USMLE Step 2 CK nephrology, fluids & electrolytes questions

Lithium accumulates in collecting duct principal cells via the epithelial sodium channel and inhibits glycogen synthase kinase-3, which impairs aquaporin-2 trafficking and expression at the apical membrane, producing resistance to antidiuretic hormone despite normal or elevated ADH levels. This causes nephrogenic diabetes insipidus, characterized by dilute urine (urine osmolality less than serum osmolality) that fails to concentrate even after exogenous desmopressin administration, distinguishing it from central diabetes insipidus. Chronic lithium use is the most common drug cause of nephrogenic diabetes insipidus. Management includes amiloride, which blocks lithium entry into principal cells, and ensuring adequate water intake.

• B. Lithium does not accelerate hepatic ADH degradation; this is not a described mechanism of its nephrotoxicity.
• C. Autoimmune destruction of the posterior pituitary causes central diabetes insipidus, which would respond to desmopressin.
• D. Osmoreceptor destruction would impair thirst sensation and ADH release, not end-organ responsiveness, and is not a recognized lithium effect.
• E. Lithium is not known to cause prolactinomas, and a mass effect would impair ADH release rather than cause renal resistance.

This patient has hypovolemic hypernatremia from inadequate free water intake compounded by insensible losses from fever, with physical findings of volume depletion (tachycardia, dry mucous membranes, decreased skin turgor) and prerenal azotemia. Initial management requires restoring intravascular volume with isotonic 0.9% saline, since volume depletion itself impairs free water excretion and must be corrected first; once volume status is restored, free water deficit is replaced with a hypotonic fluid such as 5% dextrose in water. The rate of sodium correction should not exceed 8 to 10 mEq/L per 24 hours to avoid cerebral edema from overly rapid correction of chronic hypernatremia. Desmopressin is not indicated because this is not diabetes insipidus.

• B. Oral free water alone is inappropriate given hemodynamic compromise and the need for initial volume resuscitation with isotonic fluid.
• C. Desmopressin treats central diabetes insipidus; there is no evidence of DI here, and the problem is inadequate intake, not renal water loss.
• D. Hypertonic saline would worsen hypernatremia and is used for severe hyponatremia, not hypernatremia.
• E. Rapid correction within 6 hours risks cerebral edema; correction should occur gradually over 24-48 hours.

This patient has severe hyperkalemia with life-threatening ECG changes (widened QRS, loss of P waves) indicating impending cardiac conduction failure and risk of ventricular fibrillation or asystole. The immediate first step is intravenous calcium gluconate (or calcium chloride), which stabilizes the cardiac membrane by raising the threshold potential and antagonizing the depolarizing effects of hyperkalemia within minutes, without lowering serum potassium. This buys time while potassium-lowering therapies (insulin with dextrose, albuterol, and ultimately dialysis for definitive removal) are initiated. Calcium does not treat the underlying hyperkalemia and must be followed by measures that shift or remove potassium.

• B. Dialysis is definitive therapy but takes time to arrange; calcium must be given first to prevent fatal arrhythmia while this is organized.
• C. Albuterol shifts potassium intracellularly but has a slower onset than calcium and does not protect the myocardium acutely.
• D. Insulin with dextrose lowers serum potassium over 15-30 minutes but does not immediately stabilize the cardiac membrane against the ECG changes already present.
• E. Sodium bicarbonate may help shift potassium intracellularly in the setting of acidosis but acts slower than calcium and does not provide immediate cardiac membrane stabilization.

Intravenous regular insulin drives potassium intracellularly by stimulating the Na+/K+-ATPase pump, independent of its glucose-lowering effect, and begins lowering serum potassium within 15-30 minutes with peak effect around 1 hour. Because this patient's glucose is not elevated, dextrose must be co-administered to prevent hypoglycemia. This combination is a cornerstone of acute hyperkalemia management for rapidly shifting potassium intracellularly, used alongside calcium for cardiac stabilization, while definitive potassium removal (dialysis or potassium binders) is arranged. Nebulized albuterol can be added as an adjunct but insulin-dextrose remains the primary rapid-acting shift therapy.

• A. Magnesium sulfate is not a standard treatment for shifting potassium intracellularly in hyperkalemia.
• B. Sodium zirconium cyclosilicate binds potassium in the gut for definitive removal but acts over hours, not the rapid shift needed acutely.
• D. Furosemide promotes renal potassium excretion but has unreliable and delayed effect, especially in advanced CKD, and does not shift potassium intracellularly.
• E. Sodium polystyrene sulfonate removes potassium from the gut over hours and is not used for rapid intracellular shifting; it also has a slower onset and gastrointestinal safety concerns.

In patients with end-stage renal disease who have no functioning kidneys to excrete potassium, temporizing measures such as insulin-glucose and albuterol only shift potassium intracellularly and are not sufficient as definitive therapy, especially when the response is inadequate. Hemodialysis is the definitive treatment for hyperkalemia in ESRD because it directly removes potassium from the body and should be arranged emergently given the persistently elevated and dangerous potassium level despite temporizing measures. Potassium binders such as sodium zirconium cyclosilicate or sodium polystyrene sulfonate act too slowly to be relied upon in this urgent setting. Delaying dialysis risks a fatal arrhythmia as the temporizing effects of insulin and albuterol wane.

• A. Discharging the patient without dialysis is dangerous given his inability to excrete potassium and the inadequate response to temporizing measures.
• B. Sodium polystyrene sulfonate acts slowly over many hours and is inappropriate as the sole therapy for this persistently dangerous potassium level in an anuric patient.
• C. Repeated calcium only stabilizes the myocardium transiently and does not remove potassium from the body; it is not a substitute for dialysis.
• E. Albuterol has already been given with limited effect; increasing its dose without definitive removal risks arrhythmia as its effect wanes.

