can occur in AACG due to elevated intraocular pressure, **epithelial keratopathy** specifically refers to corneal epithelial damage.
- This is not a side effect of the systemic pharmacotherapy (acetazolamide) but rather a potential complication of the disease itself.
- It is more commonly associated with conditions like dry eye or corneal exposure.
*Metabolic acidosis*
- Carbonic anhydrase inhibitors like acetazolamide **commonly cause non-anion gap metabolic acidosis** (type 2 renal tubular acidosis) by inhibiting carbonic anhydrase in the renal proximal tubules, impairing bicarbonate reabsorption.
- However, this typically develops over hours to days and is often **asymptomatic or minimally symptomatic** in the acute setting.
- While metabolic acidosis is a well-recognized side effect, **xerostomia is more immediately noticeable** to the patient, making it the better answer to "most likely to occur."
*Bradycardia*
- Beta-blockers (e.g., timolol) may be used **topically** for glaucoma management, but systemic absorption from topical drops is minimal.
- The primary initial **systemic** treatment for AACG is acetazolamide, which does not cause bradycardia.
- Bradycardia would be a concern if systemic beta-blockers were used, but this is not standard for acute management.
*Diaphoresis*
- Sweating (**diaphoresis**) is not a recognized side effect of acetazolamide or other medications typically used for acute angle-closure glaucoma.
- The patient may experience diaphoresis from pain and nausea related to the acute glaucoma itself, but not from the pharmacotherapy.
Question 15: During a study on gastrointestinal hormones, a volunteer is administered the hormone secreted by S cells. Which of the following changes most likely represent the effect of this hormone on gastric and duodenal secretions?
$$$ Gastric H+ %%% Duodenal HCO3- %%% Duodenal Cl- $$$
A. ↓ ↓ ↓
B. ↑ ↓ no change
C. ↑ ↑ ↓
D. ↓ ↑ ↓ (Correct Answer)
E. ↓ no change no change
Explanation: ***↓ ↑ ↓***
- S cells secrete **secretin**, which primarily inhibits **gastric acid (H+) secretion** to protect the duodenal mucosa from acidic chyme.
- Secretin also stimulates the pancreas and bile ducts to secrete **bicarbonate (HCO3-)**, neutralizing the acidic chyme. Duodenal **chloride (Cl-) secretion is typically reduced** as it is often exchanged for bicarbonate or water follows bicarbonate secretion for osmotic balance.
*↓ ↓ ↓*
- While **gastric H+ decreases** due to secretin, **duodenal HCO3- secretion increases**, not decreases, making this option incorrect.
- **Duodenal Cl- secretion** would likely decrease, but the other components are inconsistent with secretin's effects.
*↑ ↓ no change*
- Secretin **inhibits gastric H+ secretion**, so an increase contradicts its primary function to protect the duodenum from acid.
- **Duodenal HCO3- secretion increases**, not decreases, and no change in Cl- is unlikely given the physiological responses to secretin.
*↑ ↑ ↓*
- Secretin **inhibits gastric H+ secretion**, so an increase is incorrect.
- While **duodenal HCO3- increases** and **Cl- decreases**, the initial change in gastric H+ makes this option wrong.
*↓ no change no change*
- While **gastric H+ is indeed decreased**, secretin significantly **increases duodenal HCO3- secretion** and likely decreases duodenal Cl- secretion, making "no change" in these parameters incorrect.
- Secretin has a pronounced effect on both bicarbonate and chloride transport in the duodenum.
Question 16: A person is exercising strenuously on a treadmill for 1 hour. An arterial blood gas measurement is then taken. Which of the following are the most likely values?
Explanation: ***pH 7.57, PaO2 100, PCO2 23, HCO3 21***
- After 1 hour of strenuous exercise, this represents **respiratory alkalosis with mild metabolic compensation**, which is the expected finding in a healthy individual during sustained vigorous exercise.
- The **low PCO2 (23 mmHg)** reflects appropriate **hyperventilation** in response to increased metabolic demands and lactic acid production. During intense exercise, minute ventilation increases dramatically, often exceeding the rate of CO2 production.
- The **slightly elevated pH (7.57)** and **mildly decreased HCO3 (21 mEq/L)** indicate that respiratory compensation has slightly overshot, creating mild alkalosis, while the bicarbonate is consumed both in buffering lactate and through renal compensation.
- **Normal PaO2 (100 mmHg)** confirms adequate oxygenation maintained by increased ventilation.
*pH 7.36, PaO2 100, PCO2 40, HCO3 23*
- These are **completely normal arterial blood gas values** with no evidence of any physiological stress or compensation.
