A 12-day-old girl is brought to the emergency department by her parents due to two days of… — USMLE Step 2 CK

A 12-day-old girl is brought to the emergency department by her parents due to two days of worsening vomiting, hypotonia, and decreased responsiveness. She was born at 39 weeks gestation and was feeding well on standard infant formula until this episode. Her temperature is 36.5°C (97.7°F), heart rate is 170/min, and respiratory rate is 68/min with deep, rapid breaths. Physical examination shows a lethargic neonate with dry mucous membranes and decreased muscle tone.

Laboratory studies show the following:

Test	Result
Serum sodium	141 mEq/L
Serum potassium	4.8 mEq/L
Serum chloride	101 mEq/L
Serum bicarbonate	10 mEq/L
Blood glucose	48 mg/dL
Serum ammonia	350 µmol/L (normal: 15-50)

Arterial blood gas analysis on room air shows a pH of 7.20 and a $pCO_2$ of 23 mm Hg. Urinalysis demonstrates large ketones but is negative for reducing substances. Further metabolic testing reveals markedly elevated serum propionic acid and methylcitric acid levels, with normal methylmalonic acid levels.

Which of the following is the most appropriate specific long-term dietary management for this patient's underlying condition?

Restriction of valine, isoleucine, methionine, and threonineCevap
B
Restriction of leucine, isoleucine, and valine
C
Restriction of dietary galactose and lactose
D
Restriction of phenylalanine and supplementation with tyrosine
E
Continuous administration of bicarbonate without specific amino acid restriction

Cevap

Restriction of valine, isoleucine, methionine, and threonine

This patient's presentation of neonatal lethargy, tachypnea, vomiting, hypoglycemia, hyperammonemia, and high anion gap metabolic acidosis is the classic presentation of an organic acidemia. The specific elevation of propionic and methylcitric acids with normal methylmalonic acid confirms the diagnosis of propionic acidemia, caused by an autosomal recessive deficiency of propionyl-CoA carboxylase. Propionyl-CoA is generated from the catabolism of specific substrates, easily remembered by the mnemonic VOMIT: Valine, Odd-chain fatty acids, Methionine, Isoleucine, and Threonine. Therefore, the cornerstone of long-term management is strict dietary restriction of these precursor amino acids to prevent toxic accumulation of propionic acid and subsequent metabolic crises.

Adım Adım Çözüm

Calculate the serum anion gap to evaluate the metabolic acidosis.

Anion Gap =

Na^+ - (Cl^- + HCO_3^-)

141 - (101 + 10) = 30

mEq/L.

A high anion gap (normal 8-12 mEq/L) combined with ketosis and hyperammonemia in a neonate strongly points to an inborn error of metabolism, specifically an organic acidemia.

Interpret the specific organic acid profile.

The presence of elevated propionic and methylcitric acids with normal methylmalonic acid isolates the enzymatic defect.

This specific pattern confirms a deficiency of propionyl-CoA carboxylase, leading to the diagnosis of propionic acidemia.

Identify the metabolic precursors that must be restricted in the diet.

The precursors are Valine, Odd-chain fatty acids, Methionine, Isoleucine, and Threonine (VOMIT).

Restricting these specific dietary inputs prevents the downstream accumulation of toxic propionyl-CoA and its byproducts.

Anahtar Kavram

Propionic acidemia presents as a neonatal metabolic crisis and requires dietary restriction of amino acids that catabolize to propionyl-CoA: Valine, Methionine, Isoleucine, and Threonine.

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