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ABP Pediatric Nephrology study materials
ABP Pediatric Nephrology study resources

ABP Pediatric Nephrology practice questions

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Question 1Glomerular and Vascular Disorders

A 4-year-old boy presents with periorbital edema, frothy urine, and a serum albumin of 1.8 g/dL. Blood pressure and renal function are normal. Which histologic diagnosis is most likely?

  1. Membranous nephropathy
  2. Focal segmental glomerulosclerosis
  3. IgA nephropathy
  4. Minimal change disease Correct answer.

Explanation

Minimal change disease accounts for roughly 80-90% of nephrotic syndrome in children younger than 6 years and typically presents with isolated nephrotic features, including normal blood pressure and normal renal function. The diagnosis is supported by a dramatic response to corticosteroids, and biopsy is often reserved for atypical or steroid-resistant cases.

Question 2Glomerular and Vascular Disorders

A 9-year-old presents with cola-colored urine and hypertension 10 days after pharyngitis. Which complement profile is expected?

  1. Low C3 with normal C4 Correct answer.
  2. Elevated C3 with low C4
  3. Normal C3 and normal C4
  4. Low C3 and low C4

Explanation

Post-streptococcal glomerulonephritis activates the alternative complement pathway, so C3 is consumed while C4 remains normal. C3 levels should normalize within 6-8 weeks; persistently low C3 suggests an alternative diagnosis such as C3 glomerulopathy.

Question 3Glomerular and Vascular Disorders

Which clinical feature is most characteristic of IgA vasculitis (Henoch-Schönlein purpura) in a child?

  1. Diffuse alveolar hemorrhage with hemoptysis
  2. Palpable purpura on the buttocks and legs Correct answer.
  3. Painless jaundice with marked hepatomegaly
  4. Migratory polyarthritis of large joints only

Explanation

IgA vasculitis is a small-vessel leukocytoclastic vasculitis characterized by nonthrombocytopenic palpable purpura over dependent areas such as the buttocks and lower extremities, often with abdominal pain and arthritis. Renal involvement occurs in roughly 30-50% of children and ranges from isolated hematuria to nephritic or nephrotic syndrome.

Question 4Glomerular and Vascular Disorders

The classic triad of hemolytic uremic syndrome consists of which findings?

  1. Fever, petechial rash, and reactive thrombocytosis
  2. Gross hematuria, sensorineural hearing loss, and hypertension
  3. Recurrent hypoglycemia, generalized seizures, and cholestatic jaundice
  4. Microangiopathic hemolysis, thrombocytopenia, and acute kidney injury Correct answer.

Explanation

Typical HUS is defined by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury, usually after a diarrheal illness caused by Shiga toxin-producing E. coli. Schistocytes on the blood film and a negative direct antiglobulin test support the diagnosis.

Question 5Glomerular and Vascular Disorders

In a previously healthy child with bloody diarrhea followed by hemolytic uremic syndrome, which organism is most commonly responsible?

  1. E. coli O157:H7 Correct answer.
  2. Streptococcus pneumoniae
  3. Campylobacter jejuni
  4. Yersinia enterocolitica

Explanation

Shiga toxin-producing E. coli, classically serotype O157:H7, causes about 90% of typical childhood HUS and follows bloody diarrhea by 5-10 days. Streptococcus pneumoniae is a rare cause of atypical HUS mediated by neuraminidase exposure of the Thomsen-Friedenreich antigen.

Question 6Glomerular and Vascular Disorders

What is the most common inheritance pattern of Alport syndrome?

  1. Autosomal dominant
  2. X-linked Correct answer.
  3. Autosomal recessive
  4. Mitochondrial

Explanation

About 80% of Alport syndrome is X-linked, caused by mutations in COL4A5, which explains why males are more severely affected. Autosomal recessive forms involve COL4A3 or COL4A4 and carry similar severity, while autosomal dominant forms are generally milder.

Question 7Glomerular and Vascular Disorders

A child with new nephrotic syndrome is started on daily prednisone. By which time point must remission occur to classify the disease as steroid-sensitive?

  1. Within 1 week
  2. Within 12 weeks
  3. Within 4 weeks Correct answer.
  4. Within 24 hours

Explanation

Steroid-sensitive nephrotic syndrome is defined by remission within 4 weeks of daily prednisone at 60 mg/m2/day, and most responders do so within the first 2 weeks. Failure to respond by 4-6 weeks defines steroid resistance and prompts biopsy and genetic evaluation.