- After 1 hour of strenuous exercise, we would expect **hyperventilation with decreased PCO2**, not a normal PCO2 of 40 mmHg. This profile would be consistent with rest, not vigorous exercise.
- The absence of any respiratory or metabolic changes makes this inconsistent with the clinical scenario.
*pH 7.56, PaO2 100, PCO2 44, HCO3 38*
- This profile suggests **metabolic alkalosis** (high pH, high HCO3) with inadequate respiratory compensation (normal to slightly elevated PCO2).
- This is **not consistent with strenuous exercise**, which produces metabolic acid (lactate), not metabolic base. The elevated HCO3 suggests vomiting, diuretic use, or other causes of metabolic alkalosis.
*pH 7.32, PaO2 42, PCO2 50, HCO3 27*
- This indicates **respiratory acidosis** (low pH, high PCO2) with **severe hypoxemia** (PaO2 42 mmHg).
- During strenuous exercise, healthy individuals **increase ventilation** to enhance O2 delivery and remove CO2, so both hypoxemia and hypercapnia are unexpected and would suggest severe cardiopulmonary disease or hypoventilation.
*pH 7.38, PaO2 100, PCO2 69, HCO3 42*
- This demonstrates **compensated respiratory acidosis** (normal pH, markedly elevated PCO2 and HCO3).
- The **very high PCO2 (69 mmHg)** indicates severe **hypoventilation**, which is the opposite of what occurs during exercise. This profile suggests chronic respiratory failure with metabolic compensation, such as in severe COPD.
Question 17: Two hours after undergoing laparoscopic roux-en-Y gastric bypass surgery, a 44-year-old man complains of pain in the site of surgery and nausea. He has vomited twice in the past hour. He has hypertension, type 2 diabetes mellitus, and hypercholesterolemia. Current medications include insulin, atorvastatin, hydrochlorothiazide, acetaminophen, and prophylactic subcutaneous heparin. He drinks two to three beers daily and occasionally more on weekends. He is 177 cm (5 ft 10 in) tall and weighs 130 kg (286 lb); BMI is 41.5 kg/m2. His temperature is 37.3°C (99.1°F), pulse is 103/min, and blood pressure is 122/82 mm Hg. Examination shows five laparoscopic incisions with no erythema or discharge. The abdomen is soft and non-distended. There is slight diffuse tenderness to palpation. Bowel sounds are reduced. Laboratory studies show:
Hematocrit 45%
Serum
Na+ 136 mEq/L
K+ 3.5 mEq/L
Cl- 98 mEq/L
Urea nitrogen 31 mg/dL
Glucose 88 mg/dL
Creatinine 1.1 mg/dL
Arterial blood gas analysis on room air shows:
pH 7.28
pCO2 32 mm Hg
pO2 74 mm Hg
HCO3- 14.4 mEq/L
Which of the following is the most likely cause for the acid-base status of this patient?
A. Hypoxia (Correct Answer)
B. Uremia
C. Late dumping syndrome
D. Vomiting
E. Early dumping syndrome
Explanation: ***Hypoxia***
- The patient exhibits **metabolic acidosis** (pH 7.28, HCO3- 14.4 mEq/L) with **appropriate respiratory compensation** (pCO2 32 mm Hg using Winter's formula: expected pCO2 = 1.5 × 14.4 + 8 ± 2 = 29.6 ± 2).
- The **pO2 of 74 mm Hg is significantly low** (normal range on room air: 80-100 mm Hg), indicating **hypoxemia** that leads to **tissue hypoxia** and **anaerobic metabolism**.
- In the setting of **obesity (BMI 41.5)** and **immediate post-operative status** after laparoscopic surgery, multiple factors contribute to hypoxemia including **atelectasis, reduced functional residual capacity, pain limiting deep breathing, residual anesthetic effects, and pneumoperitoneum effects**.
- Tissue hypoxia results in **lactic acidosis** (a high anion gap metabolic acidosis), which explains the acid-base disturbance. The **elevated BUN (31 mg/dL) with relatively normal creatinine** suggests prerenal azotemia from hypoperfusion, further supporting inadequate tissue oxygenation.
- The **tachycardia (103/min)** represents a compensatory response to improve oxygen delivery to hypoxic tissues.
*Vomiting*
- Vomiting causes loss of **gastric HCl**, resulting in **hypochloremic metabolic ALKALOSIS** (elevated pH and HCO3-), not acidosis.
- While this patient is vomiting, the acid-base status shows **acidosis**, which is the opposite of what vomiting typically causes.
- The low **Cl- (98 mEq/L)** is consistent with some gastric acid loss, but the dominant acid-base disorder is metabolic acidosis from another cause.
*Uremia*
- **Uremia** causes high anion gap metabolic acidosis due to retention of organic acids and phosphates in renal failure.
