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USMLE® Step 1 Question of the Day: Frequent Urination Diagnosis

Osmosis Team
Published on Jan 13, 2021. Updated on Feb 16, 2021.

Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case involves a young boy whose worsening issues with thirst and frequent urination are worrying his parents. Can you make the correct diagnosis and help this family out?

An 8-year-old boy is brought to the clinic by his parents due to increased thirst and urination for the last month. The patient has no medical history, and he currently takes no medications. The patient’s mother notes that her father had a similar condition. The patient is at the 40th percentile for height and 20th percentile for weight. The patient’s temperature is 37.2 °C (98.9 °F), pulse is 100/min, respirations are 24/min, and blood pressure is 86/58 mmHg.

USMLE® Step 1 Question: Frequent Urination Diagnosis lab values.

Urinalysis shows elevated calcium excretion. Which of the following is most likely affected in this patient? 

A. Glomerulus

B. Proximal convoluted tubule 

C. Thick ascending limb of the loop of Henle 

D. Distal convoluted tubule Collecting duct  

E. Collecting duct 

 Scroll down to find the answer!



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The correct answer to today's USMLE® Step 1 Question is...

C. Thick ascending limb of the loop of Henle

This patient likely has Bartter syndrome. Before we go into the full answer explanation, let's review the incorrect answers.

Incorrect answer explanations

The incorrect answers to today's USMLE® Step 1 Question are...

A. Glomerulus

 Incorrect: This patient has Bartter syndrome, which affects the thick ascending limb of the loop of Henle (LoH). The glomerulus is affected in nephritic and nephrotic glomerulopathies, which present with hematuria and proteinuria.

B. Proximal convoluted tubule

Incorrect: The proximal convoluted tubule is affected in Fanconi syndrome, which causes metabolic acidosis.  

D. Distal convoluted tubule 

Incorrect: The distal convoluted tubule is affected in Gitelman syndrome. Gitelman syndrome presents similarly to thiazide diuretic therapy, meaning that urine calcium excretion would be low, not elevated as in this patient.

E. Collecting duct 

Incorrect: The collecting duct is affected in Liddle syndrome, which causes increased sodium reabsorption, leading to hypertension with hypokalemia.


Correct answer explanation

This patient with growth retardation, polyuria, polydipsia, hyponatremia, hypokalemia, hypochloremia, and metabolic alkalosis most likely has Bartter syndrome.

Bartter syndrome is an autosomal recessive renal tubular defect caused by a mutation in the Na+/K+/2Cl- cotransporter in the thick ascending limb of the loop of Henle. The loss of this cotransporter leads to excretion of sodium, potassium, and chloride in the urine, as well as excessive loss of extracellular water. The depletion of extracellular water then leads to activation of the renin-angiotensin-aldosterone system, which prompts the excretion of hydrogen and more potassium, as well as the reabsorption of bicarbonate. Therefore, common laboratory findings in patients with Bartter syndrome include metabolic alkalosis and hypokalemia. Despite the loss of sodium through the urine, there is no hyponatremia, due to the concomitant loss of water.

The thick ascending limb of the loop of Henle is also responsible for calcium reabsorption via an electrochemical gradient created by the reabsorption of sodium, potassium, and chloride. As a result, patients with Bartter syndrome commonly present with hypercalciuria. Also, due to increased reabsorption of urate and general volume depletion, patients may also present with hyperuricemia with or without gout.


It is also worth noting that loop diuretics act by blocking the same cotransporter in the thick ascending limb of the loop of Henle. Therefore, patients with Bartter syndrome present similarly to those with chronic loop diuretic use.

Major takeaways

Bartter syndrome is a renal tubular defect that is caused by an autosomal recessive mutation of the Na+/K+/2Cl- cotransporter in the thick ascending limb of the loop of Henle. Common laboratory findings in patients with Bartter syndrome include metabolic alkalosis, hypokalemia, hypercalciuria, and hyperuricemia. Patients present similarly to those with chronic use of loop diuretics, and the hyperuricemia may cause symptoms consistent with gout. 

References

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.