A 15-month-old girl is brought to the emergency department because of progressive unrespon… — USMLE Step 2 CK

A 15-month-old girl is brought to the emergency department because of progressive unresponsiveness over the past 4 hours. She had a low-grade fever and poor oral intake for the last two days due to a presumed viral gastrointestinal illness. Today, she had two episodes of non-bilious vomiting and became extremely lethargic. Her parents also mention that prior to this illness, she had just begun walking independently and using 3 specific words; they have been anxious about whether her development is on track. On physical examination, she is obtunded and difficult to arouse. Her temperature is 37.2°C (99.0°F), heart rate is 165/min, blood pressure is 80/45 mm Hg, and respiratory rate is 44/min without wheezing or retractions. Abdominal examination reveals the liver edge palpable 3 cm below the right costal margin.

Laboratory studies show:

Test	Result
Serum glucose	32 mg/dL
Serum sodium	138 mEq/L
Serum potassium	4.2 mEq/L
Serum chloride	104 mEq/L
Serum bicarbonate	14 mEq/L
Arterial blood gas	pH 7.32, pCO2 28 mm Hg, pO2 95 mm Hg
Liver enzymes	AST 215 U/L, ALT 240 U/L
Serum ammonia	75 µmol/L (normal: 15-50 µmol/L)
Urinalysis	Negative for glucose, ketones, and leukocytes

Intravenous dextrose is immediately administered, leading to rapid improvement in her mental status. In addition to supportive fluid therapy, which of the following is the most appropriate next step in the management or evaluation of this patient?

Obtain a serum acylcarnitine profileAnswer
B
Refer for a comprehensive neurodevelopmental assessment
C
Obtain blood cultures and administer broad-spectrum antibiotics
D
Administer an intravenous sodium bicarbonate infusion
E
Administer nebulized albuterol and oral corticosteroids

Answer

The most appropriate next step is to obtain a serum acylcarnitine profile to diagnose an underlying fatty acid oxidation defect.

This patient's presentation of acute lethargy, hepatomegaly, and severe hypoglycemia triggered by a period of fasting (due to viral gastroenteritis) is highly suspicious for an inborn error of metabolism, specifically a fatty acid oxidation defect (FAOD). The classic laboratory hallmark is hypoketotic hypoglycemia (low serum glucose with absent ketones in the urine), alongside mild liver inflammation (elevated AST/ALT) and mild hyperammonemia. Normally, during periods of fasting, the body breaks down fatty acids to produce ketones, which serve as an alternative energy source for the brain. A defect in this process leaves the patient entirely dependent on glucose, rapidly leading to profound hypoglycemia without the expected compensatory ketone production. The most common of these disorders is medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. The diagnosis is confirmed by obtaining a serum acylcarnitine profile, which will show characteristic elevations in C8-C10 acylcarnitines.

Step-by-Step Solution

Analyze the basic metabolic panel and arterial blood gas.

The patient has severe hypoglycemia (32 mg/dL) and an anion gap metabolic acidosis (

AG = 138 - (104 + 14) = 20

). Winter's formula (

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

mm Hg) shows the

pCO_2

of 28 mm Hg is appropriate respiratory compensation.

Accurately identifying the acid-base status prevents inappropriate therapeutic interventions like IV sodium bicarbonate.

Synthesize the laboratory toxidrome with the clinical history.

Fasting (due to viral illness) triggered severe hypoglycemia with ABSENT urine ketones, mild hepatomegaly (elevated transaminases), and mild hyperammonemia.

Hypoketotic hypoglycemia is the hallmark of a fatty acid oxidation defect (FAOD), as the body cannot break down fats into ketones when glucose stores are depleted.

Evaluate the patient's developmental milestones provided by the parents.

Walking independently and using 3 specific words is entirely normal for a 15-month-old infant.

Ruling out chronic developmental delay focuses the diagnostic pathway on the acute metabolic etiology rather than an underlying neurocognitive disorder.

Determine the definitive diagnostic step for the suspected condition.

A serum acylcarnitine profile is required to diagnose FAODs like Medium-Chain Acyl-CoA Dehydrogenase (MCAD) deficiency.

The profile will show characteristic elevations of specific acylcarnitines (e.g., C8-C10 in MCAD deficiency) necessary for a definitive diagnosis.

Key Concept

Hypoketotic hypoglycemia with mild hepatomegaly and hyperammonemia during fasting is classic for a fatty acid oxidation defect (e.g., MCAD deficiency), which is definitively diagnosed using a serum acylcarnitine profile.

Estimated Time:2m 0s

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