A 45-year-old female teacher is brought to the emergency department by her husband after s… — USMLE Step 2 CK

A $45$ -year-old female teacher is brought to the emergency department by her husband after she was unable to get out of bed this morning due to severe, generalized muscle weakness. Over the past several years, she has experienced a persistent sensation of sand in her eyes and requires frequent sips of water to swallow dry food. She has no other medical conditions and takes no medications. On physical examination, her vital signs are within normal limits. Bilateral parotid gland enlargement is noted. Neuromuscular examination reveals symmetric $\frac{2}{5}$ motor strength in the proximal muscles of the upper and lower extremities. Deep tendon reflexes are diminished globally.

Laboratory results are as follows:

Arterial blood gas (room air):
pH: $7.25$
$\text{pCO}_2$ : $26$ mm Hg
$\text{pO}_2$ : $95$ mm Hg

Serum chemistry:
Sodium: $140$ mEq/L
Potassium: $2.1$ mEq/L
Chloride: $116$ mEq/L
Bicarbonate: $12$ mEq/L
Creatinine: $0.9$ mg/dL
Blood urea nitrogen: $14$ mg/dL

Urinalysis:
pH: $6.5$
Specific gravity: $1.015$
Glucose: negative
Protein: negative
Ketones: negative

Which of the following is the most likely underlying mechanism for this patient's clinical presentation?

Impaired distal tubule hydrogen ion secretionCevap
B
Defective proximal tubule bicarbonate reabsorption
C
Decreased aldosterone production and activity
D
Concomitant primary respiratory acid-base disorder
E
Accumulation of unmeasured endogenous organic acids

Cevap

Impaired distal tubule hydrogen ion secretion

This patient's clinical presentation of sicca symptoms (dry eyes, dry mouth) and parotid gland enlargement is highly characteristic of Sjögren syndrome. Her laboratory findings demonstrate a non-anion gap metabolic acidosis (anion gap of 12) with severe hypokalemia and an inappropriately alkaline urine pH (6.5). This constellation of findings is diagnostic of distal (Type 1) renal tubular acidosis (RTA). In Type 1 RTA, there is impaired hydrogen ion secretion by the alpha-intercalated cells of the distal tubule, often due to tubulointerstitial inflammation in autoimmune conditions like Sjögren syndrome. The inability to secrete H+ causes systemic acidosis and prevents the urine pH from dropping below 5.5. The body attempts to compensate for impaired distal Na+/H+ exchange by increasing K+ wasting, leading to severe hypokalemia, which is responsible for this patient's acute flaccid paralysis.

Adım Adım Çözüm

Calculate the serum anion gap to evaluate the metabolic acidosis.

Anion gap = Na - (Cl + HCO3) = 140 - (116 + 12) = 12.

An anion gap of 12 falls within the normal range (8-12 mEq/L), indicating a non-anion gap (hyperchloremic) metabolic acidosis.

Assess the appropriateness of respiratory compensation using Winter's formula.

Expected pCO2 = (1.5 × 12) + 8 = 26 ± 2 mm Hg. The measured pCO2 is 26 mm Hg.

Because the measured pCO2 matches the expected value perfectly, there is no concomitant primary respiratory acid-base disorder.

Evaluate the potassium level and urine pH to differentiate the causes of normal anion gap metabolic acidosis.

The patient has severe hypokalemia (2.1 mEq/L) and an inappropriately alkaline urine (pH 6.5) despite severe systemic acidosis.

Hypokalemia combined with a high urine pH (> 5.5) in the setting of normal anion gap metabolic acidosis points directly to Type 1 (distal) Renal Tubular Acidosis (RTA).

Synthesize the laboratory findings with the clinical presentation.

The patient's dry eyes, dry mouth, and parotid enlargement indicate Sjögren syndrome, which classically causes Type 1 RTA.

Sjögren syndrome leads to tubulointerstitial inflammation, impairing the alpha-intercalated cells in the distal tubule from secreting hydrogen ions. The resulting severe hypokalemia causes the patient's acute flaccid paralysis.

Anahtar Kavram

Sjögren syndrome is a major cause of distal (Type 1) renal tubular acidosis, characterized by a normal anion gap metabolic acidosis, severe hypokalemia leading to muscle weakness, and an inappropriately alkaline urine pH.

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