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Difficulty: MediumPediatric Genetic and Metabolic Disorders

An infant, currently 77 months of age, presents to the emergency room with severe lethargy and tachypnea that began yesterday. The patient's diet was recently transitioned from exclusive breastfeeding to include high-protein solid foods such as meat purees. The mother notes the child can no longer sit up independently, a milestone he had successfully mastered two weeks ago.

Vital signs show a heart rate of 165/min165/\text{min} and a respiratory rate of 60/min60/\text{min}. He is afebrile. Physical examination demonstrates an encephalopathic infant with diffuse muscle weakness, significant head lag, and absent deep tendon reflexes.

Laboratory analysis reveals:
Serum sodium: 139 mEq/L139 \text{ mEq/L}
Serum chloride: 103 mEq/L103 \text{ mEq/L}
Serum bicarbonate: 11 mEq/L11 \text{ mEq/L}
Blood glucose: 48 mg/dL48 \text{ mg/dL}
Serum ammonia: 340 μmol/L340 \ \mu\text{mol/L} (normal: 1515-50 μmol/L50 \ \mu\text{mol/L})
Arterial pH: 7.237.23
Arterial pCO2: 24 mmHg24 \text{ mmHg}

Blood cultures collected two days prior at an urgent care clinic during a mild febrile illness are positive for *Streptococcus pneumoniae*; the preliminary susceptibility report confirms the organism is highly sensitive to penicillin.

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

  1. Immediate cessation of enteral protein and administration of intravenous dextroseAnswer
  2. B
    Administration of intravenous sodium bicarbonate to achieve an arterial pCO2 of 40 mmHg40 \text{ mmHg}
  3. C
    Initiation of empiric broad-spectrum therapy with intravenous vancomycin and cefepime
  4. D
    Discharge home with scheduled follow-up for normal physiological milestone progression
  5. E
    Administration of the scheduled live-attenuated rotavirus vaccine prior to hospital admission

Answer

Immediate cessation of enteral protein and administration of intravenous dextrose
The correct intervention is the immediate cessation of enteral protein and administration of intravenous dextrose. The infant's presentation of lethargy, high anion gap metabolic acidosis, hyperammonemia, and hypoglycemia following the introduction of protein-rich foods strongly suggests a decompensated organic acidemia (e.g., methylmalonic or propionic acidemia). The immediate goal of therapy is to halt the generation of toxic metabolites. This is achieved by stopping all exogenous protein intake and infusing high-dose intravenous dextrose to stimulate insulin secretion, which effectively shifts the body from a catabolic to an anabolic state.

Step-by-Step Solution

1
Interpret the clinical presentation and dietary history.
The transition to high-protein food triggering lethargy, vomiting, and milestone regression strongly suggests an inborn error of metabolism, specifically an organic acidemia.
Organic acidemias typically manifest when protein intake exceeds the body's metabolic capacity, leading to toxic accumulation.
2
Analyze the laboratory data and acid-base status.
The patient has a high anion gap (2525) metabolic acidosis with perfect respiratory compensation (pCO2 24 mmHg24 \text{ mmHg} via Winter's formula), accompanied by hypoglycemia and secondary hyperammonemia.
This specific metabolic triad (acidosis, hypoglycemia, hyperammonemia) confirms an organic acidemia crisis.
3
Determine the immediate acute management strategy.
Stop all protein intake and provide high-dose intravenous dextrose.
IV dextrose stimulates endogenous insulin release, reversing the catabolic state that drives the production of toxic organic acids, while stopping protein halts the exogenous precursor load.

Key Concept

Acute management of organic acidemia crises requires immediate cessation of dietary protein and reversal of the catabolic state using high-dose intravenous dextrose.
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Topics

Pediatric Genetic and Metabolic Disorders

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