ACE inhibitors reduce angiotensin II formation, which decreases aldosterone secretion from the adrenal cortex. Aldosterone normally stimulates sodium reabsorption and potassium secretion in the principal cells of the distal nephron through upregulation of the epithelial sodium channel and the renal outer medullary potassium channel; reduced aldosterone activity therefore impairs potassium secretion, causing hyperkalemia. This effect is more pronounced in patients with underlying CKD, who already have reduced nephron mass and impaired potassium excretory capacity. Monitoring potassium and renal function after starting or up-titrating ACE inhibitors is essential, particularly in CKD patients.

• A. Epithelial sodium channel blockade describes the mechanism of amiloride, a potassium-sparing diuretic, not ACE inhibitors.
• B. Carbonic anhydrase inhibition describes the mechanism of acetazolamide, not ACE inhibitors.
• C. ACE inhibitors do not directly inhibit the Na+/K+-ATPase pump; this is not their mechanism of causing hyperkalemia.
• E. ACE inhibitors decrease, not increase, activity of the renal outer medullary potassium channel because they reduce aldosterone-driven channel expression.

Loop diuretics like furosemide inhibit the Na+-K+-2Cl- cotransporter in the thick ascending limb, increasing distal sodium delivery and promoting potassium wasting through enhanced secretion by the renal outer medullary potassium channel and increased aldosterone activity from volume contraction. This produces hypokalemia with classic ECG findings of flattened or inverted T waves and the appearance of U waves, a distinct positive deflection following the T wave. Concomitant hypomagnesemia, also common with loop diuretics, exacerbates potassium wasting and arrhythmia risk. The metabolic alkalosis seen here (bicarbonate 30 mEq/L) is also consistent with chronic loop diuretic use from contraction alkalosis and enhanced distal hydrogen ion secretion.

• A. Hypercalcemia classically shortens the QT interval rather than producing U waves.
• B. Hypocalcemia prolongs the QT interval rather than producing U waves following flattened T waves.
• C. Hyperkalemia produces peaked T waves and widened QRS complexes, the opposite of the findings described.
• D. Hypermagnesemia causes bradycardia and prolonged conduction intervals, not the described flattened T waves with U waves, and this patient's magnesium is low, not high.

Vomiting causes loss of gastric HCl, generating metabolic alkalosis and volume depletion. Volume contraction activates the renin-angiotensin-aldosterone system, and the resulting hyperaldosteronism increases distal sodium reabsorption and potassium secretion, which is the dominant mechanism for the kaliuresis. Hypochloremia also limits chloride available for reabsorption with sodium, forcing more sodium to be reabsorbed in exchange for potassium (and hydrogen) in the collecting duct. Low urine chloride confirms a chloride-responsive (vomiting-type) process. Transcellular shift contributes only a minor, transient component.

• A. Colonic potassium losses are not significant in isolated vomiting without diarrhea.
• B. Gastric fluid itself contains relatively little potassium; renal losses from secondary hyperaldosteronism account for most of the deficit.
• D. Poor intake alone would not cause potassium this low over only 5 days without an additional renal loss mechanism.
• E. Alkalemia causes only a small, transient shift of potassium into cells and does not explain the sustained total body potassium deficit.

Peripheral IV potassium infusions are generally limited to 10 mEq/hour to avoid vein irritation and to reduce the risk of transient hyperkalemia with cardiac effects; continuous cardiac monitoring is recommended for infusion rates above 10 mEq/hour or in patients with ECG changes. Higher rates (up to 20 mEq/hour) may be used via central line in monitored settings for severe, symptomatic hypokalemia, but this patient has only mild ECG changes and is stable. IV potassium should never be given as a rapid bolus because it can precipitate fatal arrhythmias.

• A. Central line infusion at this rate is not warranted for a hemodynamically stable patient with only mild ECG changes; this rate is reserved for more severe cases.
• C. A rapid IV bolus of potassium can cause fatal hyperkalemia-induced arrhythmias and is never appropriate.
• D. Withholding treatment is inappropriate given significant hypokalemia with ECG changes already present.
• E. Peripheral infusion at 40 mEq/hour exceeds safe limits and risks local vein injury and cardiotoxicity.

Resistant hypertension with unexplained or diuretic-refractory hypokalemia and metabolic alkalosis strongly suggests primary hyperaldosteronism (Conn syndrome), caused by an aldosterone-producing adenoma or bilateral adrenal hyperplasia. Autonomous aldosterone secretion increases distal sodium reabsorption and potassium/hydrogen excretion, producing this biochemical pattern along with volume-dependent hypertension. Primary hyperaldosteronism is found in a substantial proportion of patients with resistant hypertension and should be screened for with a plasma aldosterone-to-renin ratio. Renal artery stenosis and pheochromocytoma can cause resistant hypertension but do not typically cause this degree of hypokalemia and alkalosis.

• A. Cushing syndrome can cause hypokalemia but typically presents with additional features such as central obesity, striae, and hyperglycemia, which are absent here.
• B. Pheochromocytoma typically presents with paroxysmal hypertension, headache, and sweating rather than persistent hypokalemic alkalosis.
• C. Thiazide-induced hypokalemia typically resolves or improves with potassium supplementation and diuretic adjustment, unlike this persistent case.
• D. Renal artery stenosis causes secondary hyperaldosteronism but is less commonly associated with this degree of spontaneous or diuretic-refractory hypokalemia.