Question 8Glomerular and Vascular Disorders

A 12-year-old develops gross hematuria within 24 hours of the onset of a sore throat. Which diagnosis is most likely?

  1. Hemolytic uremic syndrome
  2. Post-streptococcal glomerulonephritis
  3. IgA nephropathy Correct answer.
  4. Alport syndrome

Explanation

Gross hematuria beginning within 1-2 days of a respiratory infection, termed synpharyngitic hematuria, is characteristic of IgA nephropathy and reflects mucosal immune activation. Post-streptococcal glomerulonephritis typically has a latent period of 1-3 weeks after infection.

Question 9Glomerular and Vascular Disorders

A 7-year-old with nephrotic syndrome does not respond to 8 weeks of prednisone. Biopsy shows sclerosis of some glomeruli with the rest normal. Which finding best distinguishes FSGS from minimal change disease?

  1. Focal segmental scarring Correct answer.
  2. Lipid droplets in tubular cells
  3. Diffuse foot process effacement
  4. Mesangial IgA deposits

Explanation

FSGS is defined by sclerosis involving segments of some, but not all, glomeruli, whereas minimal change disease shows normal light microscopy with only diffuse podocyte foot process effacement on electron microscopy. Because foot process effacement occurs in both, the presence of segmental scarring is the distinguishing feature.

Question 10Glomerular and Vascular Disorders

Steroid-resistant nephrotic syndrome in children is best defined as failure to achieve remission after how long of adequate prednisone therapy?

  1. 2 weeks
  2. 12 weeks
  3. 4-6 weeks Correct answer.
  4. 6 months

Explanation

Steroid-resistant nephrotic syndrome is defined by failure to achieve remission after 4-6 weeks of adequate prednisone therapy. These children require kidney biopsy and consideration of genetic testing for podocyte gene mutations.

Question 11Glomerular and Vascular Disorders

A child had post-streptococcal glomerulonephritis with low C3 eight weeks ago. C3 remains low at follow-up. What is the most appropriate next step?

  1. Repeat the ASO titer to confirm the diagnosis
  2. Start corticosteroids for presumed active nephritis
  3. Evaluate for C3 glomerulopathy or another complement disorder Correct answer.
  4. Reassure the family that C3 will normalize over several months

Explanation

C3 levels in post-streptococcal glomerulonephritis should return to normal within 6-8 weeks, and persistent low C3 beyond about 12 weeks suggests a different disorder such as C3 glomerulopathy or a complement regulatory defect. ASO titers indicate recent streptococcal infection but do not guide complement follow-up.

Question 12Glomerular and Vascular Disorders

Which complement profile is typical of C3 glomerulopathy?

  1. Normal C3 with elevated C4
  2. Elevated C3 with low C4
  3. Low C3 and low C4
  4. Low C3 with normal C4 Correct answer.

Explanation

C3 glomerulopathy results from dysregulation of the alternative complement pathway, producing low C3 with a normal C4. Low C3 together with low C4 points to classical pathway activation, as seen in lupus nephritis.

Question 13Glomerular and Vascular Disorders

Which statement about membranous nephropathy in children is most accurate?

  1. It is always associated with anti-PLA2R antibodies in children
  2. Secondary causes are more common in children than in adults Correct answer.
  3. It is the most common cause of childhood nephrotic syndrome
  4. It typically presents with gross hematuria after a respiratory infection

Explanation

Membranous nephropathy is rare in children and, unlike the adult primary form, is more often secondary to lupus, hepatitis B, or syphilis. Anti-PLA2R antibodies are found in most adult primary cases but are uncommon in children.

Question 14Glomerular and Vascular Disorders

A 3-year-old presents with hemolytic uremic syndrome but no diarrhea, and stool testing for Shiga toxin is negative. Which diagnosis should be considered?

  1. Complement-mediated atypical HUS Correct answer.
  2. IgA nephropathy
  3. Minimal change disease
  4. Post-streptococcal glomerulonephritis

Explanation

HUS without a diarrheal prodrome and with negative Shiga toxin testing suggests complement-mediated atypical HUS, which results from mutations or autoantibodies involving factor H, factor I, MCP, or C3. These children are treated with eculizumab and require long-term complement evaluation.

Question 15Glomerular and Vascular Disorders

Which eye finding is most characteristic of Alport syndrome?

  1. Cherry-red macula
  2. Anterior lenticonus Correct answer.
  3. Papilledema
  4. Cataracts

Explanation

Anterior lenticonus and dot-and-fleck retinopathy are characteristic ocular findings in Alport syndrome. Sensorineural hearing loss and progressive kidney failure complete the classic clinical triad.

