An adult rheumatology clinic is evaluating a 48-year-old woman who reports a six-month his… — USMLE Step 3

An adult rheumatology clinic is evaluating a 48-year-old woman who reports a six-month history of profound generalized muscle weakness and fatigue. During the review of systems, she mentions requiring frequent sips of water to swallow dry foods and a constant gritty sensation in her eyes. Physical examination reveals bilateral parotid gland enlargement and dry oral mucosa.

Laboratory evaluation shows the following:

Laboratory Test	Result	Reference Range
Serum Chemistry
Sodium	$140 \text{ mEq/L}$	$136-145 \text{ mEq/L}$
Potassium	$2.4 \text{ mEq/L}$	$3.5-5.0 \text{ mEq/L}$
Chloride	$115 \text{ mEq/L}$	$95-105 \text{ mEq/L}$
Bicarbonate	$14 \text{ mEq/L}$	$22-28 \text{ mEq/L}$
Blood urea nitrogen	$14 \text{ mg/dL}$	$7-20 \text{ mg/dL}$
Creatinine	$0.8 \text{ mg/dL}$	$0.6-1.2 \text{ mg/dL}$
Urine Studies
pH	$6.8$	$4.5-8.0$
Sodium	$32 \text{ mEq/L}$	-
Potassium	$12 \text{ mEq/L}$	-
Chloride	$16 \text{ mEq/L}$	-

Based on the interpretation of these laboratory results, which of the following additional findings is most likely to be present in this patient?

Calcium phosphate nephrolithiasisAnswer
B
Glycosuria with normal serum glucose
C
Positive tissue transglutaminase antibodies
D
Suppressed plasma renin activity
E
Elevated serum lactate concentration

Answer

The most likely expected finding is calcium phosphate nephrolithiasis.

The patient's clinical presentation of sicca symptoms and parotid enlargement is classic for Sjögren syndrome, an autoimmune disorder strongly associated with Type 1 (distal) renal tubular acidosis (RTA). The laboratory evaluation demonstrates a normal anion gap metabolic acidosis (NAGMA), as the calculated serum anion gap is

11 \text{ mEq/L}

(

140 - [115 + 14]

). To determine the etiology of NAGMA, the urine anion gap (UAG) must be calculated to assess renal ammonium excretion:

UAG = (Urine Na^+ + Urine K^+) - Urine Cl^- = (32 + 12) - 16 = +28 \text{ mEq/L}

. A positive UAG indicates a failure of the kidneys to excrete acid, directing the diagnosis toward RTA rather than gastrointestinal bicarbonate loss. The presence of severe hypokalemia narrows the differential to Type 1 or Type 2 RTA. The finding of an inappropriately alkaline urine pH (

6.8

) in the setting of severe systemic acidemia confirms a defect in the alpha-intercalated cells of the distal tubule to secrete hydrogen ions, establishing the diagnosis of Type 1 RTA. The chronic acidosis in Type 1 RTA causes bone buffering and hypercalciuria, while proximal tubular citrate reabsorption is enhanced, leading to hypocitraturia. The combination of hypercalciuria, hypocitraturia, and a persistently alkaline urine creates an optimal environment for the precipitation of calcium phosphate, placing these patients at high risk for calcium phosphate nephrolithiasis and nephrocalcinosis.

Step-by-Step Solution

Calculate the serum anion gap.

Na^+ - (Cl^- + HCO_3^-) = 140 - (115 + 14) = 11 \text{ mEq/L}

To classify the metabolic acidosis as normal anion gap (NAGMA) versus high anion gap (HAGMA).

Calculate the urine anion gap (UAG).

UAG = (Urine Na^+ + Urine K^+) - Urine Cl^- = (32 + 12) - 16 = +28 \text{ mEq/L}

To differentiate between gastrointestinal bicarbonate loss (which appropriately increases renal

NH_4^+

excretion, yielding a negative UAG) and a renal tubular defect (yielding a positive UAG).

Synthesize the urine pH and serum potassium levels to identify the specific renal etiology.

The patient has severe hypokalemia (

2.4 \text{ mEq/L}

) and an inappropriately alkaline urine pH (

6.8

A positive UAG with NAGMA indicates Renal Tubular Acidosis (RTA). Hypokalemia rules out Type 4 RTA. A urine pH

> 5.5

despite severe systemic acidemia confirms Type 1 (distal) RTA.

Identify the clinical consequence of the established diagnosis.

Calcium phosphate kidney stones.

Type 1 RTA causes systemic acidosis that leads to bone buffering and hypercalciuria. Combined with hypocitraturia and a highly alkaline urine, this creates the perfect environment for calcium phosphate precipitation.

Key Concept

The synthesis of normal anion gap metabolic acidosis, a positive urine anion gap, severe hypokalemia, and an alkaline urine pH points definitively to Type 1 (distal) Renal Tubular Acidosis, a condition strongly associated with Sjögren syndrome that predisposes patients to calcium phosphate nephrolithiasis.

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