USMLE Step 2 Quiz
Practice Acid Base And Electrolyte Emergencies in USMLE Step 2 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 44-year-old woman presents 36 hours after thyroidectomy with tingling and muscle spasms. She has stridor and is anxious. Labs:
| Test | Result |
|---|---|
| ABG | pH 7.46, PaCO2 33 mm Hg, HCO3− 23 mEq/L |
| Ionized Ca2+ | 0.78 mmol/L |
| K+ | 4.1 mEq/L |
| Mg2+ | 1.8 mg/dL |
Time-sensitive context: airway symptoms are worsening, and IV access is available.
Which of the following is the most appropriate initial treatment for this electrolyte imbalance?
This quiz focuses on Acid Base And Electrolyte Emergencies, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 2.
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A 44-year-old woman presents 36 hours after thyroidectomy with tingling and muscle spasms. She has stridor and is anxious. Labs:
| Test | Result |
|---|---|
| ABG | pH 7.46, PaCO2 33 mm Hg, HCO3− 23 mEq/L |
| Ionized Ca2+ | 0.78 mmol/L |
| K+ | 4.1 mEq/L |
| Mg2+ | 1.8 mg/dL |
Time-sensitive context: airway symptoms are worsening, and IV access is available.
Which of the following is the most appropriate initial treatment for this electrolyte imbalance?
Explanation: This question tests the ability to diagnose and manage acid-base and electrolyte emergencies in critical care settings. Understanding acid-base balance is critical for managing patients with metabolic acidosis and alkalosis, as well as electrolyte imbalances like hyperkalemia or hyponatremia. In this vignette, the patient's symptoms and lab results provide crucial clues for identifying the underlying condition. The correct answer, choice A, is justified by the acute severe hypocalcemia causing laryngospasm and stridor, requiring immediate IV calcium to stabilize neuromuscular function. Choice B is incorrect due to oral calcium being insufficient for rapid correction in emergent airway-compromising symptoms. To improve student understanding, emphasize the importance of correlating clinical symptoms with laboratory findings and encourage the use of diagnostic algorithms to guide management decisions.
A 72-year-old woman is admitted to the ICU for pneumonia. Two days after admission, she develops a generalized tonic-clonic seizure. Her medical history is significant for small cell lung cancer. A stat serum sodium level is 109 mEq/L. Which of the following is the most appropriate immediate management?
Which of the following is the most appropriate immediate management?
Explanation: This patient presents with severe, symptomatic hyponatremia (seizure), likely due to the syndrome of inappropriate antidiuretic hormone (SIADH) from her small cell lung cancer. In a patient with severe neurologic symptoms such as seizures or coma, the immediate goal is to rapidly increase the serum sodium to alleviate cerebral edema. This is achieved with an intravenous bolus of 3% hypertonic saline. Fluid restriction, normal saline, and demeclocycline are treatments for chronic or asymptomatic hyponatremia and are not appropriate for this acute emergency.
A 65-year-old man with decompensated heart failure is being treated in the ICU with high-dose intravenous furosemide. He develops profound muscle weakness and constipation. His telemetry shows frequent premature ventricular contractions and flattened T waves. A stat electrolyte panel reveals a potassium level of 2.1 mEq/L. Which of the following is the most appropriate management?
Which of the following is the most appropriate management?
Explanation: The patient has severe, symptomatic hypokalemia (potassium < 2.5 mEq/L) with ECG changes, which is a medical emergency requiring intravenous potassium replacement. Oral replacement is too slow for this acuity. The standard safe rate for IV potassium replacement via a peripheral line is 10 mEq/hr to avoid venous irritation and cardiac toxicity. Rates higher than 10 mEq/hr (up to 20-40 mEq/hr) require a central line and continuous cardiac monitoring, but 10 mEq/hr is the most appropriate initial choice for urgent but controlled repletion. Spironolactone would be for long-term management, not acute replacement.
An 80-year-old woman has been intubated and sedated in the ICU for 5 days for severe pneumonia. She has been receiving enteral nutrition but has had high-volume diarrhea. Her daily labs show a progressive rise in serum sodium. Today, her sodium is 162 mEq/L. Her urine output has been 40 mL/hr. Which of the following is the most appropriate intervention?
Which of the following is the most appropriate intervention?
