During a routine well-child visit, the parents of a 5-month-old boy report that he has bec… — USMLE Step 2 CK

During a routine well-child visit, the parents of a $5$ -month-old boy report that he has become increasingly irritable over the past two weeks. The mother notes this is most pronounced in the early mornings, which coincides with him finally "sleeping through the night." She also reports he has been taking rapid, deep breaths during these fussy episodes. Additionally, he had a mild upper respiratory infection last week with rhinorrhea and diffuse wheezing; she requests a rescue inhaler, noting her older child has asthma. On examination, the infant has a "doll-like" facial appearance with prominent, rounded cheeks. His weight is at the $35$ th percentile and length is at the $20$ th percentile. He appropriately tracks objects, smiles socially, and rolls from front to back, but cannot sit unsupported. Abdominal examination reveals a massively enlarged, smooth liver palpable $6$ cm below the right costal margin.

Laboratory tests obtained after a $4$ -hour fast reveal:

Parameter	Result	Reference Range
Glucose	$42$ mg/dL	$70$ - $100$ mg/dL
Lactic acid	$8.5$ mmol/L	$0.5$ - $2.2$ mmol/L
Uric acid	$8.8$ mg/dL	$2.0$ - $5.5$ mg/dL
Triglycerides	$520$ mg/dL	$<150$ mg/dL
pH	$7.25$	$7.35$ - $7.45$
pCO $_2$	$26$ mm Hg	$35$ - $45$ mm Hg
HCO $_3^-$	$11$ mEq/L	$22$ - $28$ mEq/L
Free T4	$0.6$ ng/dL	$0.9$ - $2.3$ ng/dL
TSH	$2.0$ $\mu$ U/mL	$0.5$ - $5.0$ $\mu$ U/mL

Which of the following is the most appropriate next step in the management of this patient?

Initiation of frequent daytime carbohydrate feedings and a continuous overnight gastric glucose infusionAnswer
B
Prescription of a short-acting beta-agonist inhaler for pediatric asthma
C
Referral to physical therapy for delayed independent sitting milestones
D
Endotracheal intubation and mechanical ventilation for uncompensated respiratory failure
E
Initiation of levothyroxine therapy for primary hypothyroidism

Answer

The most appropriate next step is the initiation of frequent daytime carbohydrate feedings and a continuous overnight gastric glucose infusion.

The patient's clinical picture of a 'doll-like' face, massive hepatomegaly, and severe fasting hypoglycemia accompanied by lactic acidosis, hyperuricemia, and hypertriglyceridemia is pathognomonic for Glucose-6-phosphatase deficiency (Glycogen Storage Disease Type I, or Von Gierke disease). The symptoms began when the infant started sleeping through the night, effectively prolonging his fasting state. The foundational treatment is preventing hypoglycemia to halt the secondary metabolic derangements. In infants, this is achieved through frequent daytime feedings of complex carbohydrates (or soy-based, fructose-free formulas) and a continuous overnight gastric glucose infusion.

Step-by-Step Solution

Analyze the clinical and laboratory presentation.

The infant exhibits fasting hypoglycemia, lactic acidosis, hyperuricemia, hypertriglyceridemia, and massive hepatomegaly with a 'doll-like' face.

These findings collectively establish a diagnosis of Glucose-6-phosphatase deficiency (Glycogen Storage Disease Type I, Von Gierke disease).

Evaluate the acid-base status using Winter's formula.

The expected pCO

_2

is calculated as

(1.5 \times 11) + 8 = 24.5 \pm 2

. The measured pCO

_2

26

mm Hg shows perfect respiratory compensation.

This confirms the tachypnea is a physiologic response (Kussmaul breathing) to severe lactic acidosis, not primary respiratory failure.

Assess the developmental and endocrine findings.

Not sitting unsupported is normal for a

5

-month-old. The thyroid labs (low T4, normal TSH) indicate euthyroid sick syndrome due to severe metabolic stress.

This rules out alternative developmental or primary endocrine etiologies for the patient's symptoms.

Determine the appropriate management for Von Gierke disease.

Select the intervention that prevents fasting hypoglycemia, which is triggered by the infant sleeping through the night.

Continuous overnight glucose infusion bypasses the enzymatic defect, preventing the metabolic cascade of lactic acidosis and hyperuricemia.

Key Concept

Clinical presentation and management of Glucose-6-phosphatase deficiency (Von Gierke disease)

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