Emergency medical services transport a 2-year-old toddler to the pediatric department afte… — USMLE Step 2 CK

Emergency medical services transport a 2-year-old toddler to the pediatric department after the parents found the child unarousable in bed this morning. The parents note that the toddler had been suffering from a mild viral gastroenteritis over the last 48 hours, resulting in complete refusal of solid foods and minimal fluid intake. Medical history is unremarkable, and the child's weight and length have consistently tracked along the 50th percentile.

Upon arrival, the patient is deeply lethargic. Vital signs show a temperature of $37.4^\circ\text{C}$ ( $99.3^\circ\text{F}$ ), pulse of 160/min, and respiratory rate of 28/min. Physical assessment demonstrates a soft abdomen with the liver edge palpable 4 cm below the right costal margin. Neurological assessment reveals profound hypotonia without focal deficits.

Initial point-of-care testing and serum chemistry panel demonstrate:

Laboratory Parameter	Patient Value	Reference Range
Blood glucose	$28\text{ mg/dL}$	$70-110\text{ mg/dL}$
Sodium	$137\text{ mEq/L}$	$135-145\text{ mEq/L}$
Potassium	$4.1\text{ mEq/L}$	$3.5-5.0\text{ mEq/L}$
Bicarbonate	$23\text{ mEq/L}$	$22-28\text{ mEq/L}$
Aspartate aminotransferase (AST)	$310\text{ U/L}$	$10-40\text{ U/L}$
Alanine aminotransferase (ALT)	$345\text{ U/L}$	$10-40\text{ U/L}$
Ammonia	$45\text{ }\mu\text{mol/L}$	$10-80\text{ }\mu\text{mol/L}$

A catheterized urine sample is obtained and is notably completely negative for acetoacetate and beta-hydroxybutyrate.

An impairment in which of the following metabolic pathways best explains this patient's clinical presentation?

Fatty acid beta-oxidationAnswer
B
Hepatic glycogenolysis
C
Branched-chain amino acid degradation
D
Urea cycle nitrogen detoxification
E
Fructose metabolism

Answer

Fatty acid beta-oxidation

The correct answer is fatty acid beta-oxidation. This patient's presentation of severe hypoglycemia, lethargy, hepatomegaly, and elevated transaminases triggered by a prolonged period of fasting, combined with the critical absence of urine ketones, is classic for a fatty acid oxidation defect. The most common of these is Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. During periods of fasting, once hepatic glycogen stores are depleted, the body relies on the beta-oxidation of fatty acids to provide energy (ATP) for gluconeogenesis and to generate acetyl-CoA for ketone body synthesis. An impairment in this pathway results in an inability to synthesize ketones (hypoketotic state) and a failure to maintain blood glucose levels (hypoglycemia). The accumulation of medium-chain acylcarnitines leads to hepatic toxicity, manifesting as hepatomegaly and elevated liver enzymes.

Step-by-Step Solution

Analyze the patient's presentation and metabolic triggers.

The toddler presents with profound lethargy, hepatomegaly, and severe hypoglycemia following a 48-hour period of poor oral intake due to a viral illness.

Identifying the fasting state is crucial, as it indicates a failure of the body's compensatory mechanisms to maintain euglycemia under metabolic stress.

Evaluate the supporting laboratory findings.

The patient has severe hypoglycemia without acidosis, accompanied by elevated transaminases. Crucially, urine ketones (acetoacetate and beta-hydroxybutyrate) are completely absent.

The absence of ketones during profound fasting hypoglycemia is the hallmark diagnostic clue distinguishing this presentation from glycogen storage diseases, which typically feature hyperketonemia.

Correlate the findings with the underlying metabolic pathway.

The inability to utilize fatty acids for energy during prolonged fasting prevents the production of ketone bodies, pointing directly to a defect in fatty acid beta-oxidation.

During fasting, once glycogen stores are depleted, the body must switch to beta-oxidation of fatty acids to produce ketones for the brain and energy for gluconeogenesis. Failure of this pathway leads to hypoketotic hypoglycemia.

Key Concept

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a defect in fatty acid beta-oxidation characterized by hypoketotic hypoglycemia, lethargy, and hepatomegaly triggered by periods of fasting or illness.

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