An elevated aldosterone-to-renin ratio (typically greater than 20-30 with plasma aldosterone above 15-20 ng/dL) is a positive screening result for primary hyperaldosteronism and must be followed by a confirmatory test demonstrating failure to suppress aldosterone secretion. Confirmatory options include oral or IV sodium loading with measurement of urinary or plasma aldosterone, or a saline suppression test; failure of aldosterone to suppress appropriately confirms autonomous secretion. Adrenal CT and venous sampling are used afterward, once the diagnosis is confirmed, to localize a unilateral source for consideration of surgery. The dexamethasone suppression test is used to evaluate for Cushing syndrome, not primary hyperaldosteronism.

• A. Adrenal venous sampling is used for subtype localization after biochemical confirmation, not as the confirmatory test itself.
• B. CT imaging is used for localization after the diagnosis is biochemically confirmed and can miss small or bilateral lesions if done prematurely.
• D. A positive screening ratio requires confirmatory testing, not simple repetition, to establish autonomous aldosterone secretion.
• E. The dexamethasone suppression test evaluates cortisol excess in suspected Cushing syndrome and has no role in confirming primary hyperaldosteronism.

This patient has the classic triad of diabetic ketoacidosis: hyperglycemia (glucose 460 mg/dL), high anion gap metabolic acidosis (anion gap = 134 - (96+10) = 28 mEq/L), and ketosis (large urine ketones), precipitated by insulin omission. Kussmaul respirations (deep, labored breathing) are a compensatory response to severe metabolic acidosis. Hyperosmolar hyperglycemic state occurs mainly in type 2 diabetes with much higher glucose levels and minimal ketosis or acidosis. The clinical context of type 1 diabetes with insulin non-adherence makes DKA far more likely than alcoholic ketoacidosis or toxic ingestions.

• B. Lactic acidosis would not explain the marked hyperglycemia and heavy ketonuria seen in this patient.
• C. Salicylate toxicity causes a mixed respiratory alkalosis and metabolic acidosis without significant hyperglycemia or ketonuria.
• D. Hyperosmolar hyperglycemic state typically presents with glucose over 600 mg/dL, minimal ketoacidosis, and is seen in type 2 diabetes with preserved insulin secretion.
• E. Alcoholic ketoacidosis occurs in the setting of chronic alcohol use and starvation, not insulin omission with marked hyperglycemia.

Fomepizole is a competitive inhibitor of alcohol dehydrogenase and is the first-line antidote for ethylene glycol or methanol poisoning because it prevents conversion of the parent alcohol into toxic acid metabolites (glycolic and oxalic acid). It has a more predictable pharmacokinetic profile and fewer adverse effects than ethanol, which was historically used for the same mechanism. Thiamine and pyridoxine are given as cofactors to shunt glyoxylate metabolism away from oxalate production but are adjuncts, not primary therapy. Hemodialysis is added for severe acidosis, renal failure, or very high toxic alcohol levels.

• A. These vitamins are adjunctive cofactors for glyoxylate metabolism, not the primary antidote.
• C. Dimercaprol is a chelator used for heavy metal poisoning, not toxic alcohol ingestion.
• D. Ethanol also inhibits alcohol dehydrogenase but has a narrower therapeutic window and requires intensive monitoring, making it second-line.
• E. Bicarbonate can help correct severe acidemia but does not block further toxic metabolite formation.

Distal (type 1) RTA results from impaired hydrogen ion secretion by alpha-intercalated cells in the collecting duct, so the kidney cannot acidify the urine even in the setting of systemic metabolic acidosis, producing an inappropriately high urine pH (greater than 5.5-6). The resulting chronic acidosis leaches calcium from bone, causing hypercalciuria and alkaline urine, which together favor calcium phosphate stone formation and nephrocalcinosis. Hypokalemia occurs because the acidosis increases distal sodium delivery and aldosterone-driven potassium secretion. Proximal RTA typically presents with a lower urine pH once serum bicarbonate falls below the reduced proximal reabsorptive threshold, and it does not characteristically cause nephrocalcinosis.

• A. Proximal RTA can lower urine pH appropriately once filtered bicarbonate falls below the reabsorptive threshold and is not classically associated with nephrocalcinosis.
• C. This causes hyperkalemic normal anion gap acidosis without the alkaline urine pH or nephrocalcinosis seen here.
• D. Diarrhea causes a normal anion gap acidosis with an appropriately acidic urine pH, not nephrocalcinosis.
• E. Carbonic anhydrase inhibitors cause a proximal-type bicarbonate wasting acidosis, not nephrocalcinosis.

This patient has proximal (type 2) RTA with full Fanconi syndrome, caused here by light chain-mediated proximal tubular injury from multiple myeloma. The proximal tubule normally reabsorbs the bulk of filtered bicarbonate, glucose, phosphate, uric acid, and amino acids, so generalized proximal dysfunction produces a normal anion gap metabolic acidosis along with glucosuria at normal serum glucose, phosphaturia, hypouricemia, and aminoaciduria. Hypokalemia occurs because increased distal sodium and bicarbonate delivery enhances potassium secretion. Distal RTA impairs only hydrogen ion secretion and does not cause the broader tubular wasting of glucose, phosphate, and amino acids seen in Fanconi syndrome.

• A. Aldosterone resistance causes hyperkalemic acidosis without the multiple proximal tubular wasting defects described.
• B. Gastrointestinal bicarbonate loss would not cause glucosuria, phosphaturia, or aminoaciduria.
• C. Hyperaldosteronism causes hypokalemic metabolic alkalosis, not the acidosis and tubular wasting seen here.
• D. Distal RTA causes isolated impairment of urinary acidification without the glucosuria, phosphaturia, and aminoaciduria of Fanconi syndrome.