Explanation: This patient has hypernatremia due to a net water deficit, likely from ongoing insensible losses and hypotonic gastrointestinal losses (diarrhea) without adequate free water replacement. The treatment for hypernatremia is to replace the free water deficit. Intravenous 5% dextrose in water (D5W) is effectively free water, as the dextrose is rapidly metabolized, and is the appropriate fluid for correction. 0.9% saline would worsen the hypernatremia. Desmopressin is used for central diabetes insipidus, which is not suggested here. Decreasing protein might help with urea-induced osmotic diuresis, but the primary issue is a water deficit that needs correction.
A 58-year-old man with a history of alcohol use disorder is admitted to the ICU with alcohol withdrawal. He is treated with benzodiazepines. Despite receiving 80 mEq of intravenous potassium chloride over 8 hours, his serum potassium remains 2.4 mEq/L. He continues to have ventricular ectopy. Which of the following is the most appropriate next step?
Which of the following is the most appropriate next step?
Explanation: This patient has refractory hypokalemia, which is persistent hypokalemia despite adequate potassium replacement. In patients with a history of alcohol use disorder, concurrent hypomagnesemia is extremely common. Magnesium is a cofactor for the renal outer medullary potassium (ROMK) channels, which are responsible for potassium reabsorption. In the setting of hypomagnesemia, these channels are dysfunctional, leading to persistent renal potassium wasting. Therefore, repletion of magnesium is necessary to allow for effective correction of the hypokalemia.
A 34-year-old woman is admitted to the ICU with severe nausea and vomiting for 5 days due to hyperemesis gravidarum. She is hypotensive and tachycardic. An arterial blood gas shows: pH 7.58, PaCO2 48 mm Hg, HCO3- 40 mEq/L. Her serum chloride is 85 mEq/L. Which of the following is the most appropriate initial intravenous fluid for this patient?
Which of the following is the most appropriate initial intravenous fluid for this patient?
Explanation: This patient has a severe metabolic alkalosis (high pH, high HCO3-) with partial respiratory compensation (high PaCO2). This is a saline-responsive or 'contraction' alkalosis, caused by the loss of hydrogen and chloride ions from vomiting and subsequent volume depletion, which stimulates bicarbonate reabsorption in the kidney. The treatment is to correct the volume and chloride deficits. 0.9% saline (normal saline) is the fluid of choice as it repletes both volume and chloride, which allows the kidneys to excrete the excess bicarbonate and correct the alkalosis. Lactated Ringer's contains lactate, which is converted to bicarbonate, and would worsen the alkalosis. D5W and 0.45% saline are hypotonic and would not adequately restore volume or correct the chloride deficit.
A 66-year-old man with severe ARDS is intubated and on volume-control ventilation in the ICU. His tidal volume is set at 6 mL/kg ideal body weight. Over the past hour, his sedation was increased for agitation. A repeat arterial blood gas shows: pH 7.20, PaCO2 75 mm Hg, PaO2 85 mm Hg, HCO3- 26 mEq/L. His blood pressure is stable. Which of the following is the most appropriate next step?
Which of the following is the most appropriate next step?
Explanation: The patient has an acute respiratory acidosis (low pH, high PaCO2), likely due to a decrease in his spontaneous respiratory drive from increased sedation, leading to decreased minute ventilation. In a patient on volume-control ventilation, the most direct way to increase minute ventilation (and thus CO2 removal) is to increase the respiratory rate. Increasing the tidal volume above 6 mL/kg is generally avoided in ARDS (lung-protective strategy). Increasing PEEP primarily affects oxygenation, not ventilation. Sodium bicarbonate does not address the underlying problem of inadequate ventilation.
A 35-year-old homeless man is brought to the ICU with altered mental status. He is tachypneic and disoriented. Laboratory results show: pH 7.18, HCO3- 7 mEq/L. Serum sodium is 140 mEq/L, chloride is 100 mEq/L, and BUN is 40 mg/dL. A urine specimen fluoresces under a Wood's lamp. Which of the following is the most appropriate treatment?
Which of the following is the most appropriate treatment?
