Renal tubular acidosis: Pathology review

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Renal tubular acidosis: Pathology review

Renal system


Renal tubular acidosis: Pathology review

USMLE® Step 1 questions

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A 76-year-old woman presents to the emergency department with fatigue, decreased appetite, and muscle weakness. Past medical history includes chronic migraines, hypertension, gastroesophageal reflux, and a recent episode of podagra. Current medications include topiramate, lisinopril, acetazolamide, omeprazole, and probenecid. Temperature is 37.0°C (98.6°F), pulse is 104/min, respirations are 24/min, and blood pressure is 96/66 mmHg. Arterial blood gas and laboratory testing are obtained, and the results are shown below.

Which of the following medications most likely contributed to this patient’s disease?


In the Emergency Department, two people came in with rapid, shallow breathing and tachycardia. The first one is 45 year old Olga who also has systemic lupus erythematosus and the second one is 39 year old Fred. An arterial blood gas was taken, along with electrolytes. Results showed that Olga had low pH, low bicarbonate and pCO2 levels and her potassium level was also low. Fred also had low pH, low bicarbonate and pCO2 levels, but his potassium level was high. Based on the ABG results, the diagnosis of normal anion gap metabolic acidosis was made. In order to identify the cause of their normal gap metabolic acidosis, more investigations were done and the urine anion gap showed that both individuals had a low urinary anion gap, which suggests that the cause was renal.

Okay, now, a normal anion gap metabolic acidosis can have renal causes. Like when a lot of bicarbonate is lost through the urinary tract- which happens in type II renal tubular acidosis. A normal anion gap metabolic acidosis can also happen when too many hydrogen ions are retained, like in type I and type IV renal tubular acidosis. Now, there’s also a type III renal tubular acidosis, where both the proximal and distal tubules are affected. This is a pretty rare situation and the causes are not well understood, so it’s unlikely to be tested.

Okay, now, let’s review the physiology of the tubules. The proximal tubule is affected in RTA type II. It’s lined by brush border cells which have two surfaces: One is the apical surface that faces the tubular lumen and is lined with microvilli, and the other is the basolateral surface, which faces the peritubular capillaries. Now, a lot of bicarbonate is reabsorbed here. When bicarbonate approaches the apical surface it binds to hydrogen to form carbonic acid, which will be split into water and carbon dioxide by carbonic anhydrase. The water and carbon dioxide diffuse into the cells where carbonic anhydrase facilitates the reverse reaction and combines them to form carbonic acid, which dissociates into bicarbonate and hydrogen. Then bicarbonate will get into the blood with the help of a sodium bicarbonate cotransporter on the basolateral surface. Now, the proximal tubule is also responsible for reabsorbing glucose, as well as amino acids, sodium, chloride, potassium, phosphate, water and uric acid.


Renal tubular acidosis is a medical condition in which the kidney is unable to secrete acids or reabsorb bicarbonate from the body. When blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for the exchange of salts, acid equivalents, and other solutes before it drains into the bladder as urine. The metabolic acidosis that results from renal tubular acidosis may be caused either by failure to recover sufficient bicarbonate ions from the filtrate in the proximal tubule or by insufficient secretion of hydrogen ions into the distal tubule. If left untreated, acidemia can cause peripheral vasodilation and shock. Treatment may include alkali supplements like potassium citrate or sodium bicarbonate to neutralize the acid in the blood.


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  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "First Aid for the USMLE Step 1 2018, 28th Edition" McGraw-Hill Education / Medical (2017)
  4. "Nephrolithiasis in Renal Tubular Acidosis" Journal of Urology (1989)
  5. "Pathophysiology of Renal Tubular Acidosis: Core Curriculum 2016" American Journal of Kidney Diseases (2016)
  6. "Nephrolithiasis related to inborn metabolic diseases" Pediatric Nephrology (2009)
  7. "Renal Tubular Acidosis" Pediatric Clinics of North America (2019)
  8. "On the mechanism of renal potassium wasting in renal tubular acidosis associated with the Fanconi syndrome (type 2 RTA)" Journal of Clinical Investigation (1971)

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