A 52-year-old woman with a history of primary Sjögren syndrome, characterized by severe ke… — USMLE Step 2 CK

A 52-year-old woman with a history of primary Sjögren syndrome, characterized by severe keratoconjunctivitis sicca, xerostomia, and positive anti-Ro/SSA antibodies, is brought to the emergency department by her husband due to sudden, profound generalized muscle weakness that started this morning. She reports feeling so weak she could not get out of bed. She also complains of new-onset shortness of breath and difficulty taking a deep breath. She currently takes no medications.

Temperature is 37.1°C (98.8°F), blood pressure is 115/75 mm Hg, pulse is 95/min, and respiratory rate is 24/min with shallow breathing. Pulse oximetry is 94% on room air.

Physical examination reveals 2/5 motor strength in the proximal muscles of the upper and lower extremities. Deep tendon reflexes are 1+ bilaterally. Chest auscultation reveals clear lungs bilaterally.

Laboratory studies show:

Serum Analyte	Value	Reference Range
Sodium ( $Na^+$ )	140 mEq/L	136-145 mEq/L
Potassium ( $K^+$ )	1.8 mEq/L	3.5-5.0 mEq/L
Chloride ( $Cl^-$ )	118 mEq/L	98-106 mEq/L
Bicarbonate ( $HCO_3^-$ )	12 mEq/L	22-28 mEq/L
Blood urea nitrogen	15 mg/dL	8-20 mg/dL
Creatinine	0.9 mg/dL	0.7-1.3 mg/dL

Urine Analyte	Value
pH	6.5
Sodium ( $Na^+$ )	35 mEq/L
Potassium ( $K^+$ )	20 mEq/L
Chloride ( $Cl^-$ )	20 mEq/L

Which of the following is the most appropriate next step in the management of this patient?

Intravenous administration of potassium chlorideAnswer
B
Intravenous administration of sodium bicarbonate to correct a high anion gap acidosis
C
Initiation of furosemide and positive pressure ventilation for an acute cardiopulmonary exacerbation
D
Initiation of broad-spectrum antibiotics to empirically treat a suspected systemic infection
E
Measurement of thyroid-stimulating hormone levels to evaluate for a concomitant autoimmune endocrinopathy

Answer

Intravenous administration of potassium chloride

The correct answer addresses the most immediate, life-threatening derangement: severe hypokalemia (

K^+ = 1.8

mEq/L). This patient's presentation of profound weakness, respiratory distress (due to respiratory muscle paralysis), and a normal anion gap metabolic acidosis with a positive urine anion gap (+35) and a urine pH > 5.5 is the classic presentation of distal (Type 1) renal tubular acidosis (RTA). Type 1 RTA is a well-documented extraglandular complication of Sjögren syndrome, caused by autoimmune-mediated destruction of the

\alpha

-intercalated cells in the collecting duct. This prevents

H^+

secretion, leading to systemic acidosis, and secondarily impairs

K^+

reabsorption, leading to severe hypokalemia. The most critical step in management is intravenous potassium replacement. Administering alkali (sodium bicarbonate) to correct the acidosis before replacing potassium is strictly contraindicated, as raising the blood pH shifts potassium into the intracellular space, which can precipitate fatal cardiac arrhythmias.

Step-by-Step Solution

Calculate the serum anion gap to classify the metabolic acidosis.

The anion gap is normal (10 mEq/L).

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (118 + 12) = 10

. This indicates a normal anion gap metabolic acidosis (NAGMA).

Calculate the urine anion gap (UAG) to determine the source of the NAGMA.

The UAG is positive (+35 mEq/L).

UAG = Na^+ + K^+ - Cl^- = 35 + 20 - 20 = +35

. A positive UAG in NAGMA suggests impaired renal

NH_4^+

excretion, confirming a renal tubular acidosis (RTA).

Integrate laboratory findings with the patient's rheumatologic history.

The patient has distal (Type 1) RTA secondary to Sjögren syndrome.

Sjögren syndrome frequently causes autoantibody-mediated damage to the

\alpha

-intercalated cells of the distal tubule, leading to failure of

H^+

secretion (urine pH > 5.5) and severe hypokalemia (

1.8

mEq/L).

Identify the most critical and immediate step in management.

Intravenous potassium chloride must be administered immediately.

The profound weakness and shallow breathing are due to severe hypokalemia. Administering alkali (like bicarbonate) to correct the acidosis before replacing potassium would drive

K^+

into cells, exacerbating the hypokalemia and risking fatal arrhythmias.

Key Concept

Extraglandular manifestations of primary Sjögren syndrome include distal (Type 1) renal tubular acidosis, characterized by a normal anion gap metabolic acidosis, a positive urine anion gap, high urine pH, and severe hypokalemia. In the management of Type 1 RTA, potassium must be repleted before initiating alkali therapy.

Estimated Time:2m 30s

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