Type 4 RTA results from hyporeninemic hypoaldosteronism, commonly seen in diabetic nephropathy due to damage to the juxtaglomerular apparatus, producing low renin and consequently low aldosterone. Aldosterone deficiency impairs distal potassium and hydrogen ion secretion, causing hyperkalemia and a mild normal anion gap metabolic acidosis, as seen in this patient's low renin and aldosterone levels. Hyperkalemia itself also impairs ammoniagenesis, worsening the acidosis. Type 1 and type 2 RTA characteristically cause hypokalemia rather than hyperkalemia, which distinguishes them from this presentation.

• A. Primary hyperaldosteronism causes hypokalemic metabolic alkalosis with high, not low, aldosterone.
• B. Distal RTA typically causes hypokalemia, not hyperkalemia as seen in this patient.
• D. Proximal RTA also typically causes hypokalemia rather than hyperkalemia.
• E. Diarrhea causes hypokalemic normal anion gap acidosis with low, not low-normal, renin and aldosterone being irrelevant findings.

This patient has a hypochloremic, hypokalemic metabolic alkalosis from vomiting with a low urine chloride (less than 20 mEq/L), indicating a saline-responsive process driven by volume depletion and chloride loss. Volume depletion stimulates renin-angiotensin-aldosterone activity and bicarbonate reabsorption to maintain the alkalosis, so restoring extracellular volume and chloride with isotonic saline (plus potassium) corrects the alkalosis by removing the stimulus for bicarbonate retention. A high urine chloride (greater than 20 mEq/L) would instead suggest a saline-resistant cause such as hyperaldosteronism or severe hypokalemia, which would not respond to volume repletion. Acetazolamide is reserved for saline-resistant or edematous states where volume expansion is undesirable.

• A. Fludrocortisone would worsen the alkalosis by promoting further renal bicarbonate retention and potassium loss.
• B. Direct acid infusion is reserved for severe, refractory alkalosis and is not first-line therapy here.
• D. Water restriction treats hyponatremia from free water excess, not this chloride-responsive alkalosis.
• E. Acetazolamide is used in saline-resistant metabolic alkalosis, particularly with volume overload, not in a volume-depleted saline-responsive patient.

This patient has chronic CO2 retention from COPD, reflected in his baseline elevated PCO2 and compensatory elevated bicarbonate with a near-normal pH 6 months ago. During the current exacerbation, PCO2 has risen further from his chronic baseline, and the pH has dropped more than expected for his chronic compensation, indicating a superimposed acute worsening of respiratory acidosis rather than a new chronic steady state. Chronic respiratory acidosis alone would show a pH close to normal from full renal compensation, which is not the case here since the pH has fallen further from baseline. This pattern of acute-on-chronic respiratory acidosis is common during COPD exacerbations when ventilation acutely worsens beyond the patient's compensated baseline.

• A. There is no separate metabolic acidosis process; the bicarbonate is elevated as compensation, not reduced.
• B. Full metabolic compensation would keep the pH close to normal, but this patient's pH has dropped further from his chronic baseline, indicating acute decompensation.
• C. Chronic respiratory acidosis alone would show a stable pH near normal, not the further pH decline from baseline seen here.
• E. There is no primary metabolic alkalosis; the primary problem is CO2 retention with renal bicarbonate compensation.

Salicylates directly stimulate the medullary respiratory center causing a primary respiratory alkalosis, and they also uncouple oxidative phosphorylation and cause accumulation of organic acids, producing a primary elevated anion gap metabolic acidosis. The near-normal pH with markedly low PCO2 and low bicarbonate occurring together, rather than one process being purely compensatory for the other, indicates two simultaneous primary processes. If the respiratory alkalosis were simply compensating for the metabolic acidosis, PCO2 would not typically fall as low as 18 mmHg for this degree of bicarbonate reduction, and the near-normal pH reflects the near cancellation of two counteracting primary disorders. This classic mixed picture is highly characteristic of salicylate poisoning, especially in adults, and the elevated anion gap confirms the metabolic acidosis component.

• A. Appropriate compensation for this degree of metabolic acidosis would lower pH, not keep it near normal with such a low PCO2.
• B. There is no respiratory acidosis in this patient; PCO2 is markedly reduced, not elevated.
• D. Renal compensation for respiratory alkalosis occurs slowly over days and would not explain the markedly reduced bicarbonate seen within hours.
• E. There is no metabolic alkalosis in this patient; bicarbonate is reduced, not elevated.

The combination of elevated serum calcium with a concurrently elevated (not suppressed) PTH level is diagnostic of primary hyperparathyroidism, usually caused by a single parathyroid adenoma. The low phosphorus reflects PTH-driven renal phosphate wasting, and high urinary calcium excretion excludes the hypocalciuric variant. Many cases are detected incidentally on routine chemistry panels in asymptomatic patients. Parathyroidectomy is curative in symptomatic or qualifying asymptomatic patients.

• A. Myeloma causes hypercalcemia through osteoclastic bone resorption with a suppressed, not elevated, PTH.
• B. Secondary hyperparathyroidism occurs in response to hypocalcemia (e.g., renal failure, vitamin D deficiency), not with hypercalcemia.
• D. Sarcoidosis causes hypercalcemia via excess calcitriol production, which suppresses PTH.
• E. This condition causes low, not high, urinary calcium excretion despite elevated serum calcium and PTH.

Humoral hypercalcemia of malignancy, most often seen with squamous cell carcinomas, results from tumor secretion of parathyroid hormone-related peptide (PTHrP). PTHrP shares homology with the amino-terminal region of PTH and binds the same PTH/PTHrP receptor, increasing bone resorption and renal calcium reabsorption while suppressing endogenous PTH, which explains the low intact PTH level here. Phosphorus is low because PTHrP also promotes renal phosphate wasting, mirroring PTH's effect. This mechanism does not require bone metastases to be present.