Question 16Glomerular and Vascular Disorders

A 10-year-old has persistent isolated microscopic hematuria, a positive family history, normal renal function, and normal hearing. Biopsy shows diffuse thinning of the glomerular basement membrane. Which diagnosis is most likely?

  1. Alport syndrome with X-linked inheritance
  2. Thin basement membrane nephropathy Correct answer.
  3. Post-streptococcal glomerulonephritis
  4. IgA nephropathy

Explanation

Thin basement membrane nephropathy is a benign familial hematuria caused by heterozygous COL4A3 or COL4A4 mutations, with diffuse basement membrane thinning but no hearing loss or progressive kidney disease. Alport syndrome typically shows irregular thickening and lamellation of the basement membrane on electron microscopy.

Question 17Glomerular and Vascular Disorders

In children with systemic lupus erythematosus and severe nephritis, which ISN/RPS class is most common?

  1. Class IV (diffuse proliferative) Correct answer.
  2. Class I (minimal mesangial)
  3. Class V (membranous)
  4. Class II (mesangial proliferative)

Explanation

Class IV lupus nephritis, diffuse proliferative glomerulonephritis, is the most common and most severe form in children and requires aggressive immunosuppression. Classes I and II are mesangial and generally mild, while class V is membranous and less common in children.

Question 18Glomerular and Vascular Disorders

A child with IgA vasculitis has normal renal function, normal blood pressure, and microscopic hematuria with trace proteinuria. What is the most appropriate management?

  1. High-dose pulse methylprednisolone
  2. Supportive care with monitoring Correct answer.
  3. Plasma exchange
  4. Immediate cyclophosphamide

Explanation

Children with IgA vasculitis nephritis who have only isolated microscopic hematuria and normal renal function and blood pressure are managed with supportive care and monitoring. Corticosteroids are considered for severe or progressive nephritis, and routine steroids do not prevent renal involvement.

Question 19Glomerular and Vascular Disorders

Which immunofluorescence pattern is characteristic of anti-GBM disease?

  1. Subepithelial spikes with IgG
  2. Linear IgG along the GBM Correct answer.
  3. Starry-sky mesangial IgA deposits
  4. Granular IgG and C3 along the GBM

Explanation

Anti-GBM disease produces linear deposition of IgG along the glomerular basement membrane on immunofluorescence. Granular staining indicates immune-complex deposition, and subepithelial spikes are typical of membranous nephropathy.

Question 20Glomerular and Vascular Disorders

An infant with ambiguous genitalia, early nephrotic syndrome, and bilateral Wilms tumor most likely has a mutation in which gene?

  1. CFH
  2. COL4A5
  3. WT1 Correct answer.
  4. NPHS1

Explanation

WT1 mutations cause Denys-Drash syndrome, characterized by early nephropathy with diffuse mesangial sclerosis, ambiguous genitalia in 46,XY individuals, and a high risk of Wilms tumor. COL4A5 mutations cause Alport syndrome, and NPHS1 mutations cause congenital nephrotic syndrome of the Finnish type.

Question 21Glomerular and Vascular Disorders

A child with atypical HUS has a factor H mutation. Which statement about kidney transplantation is most accurate?

  1. Recurrence risk is negligible once the native kidneys are removed
  2. Transplantation is contraindicated in every affected child
  3. Recurrence risk is high because the graft lacks the protective factor Correct answer.
  4. Standard immunosuppression reliably prevents recurrence

Explanation

Factor H is produced mainly by the liver, and a transplanted kidney from a donor without the mutation can still be attacked by the patient's dysregulated alternative pathway, producing high recurrence rates. Eculizumab prophylaxis or combined liver-kidney transplantation is often required.

Question 22Glomerular and Vascular Disorders

What is the approximate risk that a male with X-linked Alport syndrome will develop anti-GBM nephritis after kidney transplantation?

  1. About 20%
  2. Less than 1%
  3. More than 50%
  4. About 3-5% Correct answer.

Explanation

About 3-5% of males with X-linked Alport syndrome who receive a kidney transplant develop anti-GBM nephritis, because the graft exposes normal COL4A5 epitopes that the immune system has not previously encountered. This typically presents with rising creatinine months after transplantation.

Question 23Glomerular and Vascular Disorders

A 5-year-old with frequently relapsing steroid-sensitive nephrotic syndrome has developed steroid toxicity. Which is the most appropriate steroid-sparing strategy?

