A 38-year-old woman is referred to the nephrology clinic for evaluation of a persistent ac… — USMLE Step 2 CK

A 38-year-old woman is referred to the nephrology clinic for evaluation of a persistent acid-base abnormality. Over the past 6 months, she has experienced progressive fatigue, generalized muscle weakness, and one episode of passing a small, radiopaque kidney stone. Her medical history is significant for severe dental caries and bilateral parotid gland enlargement, which has been present for 3 years. She complains of a constant sensation of "sand in her eyes." She takes no medications.

Vital signs are within normal limits. Physical examination shows dry mucous membranes and mildly swollen, non-tender parotid glands. Motor strength is 4/5 in the proximal lower extremities.

Laboratory studies:
$Na^+$ : $140\text{ mEq/L}$
$K^+$ : $2.6\text{ mEq/L}$
$Cl^-$ : $115\text{ mEq/L}$
$HCO_3^-$ : $12\text{ mEq/L}$
Blood urea nitrogen: $14\text{ mg/dL}$
Serum creatinine: $0.9\text{ mg/dL}$

Urinalysis:
pH: $6.5$
Specific gravity: $1.012$
Protein: Negative
Blood: Negative
Glucose: Negative
Ketones: Negative

Which of the following findings is most likely to be present in this patient?

Calcium phosphate deposition within the renal parenchymaAnswer
B
A negative urine anion gap
C
Fractional excretion of bicarbonate greater than $15\%$ following alkali therapy
D
Decreased plasma renin activity and low serum aldosterone concentration
E
An arterial $pCO_2$ of $32\text{ mmHg}$ due to appropriate respiratory compensation

Answer

Calcium phosphate deposition within the renal parenchyma

This patient's presentation of dry eyes, parotid enlargement, and severe dental caries strongly suggests Sjögren syndrome. Her laboratory results reveal a normal anion gap metabolic acidosis (NAGMA) with severe hypokalemia and an inappropriately alkaline urine pH (

6.5

). This triad is highly specific for Type 1 (distal) renal tubular acidosis (RTA), which is strongly associated with autoimmune disorders like Sjögren syndrome. The primary defect in Type 1 RTA is the inability of the alpha-intercalated cells in the distal tubule to secrete hydrogen ions. This chronic acid retention leads to bone buffering (which increases urinary calcium excretion) and increased proximal tubular reabsorption of citrate (hypocitraturia). The combination of hypercalciuria, hypocitraturia, and persistently alkaline urine creates an ideal environment for calcium phosphate precipitation, resulting in radiopaque kidney stones and nephrocalcinosis (calcium deposition in the renal parenchyma).

Step-by-Step Solution

Identify the underlying systemic disease based on the clinical presentation.

The patient has Sjögren syndrome.

Dry eyes, dry mouth, dental caries, and parotid gland enlargement are classic signs of autoimmune exocrine gland destruction seen in Sjögren syndrome.

Calculate the serum anion gap to classify the metabolic acidosis.

Anion Gap =

140 - (115 + 12) = 13\text{ mEq/L}

An anion gap of 13 is considered normal (non-anion gap metabolic acidosis), which narrows the differential to gastrointestinal bicarbonate loss or renal tubular acidosis (RTA).

Analyze the potassium level and urine pH to differentiate the types of RTA.

The patient has hypokalemia (

2.6\text{ mEq/L}

) and an inappropriately alkaline urine pH (

6.5

In the setting of severe academia, a normal kidney should acidify the urine (pH < 5.5). The inability to do so, combined with severe hypokalemia, confirms Type 1 (distal) RTA, a known complication of Sjögren syndrome.

Determine the downstream complications associated with this specific renal defect.

Calcium phosphate stone formation and nephrocalcinosis.

Type 1 RTA causes chronic acidemia, which buffers protons in the bone (releasing calcium) and increases proximal citrate reabsorption. The combination of hypercalciuria, hypocitraturia, and alkaline urine leads to calcium phosphate precipitation.

Key Concept

Type 1 (Distal) Renal Tubular Acidosis Secondary to Sjögren Syndrome

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