- While the **BUN is elevated (31 mg/dL)**, the **creatinine (1.1 mg/dL) is essentially normal**, especially for a patient with high muscle mass (130 kg).
- The BUN elevation is more consistent with **prerenal azotemia** (dehydration/hypoperfusion) rather than intrinsic renal failure causing uremic acidosis.
*Late dumping syndrome*
- **Late dumping syndrome** occurs **1-3 hours after eating** and results from rapid carbohydrate absorption causing hyperinsulinemia and subsequent **reactive hypoglycemia**.
- This patient's **glucose is normal (88 mg/dL)**, and symptoms began only **2 hours post-surgery** in the fasting state, not after a meal.
- Late dumping does not cause metabolic acidosis.
*Early dumping syndrome*
- **Early dumping syndrome** occurs **10-30 minutes after eating** due to rapid gastric emptying of hyperosmolar contents into the small intestine, causing fluid shifts.
- Symptoms include **cramping, diarrhea, vasomotor symptoms (flushing, palpitations, dizziness)**, not metabolic acidosis.
- This patient has not yet eaten post-operatively, making dumping syndrome impossible.
Question 18: A 25-year-old woman with an extensive psychiatric history is suspected of having metabolic acidosis after ingesting a large amount of aspirin in a suicide attempt. Labs are drawn and the values from the ABG are found to be: PCO2: 25, and HCO3: 15, but the pH value is smeared on the print-out and illegible. The medical student is given the task of calculating the pH using the pCO2 and HCO3 concentrations. He recalls from his first-year physiology course that the pKa of relevance for the bicarbonate buffering system is approximately 6.1. Which of the following is the correct formula the student should use, using the given values from the incomplete ABG?
A. 15/6.1 + log[10/(0.03*25)]
B. 6.1 + log[15/(0.03*25)] (Correct Answer)
C. 10^6.1 + 15/0.03*25
D. 6.1 + log[0.03/15*25]
E. 6.1 + log[25/(15*0.03)]
Explanation: ***6.1 + log[15/(0.03*25)]***
- This formula correctly represents the Henderson-Hasselbalch equation for the bicarbonate buffer system: **pH = pKa + log([HCO3-]/[0.03 * PCO2])**.
- Here, **pKa is 6.1**, **[HCO3-] is 15**, and **[0.03 * PCO2] is 0.03 * 25**, making this the appropriate calculation for pH.
*15/6.1 + log[10/(0.03*25)]*
- This formula incorrectly places the pKa in the denominator of the first term and introduces an arbitrary '10' in the numerator of the logarithmic term.
- The **Henderson-Hasselbalch equation** dictates that pKa is added, not divided into, another component, and the logarithmic term should reflect the ratio of bicarbonate to carbonic acid.
*10^6.1 + 15/0.03*25*
- This option incorrectly uses an exponentiation of pKa and adds it to an unrelated fractional term, which does not correspond to the Henderson-Hasselbalch equation structure.
- The formula for pH calculation is a sum of pKa and a logarithmic term, not an exponentiation and a simple fraction.
*6.1 + log[0.03/15*25]*
- This option incorrectly inverts the ratio within the logarithm, placing the carbonic acid component (0.03 * PCO2) in the numerator and bicarbonate in the denominator.
- The correct Henderson-Hasselbalch equation requires the **bicarbonate concentration in the numerator** and the carbonic acid concentration in the denominator.
*6.1 + log [25/(15*0.03)]*
- This option incorrectly places the PCO2 (25) in the numerator of the logarithmic term and the product of HCO3- and 0.03 in the denominator.
- The correct ratio for the Henderson-Hasselbalch equation is **[HCO3-] / [0.03 * PCO2]**.
Question 19: A 66-year-old man is brought to the emergency department by his daughter because of 3 days of fever, chills, cough, and shortness of breath. The cough is productive of yellow sputum. His symptoms have not improved with rest and guaifenesin. His past medical history is significant for hypertension, for which he takes hydrochlorothiazide. He has a 30-pack-year history of smoking. His temperature is 38.9 C (102.0 F), blood pressure 88/56 mm Hg, and heart rate 105/min. Following resuscitation with normal saline, his blood pressure improves to 110/70 mm Hg. His arterial blood gas is as follows:
Blood pH 7.52, PaO2 74 mm Hg, PaCO2 28 mm Hg, and HCO3- 21 mEq/L.
Which of the following acid-base disturbances best characterizes this patient's condition?
A. Metabolic acidosis
B. Respiratory acidosis
C. Respiratory alkalosis (Correct Answer)
D. Normal acid-base status
E. Metabolic alkalosis
Explanation: ***Respiratory alkalosis***
- The patient's pH of **7.52** indicates alkalemia. A **PaCO2 of 28 mm Hg** (normal range 35-45 mm Hg) is low, indicating a respiratory component.