  1. Prolonged daily prednisone indefinitely
  2. A calcineurin inhibitor Correct answer.
  3. Avoiding all immunosuppression
  4. Cyclophosphamide as first-line therapy in all children

Explanation

Steroid-sparing agents for frequently relapsing steroid-sensitive nephrotic syndrome include calcineurin inhibitors, levamisole, mycophenolate, and cyclophosphamide. Cyclosporine or tacrolimus is often chosen for its rapid steroid-sparing effect and good response rates.

Question 24Glomerular and Vascular Disorders

Which feature most strongly favors IgA nephropathy over post-streptococcal glomerulonephritis?

  1. Presence of hypertension
  2. Low C3 level at presentation
  3. Positive ASO titer
  4. Hematuria onset within 1-2 days of infection Correct answer.

Explanation

IgA nephropathy causes synpharyngitic hematuria within 1-2 days of infection, whereas post-streptococcal glomerulonephritis causes hematuria after a latent period of 1-3 weeks. Low C3 and a positive ASO titer support post-streptococcal disease, not IgA nephropathy.

Question 25Glomerular and Vascular Disorders

A child with active nephrotic syndrome develops fever, abdominal pain, and rebound tenderness. Which organism is most likely responsible?

  1. Escherichia coli
  2. Pneumococcus Correct answer.
  3. Staphylococcus aureus
  4. Candida albicans

Explanation

Nephrotic syndrome predisposes to spontaneous bacterial peritonitis with encapsulated organisms, especially Streptococcus pneumoniae, because of low IgG, impaired opsonization, and ascites. Empiric therapy should therefore cover pneumococcus promptly.

Question 26Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 2-year-old with 3 days of vomiting and diarrhea is oliguric. Serum creatinine is elevated. Urine sodium is 12 mEq/L, urine osmolality is 640 mOsm/kg, and fractional excretion of sodium is 0.4%. Which interpretation is most accurate?

  1. Prerenal azotemia with intact tubular sodium avidity Correct answer.
  2. Acute tubular necrosis with impaired sodium reabsorption
  3. Distal renal tubular acidosis with bicarbonaturia
  4. Nephrogenic diabetes insipidus with dilute urine

Explanation

A fractional excretion of sodium below 1%, urine sodium under 20 mEq/L, and a concentrated urine all indicate that the tubules are reabsorbing sodium avidly in response to hypovolemia. This pattern defines a prerenal state rather than tubular injury. Intrinsic tubular injury classically produces a fractional excretion above 2% with urine sodium above 40 mEq/L and urine osmolality near serum.

Question 27Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 3-week-old boy presents with poor feeding, vomiting, and hypotension. Serum sodium is 128 mEq/L and potassium is 7.4 mEq/L. Genital examination shows a small phallus with scrotal hyperpigmentation. Which diagnosis best explains these findings?

  1. Bartter syndrome
  2. 21-hydroxylase deficiency (classic congenital adrenal hyperplasia) Correct answer.
  3. Pseudohypoaldosteronism type 1
  4. Gitelman syndrome

Explanation

Combined hyponatremia, hyperkalemia, hypotension, and virilization in a neonate is the classic salt-wasting picture of classic 21-hydroxylase deficiency, in which aldosterone and cortisol synthesis fail while adrenal androgens rise. Hyperpigmentation reflects excess ACTH drive. The electrolyte pattern is driven by aldosterone deficiency, not by tubular resistance or renal salt wasting with hypokalemia.

Question 28Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 6-week-old girl has 2 weeks of projectile nonbilious vomiting. Perfusion and blood pressure are normal. Serum sodium is 134 mEq/L, potassium 3.0 mEq/L, chloride 86 mEq/L, and bicarbonate 36 mEq/L. Which acid-base and electrolyte picture is this?

  1. High-anion-gap metabolic acidosis
  2. Mixed respiratory and metabolic acidosis
  3. Respiratory alkalosis with compensatory metabolic acidosis
  4. Hypochloremic hypokalemic metabolic alkalosis Correct answer.

Explanation

Vomiting of gastric fluid rich in hydrochloric acid and potassium produces loss of chloride and hydrogen ions, generating a hypochloremic metabolic alkalosis with potassium depletion. The kidney tries to conserve volume by reabsorbing sodium with bicarbonate, sustaining the alkalosis. This is the classic pattern of hypertrophic pyloric stenosis.

Question 29Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 4-year-old with growth failure has a non-anion-gap metabolic acidosis. Which urine pH, measured on a fresh sample while the child remains acidemic, is most consistent with distal (type 1) renal tubular acidosis?