• A. FGF23 excess causes phosphate wasting and hypophosphatemic disorders, not hypercalcemia.
• B. Direct osteolysis from bone metastases causes local hypercalcemia but is not the dominant mechanism here, and intact PTH would not be suppressed by this alone.
• C. This mechanism causes hypercalcemia in lymphomas, not squamous cell lung cancer.
• E. Ectopic PTH secretion by non-parathyroid tumors is exceedingly rare; this patient's low intact PTH argues against it.

Severe symptomatic hypercalcemia of malignancy is treated with aggressive isotonic saline hydration followed promptly by an intravenous bisphosphonate such as zoledronic acid or pamidronate, which inhibits osteoclast-mediated bone resorption. The calcium-lowering effect of bisphosphonates takes 24-48 hours to onset, so early initiation is important even while rehydration continues. Calcitonin may be added for faster, though temporary, effect in very severe cases. Denosumab is reserved for bisphosphonate-refractory cases or renal impairment precluding bisphosphonate use.

• A. Denosumab is a second-line option, typically reserved for bisphosphonate failure or significant renal impairment.
• B. Loop diuretics are no longer recommended as routine therapy and should only be used cautiously after volume repletion, not as the next step.
• D. Glucocorticoids are useful for hypercalcemia from granulomatous disease or lymphoma, not typically for PTHrP-mediated malignancy.
• E. Cinacalcet is used for secondary hyperparathyroidism in dialysis patients or parathyroid carcinoma, not acute hypercalcemia of malignancy.

Hypocalcemia within the first 24-48 hours after total thyroidectomy is most commonly due to inadvertent removal, injury, or devascularization of one or more parathyroid glands, which are small and closely adherent to the thyroid. The resulting drop in PTH causes decreased bone resorption and renal calcium reabsorption, producing hypocalcemia with concurrent hyperphosphatemia (since PTH normally promotes phosphate excretion). Perioral and acral paresthesias are classic early symptoms, and untreated cases can progress to tetany or seizures.

• A. Hyperventilation-induced alkalosis lowers ionized calcium but total serum calcium remains normal, unlike in this patient.
• B. Adrenal insufficiency causes hyponatremia, hyperkalemia, and hypotension, not isolated hypocalcemia with hyperphosphatemia.
• D. Vitamin D deficiency typically causes hypocalcemia with low, not high, phosphorus due to secondary hyperparathyroidism.
• E. Recurrent laryngeal nerve injury causes voice changes or stridor, not paresthesias or hypocalcemia.

Chvostek sign (facial twitching upon tapping the facial nerve) and Trousseau sign (carpal spasm upon brachial cuff inflation causing transient ischemia) are classic bedside findings of neuromuscular irritability caused by hypocalcemia, specifically low ionized calcium. Hypocalcemia increases neuronal membrane excitability by lowering the threshold for sodium channel activation. Autoimmune polyglandular involvement, as suggested by this patient's Hashimoto thyroiditis, raises suspicion for autoimmune hypoparathyroidism as the underlying cause. Confirmatory total and ionized calcium levels should be obtained promptly.

• A. Hyperphosphatemia alone does not cause Chvostek or Trousseau signs; these reflect low calcium, not phosphorus.
• C. Hyponatremia causes confusion, seizures, and lethargy, not these specific neuromuscular signs.
• D. Isolated hypomagnesemia with normal calcium does not classically produce these specific signs; magnesium deficiency usually causes hypocalcemia, which then produces the findings.
• E. Hyperkalemia causes muscle weakness and cardiac conduction abnormalities, not Chvostek or Trousseau signs.

Magnesium normally inhibits the renal outer medullary potassium (ROMK) channel in the distal nephron from the intracellular side. When intracellular magnesium is depleted, this inhibition is lost, causing inappropriate potassium secretion into the tubular lumen and renal potassium wasting that persists despite potassium supplementation. This is why hypokalemia in the setting of hypomagnesemia is often refractory to potassium repletion alone and requires correction of magnesium first. Both electrolyte abnormalities are common in alcohol use disorder due to poor intake and renal magnesium wasting.

• A. Hypomagnesemia does not act primarily through aldosterone stimulation to cause potassium loss; the effect is via direct ROMK channel regulation.
• B. Hypocalcemia does not directly drive renal potassium wasting; magnesium depletion is the key mechanism here.
• D. Alcohol's effect on potassium balance in this setting is mediated mainly through magnesium depletion and renal wasting, not direct intestinal inhibition.
• E. Hypomagnesemia causes renal potassium loss, not an intracellular shift, as the dominant mechanism of refractory hypokalemia.

Loss of deep tendon reflexes, particularly the patellar reflex, is the earliest and most reliable clinical sign of magnesium toxicity and should be checked frequently during intravenous magnesium repletion, especially in patients with renal impairment who are at higher risk of accumulation. As magnesium levels rise further, toxicity progresses to respiratory depression, conduction abnormalities, and ultimately cardiac arrest. This bedside monitoring parallels its well-known use during magnesium sulfate therapy for eclampsia. Serum magnesium levels should also be checked periodically, but reflex monitoring provides immediate bedside warning.

• A. Lactate reflects tissue perfusion and metabolism, not magnesium toxicity.
• B. Peak expiratory flow assesses airway obstruction, not neuromuscular status related to magnesium excess.
• C. Creatine kinase is not a marker of magnesium toxicity and is used to assess rhabdomyolysis or myocardial injury.
• D. Pupillary reflexes are not affected by magnesium toxicity and are not a standard monitoring parameter.