Explanation: The patient has a severe high-anion-gap metabolic acidosis (Anion Gap = 140 - [100 + 7] = 33). The fluorescence of urine under a Wood's lamp is a classic clue for ethylene glycol (antifreeze) ingestion, as fluorescein is often added to these products. Ethylene glycol is metabolized by alcohol dehydrogenase to toxic metabolites (glycolic acid, oxalic acid) that cause acidosis and renal failure. The definitive treatment is to block this metabolism with an alcohol dehydrogenase inhibitor like fomepizole and to remove the parent alcohol and its toxic metabolites via hemodialysis. N-acetylcysteine is for acetaminophen toxicity. Methylene blue is for methemoglobinemia.
A 44-year-old woman with a history of anorexia nervosa is admitted to the ICU for severe malnutrition. She is started on total parenteral nutrition (TPN). On day 3 of TPN, she develops acute respiratory failure requiring intubation. Examination reveals marked generalized muscle weakness. Laboratory studies are most likely to show a severe deficiency of which of the following electrolytes?
Laboratory studies are most likely to show a severe deficiency of which of the following electrolytes?
Explanation: This patient is experiencing refeeding syndrome, a life-threatening condition that occurs when nutrition is reintroduced in severely malnourished individuals. The sudden influx of carbohydrates stimulates insulin release, which drives phosphate, potassium, and magnesium into cells for glycolysis and ATP production. The resulting acute and profound hypophosphatemia is the hallmark of the syndrome and can cause widespread organ dysfunction, including respiratory muscle weakness leading to respiratory failure, cardiac failure, and rhabdomyolysis.
A 75-year-old man is brought to the ICU with septic shock from urosepsis. He is hypotensive despite 3 liters of crystalloid and is on high-dose norepinephrine. His arterial blood gas shows a pH of 7.10 with a bicarbonate of 10 mEq/L and a lactate of 12 mmol/L. In addition to broad-spectrum antibiotics and continued hemodynamic support, which of the following interventions is most critical to reversing his metabolic acidosis?
In addition to broad-spectrum antibiotics and continued hemodynamic support, which of the following interventions is most critical to reversing his metabolic acidosis?
Explanation: The patient's severe lactic acidosis is a direct consequence of tissue hypoperfusion caused by septic shock. The three mainstays of sepsis management are early antibiotics, hemodynamic resuscitation (fluids and vasopressors), and source control. In this case of urosepsis, an obstruction (e.g., kidney stone, prostatic hypertrophy) may be causing ongoing infection and bacteremia. Identifying and draining the source of infection is critical to breaking the inflammatory cycle, improving hemodynamics, and ultimately allowing the clearance of lactate and correction of the acidosis. While the other interventions might be considered, none are as fundamental as source control in reversing the underlying pathophysiology.
An 82-year-old woman with chronic heart failure and a baseline serum sodium of 120 mEq/L is admitted to the ICU for an unrelated issue. A new physician, concerned about the hyponatremia, administers several boluses of 3% saline. Over 24 hours, her sodium increases from 118 mEq/L to 132 mEq/L. Two days later, the patient develops dysarthria, dysphagia, and spastic quadriparesis. Which of the following is the most likely diagnosis?
Which of the following is the most likely diagnosis?
Explanation: This patient with chronic hyponatremia experienced overly rapid correction of her serum sodium (14 mEq/L in 24 hours). The recommended safe limit for correction is typically 8-10 mEq/L in 24 hours to allow brain cells to adapt to the changing extracellular osmolality. Rapid correction can lead to osmotic demyelination syndrome (formerly central pontine myelinolysis), which presents with delayed neurologic symptoms (dysarthria, dysphagia, quadriparesis, altered mental status) 2-6 days after the osmotic injury. This clinical picture is classic for this iatrogenic complication.
A 55-year-old woman is in the ICU for septic shock secondary to a perforated diverticulum. Despite aggressive fluid resuscitation and vasopressor support, she remains hypotensive. Her laboratory values are notable for a pH of 7.08, PaCO2 of 30 mm Hg, and HCO3- of 8 mEq/L. Her serum lactate is 9.5 mmol/L. She is intubated and mechanically ventilated. Which of the following is the most appropriate next step in managing her severe acidemia?
Which of the following is the most appropriate next step in managing her severe acidemia?
