A 21-year-old male is brought to the emergency department by his roommate due to profound … — USMLE Step 2 CK

A 21-year-old male is brought to the emergency department by his roommate due to profound generalized weakness and episodes of confusion over the past 24 hours. The roommate mentions the patient was recently fired from his job at a paint manufacturing plant and has been spending most of his time locked in his room with a strong, unusual chemical odor. On physical examination, the patient is lethargic but arousable. Muscle strength is 2/5 in the bilateral lower extremities and 3/5 in the upper extremities. Deep tendon reflexes are diminished. His blood pressure is 110/70 mm Hg, heart rate is 98/min, and respiratory rate is 22/min.

Laboratory studies reveal the following:

Parameter	Value
Sodium	$137 \text{ mEq/L}$
Potassium	$2.1 \text{ mEq/L}$
Chloride	$115 \text{ mEq/L}$
Bicarbonate	$12 \text{ mEq/L}$
Blood urea nitrogen	$12 \text{ mg/dL}$
Creatinine	$0.8 \text{ mg/dL}$

Arterial blood gas on room air shows a pH of 7.28 and a $pCO_2$ of 26 mm Hg. Urinalysis reveals a pH of 6.8 with no protein or blood. Urine electrolyte testing reveals a sodium of 45 mEq/L, potassium of 25 mEq/L, and chloride of 30 mEq/L.

Which of the following is the most likely primary mechanism responsible for this patient's laboratory abnormalities?

Impaired distal tubular secretion of hydrogen ionsCevap
B
Decreased proximal tubular reabsorption of filtered bicarbonate
C
Reduced aldosterone synthesis or tubular responsiveness
D
Accumulation of unmeasured endogenous or exogenous organic acids
E
Excessive gastrointestinal loss of bicarbonate-rich fluids

Cevap

Impaired distal tubular secretion of hydrogen ions

The patient's presentation is highly characteristic of inhalant abuse, specifically toluene sniffing. Toluene is metabolized in the liver to hippuric acid. The rapid renal excretion of the hippurate anion, accompanied by sodium and potassium to maintain electrical neutrality, results in profound hypokalemia and severe muscle weakness. Because the hippurate anion is excreted rather than accumulating in the serum, the patient develops a hyperchloremic (normal anion gap) metabolic acidosis. Furthermore, toluene induces a functional Type 1 (distal) renal tubular acidosis, which is characterized by the inability of the distal tubule to secrete hydrogen ions. This failure of distal acidification is confirmed by the inappropriately high urine pH (6.8) in the face of systemic acidemia and a markedly positive urine anion gap (+40 mEq/L), reflecting deficient urinary ammonium (

NH_4^+

) excretion.

Adım Adım Çözüm

Calculate the serum anion gap.

Na^+ - (Cl^- + HCO_3^-) = 137 - (115 + 12) = 10 \text{ mEq/L}

This establishes that the patient has a normal anion gap metabolic acidosis (NAGMA), ruling out high-gap toxic alcohol ingestions.

Assess respiratory compensation using Winter's formula.

Expected

pCO_2 = 1.5 \times (12) + 8 \pm 2 = 26 \pm 2 \text{ mm Hg}

. The measured

pCO_2

is 26 mm Hg.

This confirms there is no superimposed primary respiratory acid-base disorder.

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

U_{Na} + U_{K} - U_{Cl} = 45 + 25 - 30 = +40 \text{ mEq/L}

A positive UAG indicates impaired renal

NH_4^+

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

Synthesize the laboratory data with the clinical presentation.

The patient has a positive UAG, severe hypokalemia, and a urine pH > 5.5, which defines Type 1 (distal) RTA. The history of inhalant abuse points to toluene toxicity.

Toluene is metabolized to hippuric acid. The protons consume bicarbonate (causing acidosis), while the hippurate anion is rapidly excreted in the urine with sodium and potassium, causing profound hypokalemia, volume depletion, and a normal serum anion gap.

Anahtar Kavram

Toluene toxicity causes a Type 1 (distal) renal tubular acidosis, characterized by a normal serum anion gap metabolic acidosis, severe hypokalemia, inappropriately alkaline urine pH, and a positive urine anion gap.

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