A 16-year-old boy is brought to the emergency department by his parents due to progressive… — USMLE Step 2 CK

A $16$ -year-old boy is brought to the emergency department by his parents due to progressive muscle weakness over the past $24$ hours. He is currently unable to stand without assistance. His parents note that he has been increasingly irritable and performing poorly in school for the past $4$ months. Two weeks ago, his primary care provider prescribed fluoxetine for suspected depression, but his behavior has not improved. A routine thyroid panel obtained at that clinic visit showed a thyroid-stimulating hormone (TSH) level of $4.5$ $\mu$ U/mL (normal: $0.4-4.0$ $\mu$ U/mL) and a normal free T4. On physical examination, temperature is $37.1^\circ$ C ( $98.8^\circ$ F), blood pressure is $110/70$ mmHg, pulse is $92$ /min, and respirations are $22$ /min. He appears lethargic. There is an erythematous, scaling rash localized around his mouth and nose. Neurologic examination reveals symmetrical proximal and distal muscle weakness in all extremities, with diminished deep tendon reflexes. Laboratory studies show:

Serum sodium: $140$ mEq/L
Serum potassium: $2.1$ mEq/L
Serum chloride: $115$ mEq/L
Serum bicarbonate: $14$ mEq/L

Arterial blood gas (room air):
pH: $7.28$
pCO $_2$ : $30$ mmHg

Which of the following is the most likely diagnosis?

Toluene inhalationAnswer
B
Ethylene glycol ingestion
C
Schizophrenia
D
Serotonin syndrome
E
Hypothyroid myopathy

Answer

The most likely diagnosis is toluene inhalation, which characteristically presents with a perioral rash, normal anion gap metabolic acidosis, and profound hypokalemia causing muscle weakness.

The correct diagnosis is toluene inhalation. Toluene is a volatile solvent found in paint thinners, glues, and rubber cements. Inhalant use disorder typically affects adolescents and presents with transient euphoria, lethargy, and behavioral changes. A classic physical finding is the "glue sniffer's rash," an erythematous dermatitis around the mouth and nares caused by the drying effects of the solvent. Metabolically, toluene is converted in the liver to hippuric acid. The rapid renal excretion of hippurate obligates the loss of sodium and potassium, leading to severe hypokalemia. This process also causes a distal (Type 1) renal tubular acidosis, which manifests as a normal anion gap metabolic acidosis (NAGMA). In this patient, the anion gap is

140 - (115 + 14) = 11

mEq/L (normal:

8-12

mEq/L). The profound hypokalemia (

2.1

mEq/L) is the direct cause of his acute, symmetrical muscle weakness and diminished deep tendon reflexes.

Step-by-Step Solution

Analyze the acid-base status using the arterial blood gas and serum electrolytes.

The pH of

7.28

and HCO

_3

14

mEq/L indicate a metabolic acidosis. The anion gap is calculated as

140 - (115 + 14) = 11

mEq/L, which is within the normal range (

8-12

mEq/L).

Differentiating between normal and high anion gap metabolic acidosis is the critical first step in determining the etiology of the metabolic derangement.

Evaluate the respiratory compensation using Winter's formula.

Expected pCO

_2

(1.5 \times 14) + 8 \pm 2 = 29 \pm 2

mmHg. The measured pCO

_2

30

mmHg, indicating appropriate respiratory compensation.

This confirms a pure normal anion gap metabolic acidosis (NAGMA) without a superimposed respiratory acid-base disorder.

Synthesize the metabolic findings (NAGMA, severe hypokalemia) with the clinical presentation (adolescent, behavioral changes, perioral rash, acute weakness).

The combination of NAGMA, profound hypokalemia (

2.1

mEq/L) leading to areflexic weakness, and a "glue sniffer's rash" points directly to toluene toxicity.

Toluene is an inhalant metabolized to hippuric acid, which obligates the renal excretion of potassium, causing a Type 1 (distal) renal tubular acidosis.

Key Concept

Toluene inhalation (glue sniffing) classically presents with a perioral rash, behavioral changes, and a distal (Type 1) renal tubular acidosis, characterized by a normal anion gap metabolic acidosis and severe hypokalemia.

Estimated Time:2m 0s

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