Explanation: The patient has a severe high-anion-gap metabolic acidosis due to lactic acidosis from septic shock. The primary treatment for this condition is to correct the underlying cause, which is poor tissue perfusion. Therefore, continuing aggressive hemodynamic support with fluids and vasopressors to improve organ perfusion is the most critical intervention. Sodium bicarbonate is controversial in lactic acidosis as it can worsen intracellular acidosis and has not been shown to improve outcomes. Increasing minute ventilation would cause a respiratory alkalosis, which is not the primary problem. Renal replacement therapy may be needed later for renal failure but is not the immediate step to address the cause of the acidosis.
A 48-year-old woman is in the surgical ICU on postoperative day 1 following a total thyroidectomy for papillary thyroid cancer. She complains of tingling around her mouth and in her fingertips. While checking her blood pressure, you observe her hand flexing at the wrist and metacarpophalangeal joints (Trousseau's sign). A stat ionized calcium level is pending. Which of the following is the most appropriate immediate intervention?
Which of the following is the most appropriate immediate intervention?
Explanation: This patient is exhibiting classic signs and symptoms of acute, symptomatic hypocalcemia (perioral paresthesias, Trousseau's sign), which is a common complication of total thyroidectomy due to iatrogenic hypoparathyroidism. Symptomatic hypocalcemia is a medical emergency that can progress to seizures, laryngospasm, and cardiac arrhythmias. The most appropriate immediate management is intravenous calcium gluconate to rapidly raise serum calcium levels and stabilize the patient. Oral calcium is for mild or chronic hypocalcemia. Awaiting lab results would dangerously delay treatment in a symptomatic patient.
A 68-year-old man with a history of end-stage renal disease on hemodialysis is brought to the intensive care unit after missing his last two dialysis sessions. He is lethargic and nauseated. His blood pressure is 150/90 mm Hg, heart rate is 50/min, and respirations are 22/min. An ECG shows peaked T waves, a widened QRS complex, and loss of P waves. A point-of-care potassium level is 8.2 mEq/L. Which of the following is the most appropriate immediate step in management?
Which of the following is the most appropriate immediate step in management?
Explanation: This patient has life-threatening hyperkalemia, evidenced by the serum potassium level and significant ECG changes (widened QRS), which indicate cardiac membrane instability. The most immediate priority is to stabilize the cardiac membrane to prevent progression to ventricular fibrillation or asystole. Intravenous calcium gluconate is the first-line agent for this purpose. It does not lower the serum potassium level but acts within minutes to antagonize the cardiotoxic effects of hyperkalemia.
A 50-year-old woman is in the ICU after a massive pulmonary embolism. She is extremely anxious and tachypneic, with a respiratory rate of 35/min. An arterial blood gas reveals: pH 7.55, PaCO2 25 mm Hg, PaO2 65 mm Hg, HCO3- 23 mEq/L. She is hemodynamically stable. Which of the following is the most appropriate management for her acid-base disturbance?
Which of the following is the most appropriate management for her acid-base disturbance?
Explanation: The patient has an acute respiratory alkalosis (high pH, low PaCO2) secondary to hypoxemia and anxiety from her pulmonary embolism. The primary management of any respiratory acid-base disorder is to treat the underlying cause. In this case, the cause is the pulmonary embolism. Therefore, initiating anticoagulation and providing supplemental oxygen to correct the hypoxemia and treat the PE is the most appropriate step. Sedating the patient or having her rebreathe CO2 does not address the life-threatening underlying pathology and could be harmful.
A 28-year-old woman is in the ICU with severe pancreatitis and has developed voluminous diarrhea from a secondary C. difficile infection. She is volume depleted. Her labs show: pH 7.28, HCO3- 15 mEq/L, sodium 138 mEq/L, potassium 3.2 mEq/L, and chloride 115 mEq/L. Which of the following is the most appropriate intravenous fluid for resuscitation?
Which of the following is the most appropriate intravenous fluid for resuscitation?
Explanation: This patient has a normal anion gap metabolic acidosis (NAGMA), calculated as 138 - (115 + 15) = 8. This is characteristic of bicarbonate loss from severe diarrhea. When resuscitating a patient with NAGMA, using large volumes of 0.9% saline (which has a high chloride content) can worsen the acidosis (hyperchloremic metabolic acidosis). Lactated Ringer's is a balanced crystalloid solution; the lactate is metabolized to bicarbonate, which helps correct the acidosis, making it the preferred resuscitation fluid in this setting. D5 1/2NS is hypotonic and inappropriate for resuscitation, and 3% saline is for severe symptomatic hyponatremia.