  1. 4.8
  2. 6.9 Correct answer.
  3. 5.0
  4. 5.4

Explanation

In distal renal tubular acidosis the collecting duct cannot secrete hydrogen ions, so the urine cannot be maximally acidified even in the face of systemic acidosis and the pH stays above 5.5. A urine pH below 5.5 during acidemia effectively excludes a distal acidification defect. The value of 6.9 is therefore the expected finding.

Question 30Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 7-year-old on maintenance dialysis arrives with weakness and a serum potassium of 7.8 mEq/L. The ECG shows peaked T waves and a widened QRS complex. Which intervention should be given first?

  1. Intravenous calcium gluconate Correct answer.
  2. Intravenous insulin with glucose
  3. Intravenous furosemide
  4. Oral sodium polystyrene sulfonate

Explanation

Intravenous calcium stabilizes cardiac membranes within minutes and is the first treatment when hyperkalemia produces ECG changes such as QRS widening. Insulin with glucose and beta agonists shift potassium intracellularly but do not immediately protect the myocardium. Potassium removal with binders or diuretics works over hours and is not an emergency first step in an anuric child.

Question 31Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 10-year-old with new-onset diabetes mellitus presents with vomiting and Kussmaul breathing. Which acid-base pattern is expected?

  1. Hyperchloremic non-anion-gap metabolic acidosis
  2. Metabolic alkalosis with compensatory hypoventilation
  3. High-anion-gap metabolic acidosis Correct answer.
  4. Acute respiratory acidosis

Explanation

Diabetic ketoacidosis generates acetoacetate and beta-hydroxybutyrate, which are unmeasured anions, producing a high-anion-gap metabolic acidosis. Deep Kussmaul breathing is respiratory compensation that lowers the partial pressure of carbon dioxide. A non-anion-gap pattern can appear later during treatment but is not the presenting picture.

Question 32Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 12-year-old taking lithium for bipolar disorder develops polyuria, nocturia, and a serum sodium of 150 mEq/L. Urine osmolality is 85 mOsm/kg and does not rise after desmopressin. Which diagnosis fits?

  1. Osmotic diuresis from glucosuria
  2. Primary polydipsia
  3. Central diabetes insipidus
  4. Nephrogenic diabetes insipidus Correct answer.

Explanation

Lithium impairs aquaporin-2 signaling in the collecting duct, producing renal resistance to antidiuretic hormone. The result is a failure of urine concentration that persists after desmopressin administration, together with hypernatremia when water intake cannot keep up with losses. Central diabetes insipidus would respond to desmopressin with a rise in urine osmolality.

Question 33Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 14-month-old has failure to thrive and rickets. Fasting glucose is normal, but urinalysis shows 3+ glucose. Serum potassium is 2.9 mEq/L, phosphate 2.1 mg/dL, bicarbonate 15 mEq/L, and alkaline phosphatase is elevated. Which diagnosis fits best?

  1. Distal renal tubular acidosis
  2. Fanconi syndrome Correct answer.
  3. Isolated renal glucosuria
  4. Diabetic ketoacidosis

Explanation

Generalized proximal tubular dysfunction causes glycosuria with normal blood glucose plus renal losses of phosphate, potassium, amino acids, and bicarbonate, producing hypophosphatemic rickets and a proximal renal tubular acidosis. This constellation is Fanconi syndrome. Isolated glucosuria or a purely distal acidification defect would not explain the phosphaturia and rickets.

Question 34Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 3-year-old with prolonged vomiting has a metabolic alkalosis that persists despite improvement in oral intake. Urine chloride is 5 mEq/L. Which mechanism best maintains the alkalosis?

  1. Impaired renal bicarbonate excretion due to type 2 RTA
  2. Excess aldosterone secretion from an adrenal adenoma
  3. Reduced effective circulating volume causing lactic acidosis
  4. Chloride depletion and volume contraction sustaining bicarbonate reabsorption Correct answer.

Explanation

A urine chloride below 10 mEq/L identifies a chloride-responsive alkalosis: gastric chloride losses plus volume contraction increase distal sodium delivery and enhance collecting duct hydrogen ion and bicarbonate reabsorption. Sodium bicarbonate is therefore retained rather than excreted. Giving sodium chloride reverses the process by restoring chloride and expanding volume.

Question 35Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 5-year-old with a neurogenic bladder and recurrent urinary obstruction has hyperkalemia, a non-anion-gap metabolic acidosis, and a low serum aldosterone. Which type of renal tubular acidosis is this?

