A 42-year-old woman is evaluated in the emergency department for severe, colicky left flan… — USMLE Step 2 CK

A 42-year-old woman is evaluated in the emergency department for severe, colicky left flank pain radiating to the groin and gross hematuria. For the past year, she has experienced progressive generalized muscle weakness, persistent dry eyes, and a dry mouth. Blood pressure is 120/76 mm Hg, and pulse is 88/min. Laboratory studies show:

Laboratory Test	Result
Serum sodium	$139$ mEq/L
Serum potassium	$2.8$ mEq/L
Serum chloride	$115$ mEq/L
Serum bicarbonate	$14$ mEq/L
Serum creatinine	$1.1$ mg/dL
Arterial pH	$7.27$
Arterial $PaCO_2$	$29$ mm Hg
Arterial $PaO_2$	$98$ mm Hg

Urinalysis	Result
pH	$6.8$
Blood	Large
Protein	Trace

A non-contrast CT of the abdomen and pelvis reveals a 5-mm calculus in the left ureter and bilateral medullary nephrocalcinosis. Which of the following is the most likely primary pathophysiologic mechanism responsible for this patient's laboratory and imaging findings?

Impaired apical hydrogen ion secretion in the distal tubuleAnswer
B
Defective sodium-bicarbonate cotransport in the proximal tubule
C
Decreased epithelial sodium channel activity in the cortical collecting duct
D
Increased gastrointestinal loss of bicarbonate-rich fluids
E
Accumulation of unmeasured organic anions due to decreased glomerular filtration

Answer

The primary pathophysiologic mechanism is impaired apical hydrogen ion secretion in the distal tubule (Type 1 Renal Tubular Acidosis).

The correct option identifies the defect seen in Type 1 (distal) RTA: an inability of the

\alpha

-intercalated cells in the distal tubule to secrete hydrogen ions. This defect leads to a normal anion gap metabolic acidosis, profound hypokalemia (as the kidney wastes potassium in an attempt to retain sodium without exchanging hydrogen), and a persistently alkaline urine (pH > 5.5). The combination of alkaline urine and hypocitraturia creates an environment highly susceptible to calcium phosphate precipitation, leading to bilateral medullary nephrocalcinosis and urolithiasis. Furthermore, Type 1 RTA is classically associated with autoimmune diseases, specifically Sjögren syndrome and rheumatoid arthritis, perfectly matching the patient's sicca symptoms and joint history.

Step-by-Step Solution

Calculate the serum anion gap to classify the metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 139 - (115 + 14) = 10

mEq/L.

An anion gap between 8 and 12 mEq/L indicates a normal anion gap metabolic acidosis (NAGMA), ruling out etiologies like lactic acidosis, ketoacidosis, and uremia.

Evaluate the respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2

mm Hg. The measured

PaCO_2

29

mm Hg.

Confirming appropriate respiratory compensation ensures there is no mixed acid-base disorder (e.g., concurrent respiratory alkalosis or acidosis) complicating the clinical picture.

Analyze the serum potassium and urine pH in the context of a NAGMA.

The patient has severe hypokalemia (

2.8

mEq/L) and an inappropriately alkaline urine (pH

6.8

In the setting of NAGMA, a high urine pH (

> 5.5

) and hypokalemia specifically point toward a distal renal tubular defect (Type 1 RTA), as the kidney fails to excrete

H^+

Integrate the clinical and imaging findings to confirm the etiology.

The patient has signs of Sjögren syndrome (dry eyes/mouth, muscle weakness from hypokalemia) and nephrocalcinosis with kidney stones.

Type 1 RTA is highly associated with autoimmune diseases like Sjögren syndrome and uniquely causes nephrocalcinosis/calcium phosphate stones due to alkaline urine and low urinary citrate.

Key Concept

Type 1 (distal) renal tubular acidosis presents with a normal anion gap metabolic acidosis, hypokalemia, urine pH > 5.5, and nephrocalcinosis, often in the setting of autoimmune disorders like Sjögren syndrome.

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