This patient has tumor lysis syndrome, marked by hyperkalemia, hyperphosphatemia, hypocalcemia (from calcium-phosphate precipitation and chelation), and acute kidney injury from massive tumor cell death after chemotherapy for a high-grade, rapidly proliferating lymphoma. Prevention centers on aggressive intravenous hydration to maintain high urine output and reduce intratubular crystal precipitation, combined with rasburicase (recombinant urate oxidase) to rapidly lower uric acid levels in high-risk patients. Allopurinol is used for lower-risk patients but acts more slowly and does not reduce existing uric acid. Early identification of high-risk tumors, such as Burkitt lymphoma with high tumor burden, should prompt prophylactic measures before chemotherapy begins.

• A. Calcium supplementation does not prevent tumor lysis syndrome and is reserved for symptomatic hypocalcemia after it develops.
• B. Fluid restriction is incorrect and dangerous; aggressive hydration, not restriction, is key to prevention.
• C. Bisphosphonates are not used for tumor lysis prevention; they are used for hypercalcemia of malignancy.
• E. Potassium-binding resins treat existing hyperkalemia but do not prevent the syndrome from occurring.

This patient has a high tumor burden, bulky disease, and markedly elevated LDH, placing him at high risk for tumor lysis syndrome once chemotherapy begins. Rasburicase, a recombinant urate oxidase, rapidly converts uric acid to allantoin, which is far more soluble and easily excreted, preventing urate nephropathy. Aggressive intravenous hydration promotes a high urine output to flush out uric acid, potassium, and phosphate released from lysed tumor cells. Allopurinol is used for low- to intermediate-risk patients but works too slowly and does not reduce existing uric acid in high-risk cases. Urine alkalinization is no longer routinely recommended because it can promote calcium phosphate precipitation in the kidneys.

• A. Urinary alkalinization is no longer recommended because it increases the risk of calcium phosphate crystal deposition in renal tubules.
• B. Dialysis is reserved for established tumor lysis syndrome with severe electrolyte derangements or renal failure, not as routine prophylaxis before chemotherapy.
• D. Allopurinol only blocks new uric acid formation and is reserved for lower-risk patients, not high-risk cases with high LDH and bulky disease.
• E. Tumor lysis syndrome causes hyperphosphatemia; giving additional phosphate would worsen the risk of calcium phosphate precipitation and AKI.

The fractional excretion of sodium (FENa) here is calculated as (urine Na x serum creatinine) / (serum Na x urine creatinine) x 100, which equals (8 x 2.1) / (140 x 60) x 100, roughly 0.2%, well below the 1% threshold that suggests a prerenal cause. In prerenal azotemia from volume depletion, intact tubules avidly reabsorb sodium and water in response to decreased renal perfusion, producing a low urine sodium and low FENa. The clinical picture of vomiting, dry mucous membranes, tachycardia, and hypotension supports true volume depletion as the cause of decreased renal perfusion. A bland urinalysis without casts or protein argues against intrinsic renal disease.

• A. Glomerulonephritis typically presents with hematuria, red cell casts, or proteinuria, none of which are seen here.
• B. Obstructive uropathy would not produce this degree of sodium avidity and is not supported by the history or exam.
• C. ATN typically produces a FENa greater than 2% and muddy brown casts on urinalysis, neither of which is present here.
• D. AIN usually shows pyuria, white cell casts, or eosinophils on urinalysis, which are absent in this bland sediment.

Aminoglycosides such as gentamicin accumulate in proximal tubular cells and cause direct toxic injury, leading to acute tubular necrosis, typically after 5 to 10 days of therapy. The urinalysis finding of granular, muddy brown casts reflects sloughed tubular epithelial cells and is a hallmark of ATN. A FENa above 2% indicates loss of tubular concentrating ability, consistent with intrinsic tubular injury rather than a prerenal process. Nephrotoxic ATN is typically nonoliguric and develops gradually over the course of treatment, as seen in this patient on hospital day 8.

• A. Prerenal azotemia would show a FENa below 1% and a bland urinalysis, not muddy brown casts.
• C. AIN from beta-lactams classically presents with fever, rash, eosinophilia, and white cell casts or eosinophiluria, not muddy brown casts.
• D. Obstruction would show hydronephrosis on imaging and would not produce muddy brown casts.
• E. There is no mention of a recent contrast-enhanced imaging study, making this diagnosis unlikely.

In patients with reduced kidney function undergoing a procedure requiring iodinated contrast, periprocedural administration of isotonic intravenous fluids, typically normal saline, is the most effective and evidence-supported strategy to reduce the risk of contrast-induced nephropathy. Adequate volume expansion improves renal medullary blood flow and increases urinary flow rate, diluting the contrast agent and limiting direct tubular toxicity. N-acetylcysteine has not shown consistent benefit in large randomized trials and is not recommended as a primary preventive strategy. Diuretics should be avoided before contrast administration because they can cause volume depletion and worsen renal perfusion.

• A. Dialysis immediately after contrast exposure does not prevent nephropathy and is not indicated for prophylaxis in this setting.
• B. Furosemide before contrast administration can cause volume depletion and increase the risk of renal injury rather than prevent it.
• C. Sodium bicarbonate alone without volume expansion has not shown clear superiority and isotonic saline remains the standard approach.
• E. N-acetylcysteine lacks consistent evidence of benefit in large trials and is not a first-line preventive measure.

This patient's presentation of fever, rash, peripheral eosinophilia, and urinary eosinophils occurring after initiation of a nonsteroidal anti-inflammatory drug is classic for acute interstitial nephritis, a hypersensitivity reaction within the renal interstitium. White blood cell casts reflect interstitial inflammation rather than glomerular or tubular epithelial injury. Common culprits include NSAIDs, beta-lactam antibiotics, and proton pump inhibitors, with onset typically 7 to 10 days after drug exposure. Diagnosis is often made clinically, though a kidney biopsy showing interstitial infiltrate with eosinophils confirms the diagnosis in unclear cases; the mainstay of management is prompt discontinuation of the offending drug.