  1. Type 2 (proximal) renal tubular acidosis
  2. Proximal renal tubular acidosis with Fanconi syndrome
  3. Type 4 renal tubular acidosis Correct answer.
  4. Type 1 (distal) renal tubular acidosis

Explanation

Type 4 renal tubular acidosis results from aldosterone deficiency or resistance, so potassium is retained and ammonium excretion falls, giving hyperkalemia with a non-anion-gap acidosis. Obstructive uropathy damages the distal nephron and impairs renin and aldosterone production. Types 1 and 2 are characteristically associated with hypokalemia.

Question 36Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 15-year-old has blood pressure above the 99th percentile, potassium 2.8 mEq/L, bicarbonate 32 mEq/L, undetectable plasma renin activity, and low aldosterone. He eats a large bag of licorice-flavored candy every day. Which mechanism explains these findings?

  1. Primary hyperaldosteronism due to bilateral adrenal hyperplasia
  2. A renin-secreting juxtaglomerular cell tumor of the kidney
  3. 11β-HSD2 inhibition letting cortisol activate mineralocorticoid receptors Correct answer.
  4. Gain-of-function mutation of the epithelial sodium channel

Explanation

Licorice contains glycyrrhetinic acid, which inhibits 11β-hydroxysteroid dehydrogenase type 2 in the distal nephron. Cortisol is no longer converted to cortisone and therefore stimulates mineralocorticoid receptors, producing sodium retention, hypertension, hypokalemia, alkalosis, and suppression of renin and aldosterone. Liddle syndrome mimics the electrolyte pattern but is genetic and unrelated to licorice.

Question 37Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A child with nephrotic syndrome has sodium 135 mEq/L, chloride 108 mEq/L, bicarbonate 20 mEq/L, and albumin 1.8 g/dL. The measured anion gap is 7 mEq/L. What is the albumin-corrected anion gap?

  1. 9 mEq/L
  2. 7 mEq/L
  3. 12.5 mEq/L Correct answer.
  4. 17 mEq/L

Explanation

Albumin carries most of the unmeasured anion charge, so hypoalbuminemia lowers the measured anion gap and can hide a high-anion-gap acidosis. The correction adds 2.5 mEq/L for every 1 g/dL that albumin falls below 4 g/dL: 7 + 2.5 × (4.0 − 1.8) = 12.5 mEq/L. This corrected value indicates a coexisting high-anion-gap process.

Question 38Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 14-year-old with SIADH has a serum sodium of 116 mEq/L and has just had a seizure treated with hypertonic saline. Which limit on the rise in serum sodium over the first 24 hours is most appropriate to prevent osmotic demyelination?

  1. No limit once seizures have occurred
  2. 8 to 10 mEq/L Correct answer.
  3. 15 to 18 mEq/L
  4. 20 to 25 mEq/L

Explanation

Osmotic demyelination results from overly rapid correction of chronic hyponatremia, so the rise should be limited to roughly 8 to 10 mEq/L in 24 hours, with an even slower rate in high-risk patients. Symptomatic hyponatremia still warrants prompt hypertonic saline, but once symptoms abate the rate is deliberately slowed. Seizures justify urgent partial correction, not unrestricted correction.

Question 39Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 6-month-old with diarrhea has a serum sodium of 170 mEq/L and is now hemodynamically stable. Free water is replaced slowly with a plan to lower sodium by about 0.5 mEq/L per hour. Which complication is most likely if correction proceeds faster than this?

  1. Cerebral edema with seizures Correct answer.
  2. Hypokalemic periodic paralysis
  3. Osmotic demyelination syndrome
  4. Acute subdural hematoma

Explanation

Chronic hypernatremia prompts brain cells to generate idiogenic osmoles so they do not shrink; when sodium falls rapidly, these retained osmoles draw water into the brain. The result is cerebral edema, raised intracranial pressure, and seizures. The serum sodium should therefore be lowered by no more than about 10 to 12 mEq/L per day.

Question 40Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 3-year-old has a non-anion-gap metabolic acidosis. Urine sodium is 30 mEq/L, potassium 25 mEq/L, and chloride 60 mEq/L, giving a urine anion gap of −5. Which condition best fits this picture?

  1. Diarrhea with gastrointestinal bicarbonate loss Correct answer.
  2. Ethylene glycol ingestion
  3. Distal renal tubular acidosis
  4. Type 2 proximal renal tubular acidosis

Explanation

A negative urine anion gap indicates that ammonium is being excreted in abundance with chloride, which is the normal renal response to acidosis caused by bicarbonate loss from the gut. Diarrhea therefore fits this result. When the kidney itself cannot excrete ammonium, as in distal renal tubular acidosis, the urine anion gap becomes positive.

