An outpatient evaluation is requested for a 35-year-old woman complaining of new-onset pro… — USMLE Step 2 CK

An outpatient evaluation is requested for a 35-year-old woman complaining of new-onset profound generalized weakness and intermittent palpitations over the past two weeks. Her medical history is notable for rheumatoid arthritis and severe dry eyes, for which she frequently uses artificial tears. Her blood pressure is $118/76 \text{ mmHg}$ and heart rate is $92\text{/min}$ . Laboratory studies show the following:

Parameter	Value
Serum sodium	$140 \text{ mEq/L}$
Serum potassium	$2.2 \text{ mEq/L}$
Serum chloride	$116 \text{ mEq/L}$
Serum bicarbonate	$14 \text{ mEq/L}$
Blood urea nitrogen	$12 \text{ mg/dL}$
Serum creatinine	$0.9 \text{ mg/dL}$
Arterial $pH$	$7.25$
Arterial $PaCO_2$	$29 \text{ mmHg}$

Urinalysis demonstrates a $pH$ of $6.5$ , with no glucose, protein, or blood. Urine electrolytes reveal a sodium of $45 \text{ mEq/L}$ , potassium of $25 \text{ mEq/L}$ , and chloride of $50 \text{ mEq/L}$ . Which of the following best describes the underlying pathophysiologic defect in this patient?

Impaired hydrogen ion secretion by alpha-intercalated cellsAnswer
B
Defective bicarbonate reabsorption in the proximal convoluted tubule
C
Decreased collecting duct responsiveness to aldosterone
D
Gastrointestinal loss of bicarbonate-rich secretions
E
Accumulation of endogenous unmeasured organic anions

Answer

Impaired hydrogen ion secretion by alpha-intercalated cells

This patient presents with a normal anion gap metabolic acidosis (

140 - 130 = 10 \text{ mEq/L}

) and a positive urine anion gap (

[45 + 25] - 50 = +20 \text{ mEq/L}

). A positive urine anion gap indicates impaired renal excretion of ammonium (

NH_4^+

), effectively ruling out extrarenal causes of acidosis like diarrhea. Among the renal tubular acidoses (RTAs), this patient's severe hypokalemia (

2.2 \text{ mEq/L}

) rules out Type 4 RTA (which features hyperkalemia). Her inappropriately alkaline urine (

pH

6.5

, well above

5.5

) in the face of severe systemic acidosis strongly points to Type 1 (distal) RTA. Her history of rheumatoid arthritis and keratoconjunctivitis sicca (Sjogren syndrome) is a classic trigger for this disorder. Type 1 RTA is caused by the impaired secretion of hydrogen ions by the alpha-intercalated cells of the cortical collecting duct.

Step-by-Step Solution

Calculate the serum anion gap (AG).

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (116 + 14) = 10 \text{ mEq/L}

(Normal).

Establishing whether the metabolic acidosis is due to unmeasured anions (high AG) or loss of bicarbonate (normal AG) is the first step in formulating a differential diagnosis.

Check respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 \text{ mmHg}

. The patient's

PaCO_2

is exactly

29 \text{ mmHg}

To ensure there is no secondary concomitant respiratory acid-base disorder.

Calculate the urine anion gap (UAG) for the normal anion gap metabolic acidosis.

UAG = (U_{Na} + U_K) - U_{Cl} = (45 + 25) - 50 = +20 \text{ mEq/L}

(Positive).

A positive UAG indicates a failure of the kidneys to excrete sufficient ammonium (

NH_4^+

), confirming a renal etiology (Renal Tubular Acidosis) rather than a gastrointestinal etiology.

Differentiate the type of RTA based on serum potassium and urine pH.

The patient has severe hypokalemia (

2.2 \text{ mEq/L}

) and an inappropriately alkaline urine

pH

(

6.5

This specific combination, often associated with autoimmune conditions like Sjogren syndrome, diagnoses Type 1 (distal) RTA, which is caused by a failure of alpha-intercalated cells to secrete

H^+

Key Concept

Type 1 (distal) renal tubular acidosis presents as a normal anion gap metabolic acidosis with hypokalemia, a positive urine anion gap, and a urine pH > 5.5, caused by impaired hydrogen ion secretion in the distal nephron.

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