- The **primary disturbance is respiratory alkalosis** due to hyperventilation from pneumonia/sepsis causing tachypnea and increased CO2 elimination.
- The HCO3- of 21 mEq/L (normal range 22-26 mEq/L) is at the lower limit of normal. In acute respiratory alkalosis, bicarbonate remains near normal since **metabolic compensation takes 2-3 days** to develop significantly.
- With a 3-day history, minimal renal compensation is expected, consistent with the near-normal bicarbonate.
*Metabolic acidosis*
- Metabolic acidosis would present with a **low pH** and a **low HCO3-**, which is not seen here.
- The patient's pH is **alkaline (7.52)**, not acidic, ruling out this diagnosis.
*Respiratory acidosis*
- Respiratory acidosis would be characterized by a **low pH** and a **high PaCO2**, indicating hypoventilation.
- The patient's **PaCO2 is low (28 mm Hg)** and the **pH is high**, directly contradicting respiratory acidosis.
*Normal acid-base status*
- A normal acid-base status would have a **pH between 7.35 and 7.45** and PaCO2 between 35-45 mm Hg.
- The patient's **pH of 7.52** and **PaCO2 of 28 mm Hg** are both abnormal, specifically indicating alkalemia and hypocapnia.
*Metabolic alkalosis*
- Metabolic alkalosis would feature a **high pH** and a **high HCO3-** (typically >26 mEq/L), often resulting from conditions like vomiting or diuretic use.
- While the patient is on hydrochlorothiazide, his **HCO3- is 21 mEq/L** (low-normal, not elevated), indicating this is not a primary metabolic alkalosis.
Question 20: A 57-year-old male presents to his primary care physician with upper abdominal pain. He reports a 3-month history of mild epigastric pain that improves with meals. He has lost 15 pounds since his symptoms started. His past medical history is notable for gynecomastia in the setting of a prolactinoma for which he underwent surgical resection over 10 years prior. He has a 15-pack-year smoking history, a history of heroin abuse, and is on methadone. His family history is notable for parathyroid adenoma in his father. His temperature is 98.8°F (37.1°C), blood pressure is 125/80 mmHg, pulse is 78/min, and respirations are 18/min. This patient’s symptoms are most likely due to elevations in a substance with which of the following functions?
A. Promote gastric mucosal growth
B. Increase pancreatic bicarbonate secretion
C. Decrease gastrin secretion
D. Increase pancreatic exocrine secretion
E. Increase gastric acid secretion (Correct Answer)
Explanation: ***Increase gastric acid secretion***
- The patient's history of a prolactinoma, family history of parathyroid adenoma, and current symptoms suggest **Multiple Endocrine Neoplasia type 1 (MEN1)**.
- **MEN1** often involves **pancreatic neuroendocrine tumors**, specifically **gastrinomas**, which autonomously secrete **gastrin**, leading to **Zollinger-Ellison syndrome (ZES)**.
- **Gastrin's primary function** is to stimulate **parietal cells** in the gastric mucosa to secrete **hydrochloric acid (HCl)**, leading to **massive gastric acid hypersecretion**.
- This acid hypersecretion causes **severe peptic ulcer disease** (explaining the epigastric pain), **refractory ulcers**, and often **diarrhea** from acid overload in the small intestine.
*Promote gastric mucosal growth*
- While gastrin does have a **trophic effect** on gastric mucosa (promoting mucosal growth), this is a **secondary/chronic effect**, not the primary pathophysiologic mechanism.
- The acute symptoms (epigastric pain, ulcers, weight loss) are directly caused by **acid hypersecretion**, not mucosal growth.
- In clinical practice, the focus in Zollinger-Ellison syndrome is on managing the **acid hypersecretion** with high-dose proton pump inhibitors.
*Increase pancreatic bicarbonate secretion*
- **Secretin** is the primary hormone responsible for increasing pancreatic bicarbonate secretion in response to duodenal acidification.
- Gastrin does not significantly affect pancreatic bicarbonate secretion.
*Decrease gastrin secretion*
- This is incorrect; the problem in gastrinomas is **autonomous hypersecretion of gastrin**.
- Normally, low gastric pH inhibits gastrin release via negative feedback, but gastrinomas secrete gastrin autonomously, independent of feedback mechanisms.
*Increase pancreatic exocrine secretion*
- **Cholecystokinin (CCK)** is the primary hormone that stimulates pancreatic enzyme secretion.
- While the excessive acid entering the duodenum in ZES may indirectly stimulate pancreatic secretion, gastrin itself does not directly increase pancreatic exocrine secretion significantly.