Question 41Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 2-year-old with polyuria and failure to thrive has blood pressure 88/52 mmHg, potassium 2.5 mEq/L, bicarbonate 32 mEq/L, magnesium 1.9 mg/dL, and urine chloride 45 mEq/L. Which diagnosis is most likely?

  1. Gitelman syndrome
  2. Liddle syndrome
  3. Distal renal tubular acidosis
  4. Bartter syndrome Correct answer.

Explanation

Bartter syndrome results from defective sodium chloride reabsorption in the thick ascending limb, producing hypokalemic metabolic alkalosis, salt wasting with normal or low blood pressure, and high urine chloride from early childhood. Hypercalciuria and nephrocalcinosis are common, while magnesium is usually normal. Gitelman syndrome, in contrast, causes hypomagnesemia and hypocalciuria and presents later.

Question 42Tubular, Fluid, Electrolyte, and Acid-Base Disorders

Which combination of findings best distinguishes Gitelman syndrome from Bartter syndrome?

  1. Hypertension with suppressed plasma renin activity
  2. Hypercalciuria with nephrocalcinosis in infancy
  3. Hyperkalemia with a non-anion-gap metabolic acidosis
  4. Hypomagnesemia with hypocalciuria in later childhood Correct answer.

Explanation

Gitelman syndrome is caused by loss-of-function of the distal convoluted tubule sodium chloride cotransporter, which produces hypokalemic metabolic alkalosis together with hypomagnesemia and hypocalciuria. Presentation is usually in later childhood or adulthood, and nephrocalcinosis is uncommon. Bartter syndrome, by contrast, causes hypercalciuria with nephrocalcinosis and presents earlier.

Question 43Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 9-year-old with chronic diarrhea has a potassium of 2.4 mEq/L that does not improve despite intravenous potassium chloride. Serum magnesium is 1.0 mg/dL. What best explains the refractory hypokalemia?

  1. Magnesium depletion increases distal potassium secretion Correct answer.
  2. Hypokalemia itself causes renal magnesium wasting
  3. Potassium chloride infusion blocks magnesium reabsorption
  4. Low magnesium increases aldosterone secretion

Explanation

Magnesium depletion removes the normal magnesium-dependent block on renal outer medullary potassium channels in the distal nephron, so potassium continues to be secreted into the urine. Potassium given without magnesium repletion is therefore largely wasted. Hypokalemia will not correct until the magnesium deficit is repaired.

Question 44Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 6-year-old with polyuria undergoes a water deprivation test. Urine osmolality remains at 110 mOsm/kg, and after desmopressin administration it rises to 620 mOsm/kg. Which diagnosis is most likely?

  1. Osmotic diuresis
  2. Nephrogenic diabetes insipidus
  3. Primary polydipsia
  4. Central diabetes insipidus Correct answer.

Explanation

Concentrating ability that is poor during water deprivation but restored after desmopressin indicates that the kidneys respond to antidiuretic hormone and that the defect is inadequate hormone secretion. This is central diabetes insipidus. Nephrogenic diabetes insipidus shows no response to desmopressin, and primary polydipsia usually concentrates partially during deprivation itself.

Question 45Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 10-day-old boy with normal male genitalia has sodium 125 mEq/L and potassium 8.1 mEq/L. Plasma renin and aldosterone are both markedly elevated, while cortisol and 17-hydroxyprogesterone are normal and renal function is normal. Which diagnosis fits best?

  1. Gordon syndrome (pseudohypoaldosteronism type 2)
  2. Classic 21-hydroxylase deficiency
  3. Type 4 renal tubular acidosis
  4. Pseudohypoaldosteronism type 1 Correct answer.

Explanation

Hyponatremia with hyperkalemia in a neonate whose aldosterone and renin are very high indicates end-organ resistance to aldosterone, which is pseudohypoaldosteronism type 1. Normal genitalia and normal adrenal steroid precursors argue against congenital adrenal hyperplasia. The salt wasting is severe and typically requires high-dose sodium supplementation.

Question 46Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 16-year-old arrives 3 hours after ingesting a large amount of aspirin. Arterial blood gas shows pH 7.50, partial pressure of carbon dioxide 22 mmHg, bicarbonate 17 mEq/L, sodium 138 mEq/L, chloride 100 mEq/L, and normal albumin. Which description of the acid-base disorder is correct?