A 70-year-old woman with a history of metastatic breast cancer to the bone is brought to the ICU for profound lethargy and confusion. Her blood pressure is 90/50 mm Hg and heart rate is 110/min. Laboratory studies show a total serum calcium of 16.2 mg/dL and a creatinine of 2.5 mg/dL. Her ECG shows a shortened QT interval. Which of the following is the most critical initial step in management?
Which of the following is the most critical initial step in management?
Explanation: This patient is in a hypercalcemic crisis, a life-threatening emergency. The primary cause of hypercalcemia in this setting is profound volume depletion from calcium-induced nephrogenic diabetes insipidus. Therefore, the most critical initial step is aggressive volume repletion with intravenous 0.9% saline. This restores intravascular volume, improves glomerular filtration rate, and enhances renal calcium excretion. Bisphosphonates are essential for long-term control but take 2-3 days to work. Furosemide should only be given after volume status has been restored, as it can worsen dehydration. Hemodialysis is reserved for severe, refractory cases or those with severe renal failure unable to tolerate fluids.
A 45-year-old man is in the ICU on postoperative day 3 after an exploratory laparotomy. Over the past 48 hours, he has received 6 liters of 0.45% saline for maintenance fluids. He is now confused and lethargic. His serum sodium is 122 mEq/L, and his urine osmolality is 150 mOsm/kg. Which of the following is the most likely cause of his hyponatremia?
Which of the following is the most likely cause of his hyponatremia?
Explanation: This patient's hyponatremia is most likely iatrogenic. Postoperative patients often have elevated levels of ADH due to surgical stress and pain. The administration of large volumes of hypotonic fluid (0.45% saline) in the setting of high ADH levels leads to free water retention and subsequent dilutional hyponatremia. While SIADH also involves high ADH, the primary iatrogenic driver here is the administration of hypotonic fluids. Cerebral salt wasting is less common and characterized by volume depletion. Adrenal insufficiency would typically present with hyperkalemia.
A 62-year-old man with a history of severe COPD is admitted to the ICU with diabetic ketoacidosis (DKA). He is obtunded and tachypneic. Arterial blood gas analysis shows: pH 7.15, PaCO2 60 mm Hg, PaO2 70 mm Hg, HCO3- 16 mEq/L. His serum glucose is 550 mg/dL, and serum ketones are positive. Which of the following best describes his acid-base status?
Which of the following best describes his acid-base status?
Explanation: The patient's pH is 7.15, indicating a significant acidemia. The elevated PaCO2 of 60 mm Hg (normal ~40) indicates a primary respiratory acidosis, likely due to his severe COPD exacerbation. The low bicarbonate of 16 mEq/L (normal ~24) indicates a concurrent metabolic acidosis, consistent with DKA. In a simple metabolic acidosis, the expected respiratory compensation (Winter's formula: PaCO2 = 1.5 * HCO3 + 8 ± 2) would be a PaCO2 of approximately 32 mm Hg. Since the actual PaCO2 is much higher, there is a superimposed respiratory acidosis. Therefore, this is a mixed disorder.
A 59-year-old man with essential thrombocythemia (platelet count 1.2 million/μL) is in the ICU for monitoring. A routine metabolic panel drawn from a peripheral vein shows a potassium level of 7.1 mEq/L. The patient is asymptomatic, and his ECG is entirely normal with no T-wave changes or QRS widening. A repeat sample drawn into a heparinized tube and analyzed immediately shows a potassium of 4.3 mEq/L. What is the most likely cause of the initial laboratory finding?
What is the most likely cause of the initial laboratory finding?
Explanation: Pseudohyperkalemia is an artificially elevated potassium level due to the in vitro release of potassium from cells during or after phlebotomy. It is a common finding in patients with extreme thrombocytosis or leukocytosis. During clotting in the collection tube, the large number of platelets release intracellular potassium, falsely elevating the measured serum level. The discrepancy between the high lab value and the normal ECG, along with the normal result from a whole blood (heparinized) sample, confirms this diagnosis. It is a lab artifact, not a true representation of the patient's in vivo potassium level.