• A. RPGN typically presents with red cell casts and hematuria from glomerular injury, not eosinophiluria and white cell casts.
• B. Papillary necrosis presents with gross hematuria and flank pain, often in the setting of chronic NSAID use, diabetes, or sickle cell disease, not this acute hypersensitivity picture.
• C. ATN presents with granular muddy brown casts rather than white cell casts and urinary eosinophils.
• D. Minimal change disease causes nephrotic-range proteinuria without the systemic hypersensitivity features seen here.

This patient has acute urinary retention from bladder outlet obstruction due to benign prostatic hyperplasia, confirmed by the distended bladder, bilateral hydronephrosis, and acute rise in creatinine, representing postrenal acute kidney injury. The most appropriate immediate step is bladder decompression with a urethral catheter, or a suprapubic catheter if urethral catheterization is unsuccessful, to relieve the obstruction and allow renal function to recover. Prompt decompression is both diagnostic and therapeutic, and renal function typically improves once the obstruction is relieved. Imaging with intravenous contrast should be avoided in a patient with significant AKI because of the added risk of nephrotoxicity.

• A. Hemodialysis is not indicated before attempting decompression, which is likely to reverse the kidney injury on its own.
• B. Intravenous fluids do not address the underlying mechanical obstruction and will not relieve the AKI.
• C. Finasteride takes weeks to months to reduce prostate size and does not address the acute obstruction.
• E. Contrast-enhanced imaging adds unnecessary nephrotoxic risk in a patient with acute kidney injury and is not needed when ultrasound has already shown hydronephrosis.

This patient has refractory volume overload with pulmonary edema and hypoxemia unresponsive to diuretics, which is an absolute indication for emergent hemodialysis in acute kidney injury, captured by the fluid overload component of the AEIOU mnemonic for dialysis indications (acidosis, electrolyte abnormalities, intoxications, volume overload, uremic symptoms). Although his acidosis and mild hyperkalemia also support dialysis, the dominant and most urgent indication here is diuretic-refractory pulmonary edema causing respiratory compromise. Continued diuretic escalation is unlikely to be effective given the severity of oliguria and would delay definitive treatment. Supportive measures like calcium gluconate or bicarbonate do not remove excess volume and are not substitutes for dialysis in this setting.

• A. Calcium gluconate stabilizes cardiac membranes in hyperkalemia but does not treat volume overload or acidosis and is not a definitive treatment here.
• C. Bicarbonate may transiently raise pH but does not address the volume overload driving respiratory failure and does not substitute for dialysis.
• D. Further diuretic escalation is unlikely to succeed in this oliguric patient and would unnecessarily delay definitive treatment of fluid overload.
• E. Noninvasive ventilation may support oxygenation temporarily but does not remove excess fluid or correct the underlying metabolic derangements.

Active urinary sediment (hematuria with red cell casts), proteinuria, rising creatinine, and hypocomplementemia with elevated anti-dsDNA strongly suggest active lupus nephritis. A renal biopsy is required to confirm the diagnosis, determine the ISN/RPS histologic class, and assess activity and chronicity indices, since treatment intensity (e.g., mycophenolate or cyclophosphamide plus glucocorticoids versus less aggressive therapy) depends directly on the biopsy class. Clinical and serologic findings alone cannot reliably distinguish proliferative from non-proliferative classes. Biopsy findings also provide prognostic information and a baseline for monitoring treatment response.

• A. Ultrasound evaluates size, obstruction, or vascular flow but cannot characterize glomerular pathology.
• B. Quantifying proteinuria is useful but does not determine histologic class or guide immunosuppressive choice.
• C. Starting aggressive immunosuppression without a biopsy risks over- or under-treatment since histologic class is unknown.
• D. Changing hydroxychloroquine dosing without addressing active nephritis risks progression to chronic kidney damage.

In CKD, failing kidneys produce less 1,25-dihydroxyvitamin D (calcitriol) due to reduced 1-alpha-hydroxylase activity, which decreases intestinal calcium absorption and lowers serum calcium. Phosphate retention from reduced renal excretion further suppresses calcitriol synthesis and directly stimulates parathyroid hormone secretion. The resulting hypocalcemia and hyperphosphatemia remove the normal negative feedback on the parathyroid glands, causing secondary hyperparathyroidism with compensatory parathyroid gland hyperplasia. Chronically elevated parathyroid hormone increases osteoclastic bone resorption, producing the bone pain and osteitis fibrosa cystica seen in renal osteodystrophy.

• B. Elevated creatinine does not directly stimulate parathyroid hormone secretion; the stimulus is hypocalcemia and hyperphosphatemia.
• C. Ectopic PTHrP secretion causes primary hyperparathyroidism-like hypercalcemia and is not detected by standard intact PTH assays.
• D. CKD causes decreased, not increased, calcium reabsorption and this patient has hypocalcemia, not hypercalcemia.
• E. An autonomous adenoma causing primary hyperparathyroidism would present with hypercalcemia, not the hypocalcemia seen here.

This patient has diabetic nephropathy with moderately increased albuminuria (A2 category) and hypertension. ACE inhibitors such as lisinopril reduce intraglomerular pressure by dilating the efferent arteriole, which decreases albuminuria and slows progression of diabetic kidney disease independent of their blood pressure-lowering effect. Angiotensin receptor blockers provide equivalent renoprotection and are used when ACE inhibitors are not tolerated due to cough. Current guidelines recommend an ACE inhibitor or ARB as first-line therapy in diabetic patients with hypertension and albuminuria, with potassium and creatinine monitored after initiation.

