A 65-year-old woman is evaluated in the clinic for progressive fatigue and generalized wea… — USMLE Step 2 CK

A 65-year-old woman is evaluated in the clinic for progressive fatigue and generalized weakness over the past month. Her past medical history is significant for type 2 diabetes mellitus, essential hypertension, and heart failure with preserved ejection fraction. Two months ago, a new medication was added to her regimen to improve fluid balance and blood pressure control. Her current medications include metformin, amlodipine, empagliflozin, and the newly prescribed drug.

Her temperature is 37.0°C (98.6°F), blood pressure is 128/78 mm Hg, pulse is 72/min, and respirations are 16/min. Physical examination reveals clear lungs and no lower extremity edema.

Laboratory studies are obtained:

Test	Value
Serum sodium	$138 \text{ mEq/L}$
Serum potassium	$5.8 \text{ mEq/L}$
Serum chloride	$109 \text{ mEq/L}$
Serum bicarbonate	$17 \text{ mEq/L}$
Blood urea nitrogen	$28 \text{ mg/dL}$
Serum creatinine	$1.6 \text{ mg/dL}$
Serum glucose	$145 \text{ mg/dL}$

An arterial blood gas on room air shows:

Test	Value
pH	$7.33$
PaCO $_2$	$33 \text{ mm Hg}$
PaO $_2$	$92 \text{ mm Hg}$

Which of the following best explains the underlying pathophysiology of this patient's current acid-base and electrolyte abnormalities?

Decreased apical sodium channel activity in the cortical collecting ductCevap
B
Defective hydrogen ion secretion by alpha-intercalated cells in the distal tubule
C
Impaired carbonic anhydrase-dependent bicarbonate reabsorption in the proximal tubule
D
Concurrent primary respiratory alkalosis driven by an exaggerated ventilatory response
E
Accumulation of unmeasured endogenous organic anions in the serum

Cevap

The correct answer is decreased apical sodium channel activity in the cortical collecting duct.

The correct answer accurately describes the pathophysiology of Type 4 renal tubular acidosis (RTA). The patient presents with a primary metabolic acidosis (pH 7.33, HCO3 17), a normal anion gap (138 - 126 = 12), appropriate respiratory compensation (Winter's formula:

1.5 \times 17 + 8 = 33.5

), and hyperkalemia (K+ 5.8). This triad of NAGMA, hyperkalemia, and mild renal impairment in a diabetic patient on a newly added heart failure medication (e.g., spironolactone or an ACE inhibitor) strongly points to hypoaldosteronism or aldosterone resistance (Type 4 RTA). Aldosterone normally stimulates the epithelial sodium channel (ENaC) in the principal cells of the cortical collecting duct. Decreased ENaC activity reduces sodium reabsorption, impairing the generation of the lumen-negative transepithelial voltage. This loss of electrical gradient prevents the normal secretion of potassium and hydrogen ions, resulting in hyperkalemia and metabolic acidosis.

Adım Adım Çözüm

Analyze the primary acid-base disturbance using the arterial blood gas and serum chemistry.

The pH of 7.33 indicates acidemia, and the low serum bicarbonate (17 mEq/L) confirms a primary metabolic acidosis.

Identifying the primary derangement is the first step in approaching any acid-base disorder.

Calculate the serum anion gap to classify the metabolic acidosis.

Anion Gap = Na - (Cl + HCO3) = 138 - (109 + 17) = 12 mEq/L. This is a normal anion gap metabolic acidosis (NAGMA).

Differentiating between normal and high anion gap narrows down the differential diagnosis significantly.

Apply Winter's formula to determine if respiratory compensation is appropriate.

Expected PaCO2 = 1.5(HCO3) + 8 = 1.5(17) + 8 = 33.5 ± 2 mm Hg. The actual PaCO2 is 33 mm Hg, indicating perfect respiratory compensation with no concurrent respiratory disorder.

This rules out a mixed acid-base disorder.

Evaluate the serum potassium level in the context of the NAGMA.

The patient has hyperkalemia (5.8 mEq/L) alongside the NAGMA.

A hyperkalemic normal anion gap metabolic acidosis is the hallmark of Type 4 renal tubular acidosis (RTA).

Identify the cellular mechanism causing Type 4 RTA in this clinical vignette.

The patient recently started a heart failure medication (likely spironolactone, a mineralocorticoid receptor antagonist). This blocks aldosterone action, leading to decreased epithelial sodium channel (ENaC) activity in the cortical collecting duct.

Reduced ENaC activity diminishes the lumen-negative potential, which is the driving force for both potassium and hydrogen ion secretion, culminating in hyperkalemia and acidosis.

Anahtar Kavram

Type 4 Renal Tubular Acidosis (Hyperkalemic RTA)

Tahmini Süre:1m 30s

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