  1. Mixed metabolic alkalosis and respiratory acidosis
  2. Simple high-anion-gap metabolic acidosis with appropriate respiratory compensation
  3. Simple respiratory alkalosis with appropriate metabolic compensation
  4. Mixed high-anion-gap metabolic acidosis and respiratory alkalosis Correct answer.

Explanation

The anion gap is 21 mEq/L, indicating a high-anion-gap metabolic acidosis, and salicylates also stimulate the medullary respiratory center directly. For a bicarbonate of 17 mEq/L, Winter's formula predicts a partial pressure of carbon dioxide of about 33 ± 2 mmHg, so the measured 22 mmHg is far too low and represents a superimposed respiratory alkalosis. The mixed disorder is characteristic of salicylate toxicity.

Question 47Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 12-year-old in diabetic ketoacidosis has received 12 hours of treatment. pH is 7.28, sodium 140 mEq/L, chloride 114 mEq/L, bicarbonate 13 mEq/L, glucose 210 mg/dL, and beta-hydroxybutyrate is nearly normal. The anion gap is 13 mEq/L. What best explains this acid-base picture?

  1. Distal renal tubular acidosis induced by insulin
  2. Hyperchloremic non-anion-gap acidosis after saline resuscitation Correct answer.
  3. Recurrent ketoacidosis with a rising anion gap
  4. Lactic acidosis from tissue hypoperfusion

Explanation

As ketones are metabolized and the anion gap closes, chloride from large-volume saline resuscitation remains behind, producing a hyperchloremic non-anion-gap acidosis. This phase is expected during recovery and does not by itself indicate treatment failure. The acidosis resolves as the kidneys excrete the chloride load and bicarbonate is regenerated.

Question 48Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 9-year-old is lethargic 2 days after resection of a craniopharyngioma. Weight is down 1.5 kg, central venous pressure is low, urine output is 5 mL/kg/hour, urine sodium is 90 mEq/L, and serum sodium has fallen from 141 to 128 mEq/L. Which is the most likely cause?

  1. SIADH from surgical injury
  2. Diuretic-induced hyponatremia
  3. Primary polydipsia
  4. Cerebral salt wasting Correct answer.

Explanation

Hyponatremia with high urine sodium, polyuria, weight loss, and low filling pressures after neurosurgery indicates renal salt wasting with intravascular depletion, which is cerebral salt wasting. SIADH produces high urine sodium but with euvolemia or mild volume expansion and usually oliguria. Treatment is volume and salt replacement rather than fluid restriction.

Question 49Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 1-year-old with failure to thrive has bicarbonate 14 mEq/L, an anion gap of 11 mEq/L, and a urine pH of 7.4 while acidemic. After sodium bicarbonate loading, the fractional excretion of bicarbonate is 12%. Which diagnosis is most likely?

  1. Distal (type 1) renal tubular acidosis
  2. Proximal (type 2) renal tubular acidosis Correct answer.
  3. Type 4 renal tubular acidosis
  4. Chronic diarrhea

Explanation

Bicarbonate wasting with a fractional excretion above 10 to 15% during bicarbonate loading indicates a lowered proximal reabsorption threshold, which defines proximal renal tubular acidosis. The inappropriately alkaline urine reflects bicarbonaturia while serum bicarbonate is above the reabsorptive threshold. Distal renal tubular acidosis would not show substantial bicarbonate wasting when the serum bicarbonate is only modestly reduced.

Question 50Tubular, Fluid, Electrolyte, and Acid-Base Disorders

A 13-year-old has hypertension, potassium 5.6 mEq/L, bicarbonate 19 mEq/L, chloride 110 mEq/L, normal glomerular filtration rate, suppressed plasma renin, and low-normal aldosterone. Blood pressure and potassium normalize with a thiazide diuretic. Which disorder is most likely?

  1. Liddle syndrome
  2. Apparent mineralocorticoid excess
  3. Classic 21-hydroxylase deficiency
  4. Gordon syndrome (pseudohypoaldosteronism type 2) Correct answer.

Explanation

Gordon syndrome is caused by gain-of-function variants in WNK kinases, producing hypertension with hyperkalemia, a non-anion-gap metabolic acidosis, and suppressed renin, all of which respond to thiazides. Aldosterone is low-normal because volume expansion suppresses the axis. Liddle syndrome and apparent mineralocorticoid excess share the hypertension and suppressed renin but cause hypokalemia rather than hyperkalemia.