• A. Clonidine is a centrally acting antihypertensive with no specific benefit for diabetic nephropathy progression.
• B. Hydrochlorothiazide treats hypertension but has no specific renoprotective effect on diabetic nephropathy.
• C. Amlodipine lowers blood pressure but does not reduce intraglomerular pressure or albuminuria to the same degree as ACE inhibitors.
• E. Beta-blockers control blood pressure and heart rate but do not provide the afferent/efferent arteriolar effects needed for renoprotection.

SGLT2 inhibitors such as dapagliflozin reduce intraglomerular pressure by causing afferent arteriolar vasoconstriction through restoration of tubuloglomerular feedback, independent of their glucose-lowering effect. Large trials have demonstrated that SGLT2 inhibitors reduce the composite risk of sustained eGFR decline, progression to end-stage renal disease, and renal or cardiovascular death in patients with diabetic and non-diabetic CKD with albuminuria, when added to an ACE inhibitor or ARB. Current guidelines recommend SGLT2 inhibitors for patients with type 2 diabetes and CKD with an eGFR generally above 20-25 mL/min/1.73 m², regardless of glycemic control, because the renal and cardiovascular benefits occur independent of hemoglobin A1c improvement.

• A. Pioglitazone can cause fluid retention and has no proven benefit in slowing progression of diabetic nephropathy.
• B. Sulfonylureas lower glucose but do not have renoprotective effects independent of glycemic control.
• C. Insulin improves glycemic control but has not been shown to independently slow progression of diabetic kidney disease.
• D. DPP-4 inhibitors like sitagliptin lower glucose but have not demonstrated the renoprotective effects seen with SGLT2 inhibitors.

This patient has hyperphosphatemia with secondary hyperparathyroidism in the setting of advanced CKD, and poor adherence to calcium carbonate due to constipation. Sevelamer is a non-calcium-based phosphate binder that lowers serum phosphorus by binding dietary phosphate in the gut without contributing to calcium load or vascular calcification risk, and it has a more favorable gastrointestinal side effect profile than calcium-based binders. Switching to sevelamer addresses both the inadequate phosphate control and the adherence barrier caused by constipation. Dietary phosphorus restriction should also be reinforced, but a binder is still needed given the degree of hyperphosphatemia.

• A. Increasing dietary phosphorus would worsen this patient's hyperphosphatemia and is contraindicated.
• B. Thiazide diuretics are not used to manage hyperphosphatemia in CKD and have minimal effect on phosphate excretion at this GFR.
• D. Discontinuing phosphate binder therapy would worsen hyperphosphatemia and secondary hyperparathyroidism.
• E. Calcitriol increases intestinal calcium and phosphate absorption and would worsen hyperphosphatemia if given without a phosphate binder.

The most important intervention for preventing calcium oxalate stone recurrence is increasing fluid intake to achieve a urine output of at least 2 to 2.5 liters per day, which dilutes urinary solutes and reduces supersaturation. Dietary calcium should not be restricted because normal calcium intake binds oxalate in the gut, reducing its intestinal absorption and subsequent urinary excretion; low calcium intake paradoxically increases oxalate absorption and stone risk. Patients should also be counseled to reduce sodium and animal protein intake and avoid excessive oxalate-rich foods, but adequate hydration remains the cornerstone of prevention. High-dose vitamin C supplementation should be avoided because it is metabolized to oxalate and can increase urinary oxalate excretion.

• A. High animal protein intake increases urinary calcium and uric acid excretion and lowers urinary citrate, increasing stone risk.
• B. High-dose vitamin C is metabolized to oxalate and increases urinary oxalate excretion, raising stone risk.
• D. Restricting dietary calcium increases intestinal oxalate absorption and urinary oxalate excretion, raising stone risk.
• E. Restricting fluid intake increases urinary solute concentration and promotes further stone formation.

Struvite (magnesium ammonium phosphate) staghorn calculi form in the setting of chronic infection with urease-producing organisms, classically Proteus mirabilis, which hydrolyzes urea into ammonia, raising urine pH above 7.2 and promoting precipitation of struvite crystals. The alkaline urine pH and branching calculus filling the collecting system in this vignette are characteristic findings. Treatment requires complete stone removal, typically via percutaneous nephrolithotomy, because residual fragments harbor bacteria and lead to recurrence. Proteus is also lactose-negative and characteristically motile, producing a distinctive swarming pattern on culture media.

• A. Escherichia coli is the most common cause of uncomplicated UTIs but is not a strong urease producer and does not typically cause staghorn calculi.
• B. Klebsiella pneumoniae can produce urease but is a less common cause of staghorn calculi than Proteus mirabilis.
• D. Staphylococcus saprophyticus is a gram-positive coccus causing UTIs in young women but is not a classic urease-producing stone-former.
• E. Enterococcus faecalis is a gram-positive coccus, not a gram-negative rod, and is not a major urease producer.

In pregnant patients with suspected nephrolithiasis, ultrasonography is the preferred initial imaging modality because it avoids ionizing radiation and is widely available and inexpensive. It can detect hydronephrosis and sometimes the calculus itself, and findings often guide management without further imaging. If ultrasound is inconclusive and the diagnosis remains uncertain, non-contrast MRI can be used as a second-line study, reserving CT for cases where the benefit clearly outweighs fetal radiation risk. This stepwise approach minimizes fetal radiation exposure while still allowing timely diagnosis.

• B. Contrast-enhanced CT adds iodinated contrast exposure without clear benefit over non-contrast imaging and is not appropriate as an initial test, especially in pregnancy.
• C. Intravenous pyelography involves both ionizing radiation and iodinated contrast and is not used as the initial imaging test in pregnancy.
• D. Non-contrast CT is the preferred initial test for nephrolithiasis in non-pregnant patients but is avoided as a first-line study in pregnancy due to ionizing radiation.
• E. MRI without contrast is a reasonable second-line test if ultrasound is nondiagnostic but is not the first-line study due to cost and limited availability.