A 7-month-old boy is brought to the pediatric emergency department for extreme lethargy, d… — USMLE Step 2 CK

A $7$ -month-old boy is brought to the pediatric emergency department for extreme lethargy, diaphoresis, and recurrent vomiting. The parents state that he was completely healthy and exclusively breastfed until one month ago, when they began introducing fruit purees and juices. He frequently vomits shortly after these meals. The parents also mention they are worried about his development; although he can roll over in both directions and passes toys from one hand to the other, he is not yet able to pull himself up to a standing position. Physical examination reveals a lethargic and mildly jaundiced infant. The liver edge is palpable $4$ cm below the right costal margin.

Laboratory studies show:
Sodium: $140$ mEq/L
Potassium: $3.5$ mEq/L
Chloride: $104$ mEq/L
Bicarbonate: $14$ mEq/L
Glucose: $45$ mg/dL
Aspartate aminotransferase (AST): $180$ U/L
Alanine aminotransferase (ALT): $210$ U/L

Arterial blood gas on room air:
pH: $7.29$
PaCO2: $29$ mm Hg

Which of the following is the most accurate statement regarding this patient's presentation?

A
The arterial blood gas demonstrates a mixed primary metabolic acidosis and primary respiratory alkalosis, indicating impending respiratory failure.
B
The presentation of lethargy, vomiting, and acidosis is definitively due to severe bacterial sepsis, requiring immediate empiric broad-spectrum antibiotics as the primary intervention.
The condition is caused by a deficiency of aldolase B, and the arterial blood gas reflects an appropriately compensated simple metabolic acidosis.Answer
D
The patient's inability to pull to a stand at $7$ months of age indicates a significant gross motor delay warranting a neurological evaluation.
E
The basic metabolic panel indicates a non-anion gap metabolic acidosis, suggesting an underlying renal tubular defect.

Answer

The condition is caused by a deficiency of aldolase B, and the arterial blood gas reflects an appropriately compensated simple metabolic acidosis.

This patient presents with classic signs of Hereditary Fructose Intolerance (HFI), an autosomal recessive disorder caused by a deficiency of aldolase B. Symptoms typically begin when fructose-containing foods (e.g., fruit purees, juices) are introduced into the infant's diet. The accumulation of fructose-1-phosphate depletes intracellular ATP, severely impairing gluconeogenesis and glycogenolysis, which leads to profound hypoglycemia, vomiting, hepatomegaly, and jaundice. The lactic acidosis resulting from impaired gluconeogenesis causes an elevated anion gap metabolic acidosis. The patient's anion gap is calculated as

140 - (104 + 14) = 22

mEq/L (normal is

8-12

mEq/L). To determine if the respiratory compensation is appropriate, Winter's formula is applied: Expected PaCO2

= (1.5 \times [HCO3-]) + 8 \pm 2

. For this patient,

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

mm Hg. The actual PaCO2 of

29

mm Hg falls exactly within this expected range, confirming a fully compensated simple metabolic acidosis.

Step-by-Step Solution

Identify the underlying metabolic disorder based on the clinical presentation and dietary history.

The introduction of fructose (fruit purees) causing profound hypoglycemia, hepatomegaly, jaundice, and vomiting points to Hereditary Fructose Intolerance (Aldolase B deficiency).

Recognizing the trigger and metabolic consequences is required to diagnose the primary condition.

Calculate the serum anion gap to classify the metabolic acidosis.

Anion gap =

140 - (104 + 14) = 22

mEq/L. This is an elevated anion gap metabolic acidosis (due to lactic acidosis from impaired gluconeogenesis).

Accurate interpretation of the metabolic panel rules out a non-anion gap (renal tubular) etiology.

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

Expected PaCO2 =

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

mm Hg. The actual PaCO2 is

29

mm Hg, confirming a fully compensated simple metabolic acidosis.

This calculation differentiates a simple compensated disorder from a mixed acid-base disorder.

Evaluate the patient's developmental milestones for his age (

7

months).

Rolling over and transferring objects are age-appropriate. Pulling to a stand is not expected until

9

12

months, so development is normal.

Proper milestone knowledge prevents the misdiagnosis of a neurological delay.

Key Concept

Hereditary Fructose Intolerance (aldolase B deficiency) presents upon the dietary introduction of fructose and can cause an elevated anion gap metabolic acidosis; pediatric milestones must be accurately recognized, and Winter's formula must be used to verify respiratory compensation.

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