A 12-month-old boy is evaluated in the pediatric emergency department for profound letharg… — USMLE Step 2 CK

A $12$ -month-old boy is evaluated in the pediatric emergency department for profound lethargy, frequent episodes of non-bilious vomiting, and rapid breathing over the past $24$ hours. His parents report that he had a mild runny nose and cough for three days prior to this acute deterioration. During the medical history, the parents mention that he has not yet begun to stand while holding onto furniture and cannot pick up small pieces of food using his thumb and index finger. Vital signs show a temperature of $37.8^\circ\text{C}$ ( $100^\circ\text{F}$ ), pulse of $158/\text{min}$ , respirations of $48/\text{min}$ , and blood pressure of $86/52\text{ mm Hg}$ . Physical examination reveals a somnolent infant with generalized hypotonia. The liver is palpated $3\text{ cm}$ below the right costal margin.

Laboratory evaluation reveals the following:

Serum sodium: $140\text{ mEq/L}$
Serum potassium: $4.2\text{ mEq/L}$
Serum chloride: $102\text{ mEq/L}$
Serum bicarbonate: $12\text{ mEq/L}$
Blood urea nitrogen: $18\text{ mg/dL}$
Serum creatinine: $0.4\text{ mg/dL}$
Blood glucose: $48\text{ mg/dL}$
Serum ammonia: $145\text{ }\mu\text{mol/L}$ (Normal: $15-50\text{ }\mu\text{mol/L}$ )

Arterial blood gas (room air):
pH: $7.26$
$\text{PaCO}_2$ : $26\text{ mm Hg}$
$\text{PaO}_2$ : $96\text{ mm Hg}$

Urinalysis is strongly positive for ketones.

Based on the clinical presentation and laboratory findings, which of the following statements is most accurate regarding this patient's condition?

It is caused by an underlying deficiency of a mitochondrial carboxylase enzymeCevap
B
The acid-base status reveals an inadequately compensated metabolic acidosis indicating impending respiratory failure
C
The patient's baseline gross and fine motor milestones are appropriate for his age, suggesting an exclusively acute encephalopathy
D
The severe tachypnea is primarily the result of acute viral inflammation of the lower bronchioles
E
The elevated ammonia level confirms a primary urea cycle defect as the root cause of the patient's symptoms

Cevap

The presentation is most consistent with an organic acidemia (such as propionic acidemia), which is caused by a deficiency in a mitochondrial carboxylase enzyme.

The correct statement identifies that the patient's condition is caused by an underlying deficiency of a mitochondrial carboxylase enzyme. The patient presents with a classic metabolic crisis: an elevated anion gap metabolic acidosis (

AG = 26

), profound hypoglycemia, hyperammonemia, and ketosis. This constellation of laboratory findings is characteristic of an organic acidemia, such as propionic acidemia or methylmalonic acidemia. Propionic acidemia is specifically caused by an autosomal recessive deficiency in propionyl-CoA carboxylase, a biotin-dependent mitochondrial enzyme responsible for converting propionyl-CoA to methylmalonyl-CoA. These crises are typically triggered by catabolic stress, such as the patient's recent viral upper respiratory infection, in a child who may already show signs of developmental delay (e.g., failing to pull to stand or use a pincer grasp at

12

months).

Adım Adım Çözüm

Calculate the anion gap and evaluate the primary acid-base disorder using Winter's formula.

Anion gap =

140 - (102 + 12) = 26

(Elevated). Winter's formula: expected

\text{PaCO}_2 = 1.5 \times 12 + 8 = 26 \pm 2

. Actual

\text{PaCO}_2

26

, confirming a fully compensated high anion gap metabolic acidosis.

Determining the precise acid-base status helps narrow the differential diagnosis for an acutely ill, encephalopathic child.

Synthesize the laboratory findings to identify the underlying metabolic derangement.

The patient has a triad of elevated anion gap metabolic acidosis, hypoglycemia (

48\text{ mg/dL}

), and hyperammonemia (

145\text{ }\mu\text{mol/L}

) combined with strong ketonuria.

This specific combination of laboratory abnormalities distinguishes organic acidemias from other inborn errors of metabolism, such as urea cycle defects (which lack acidosis/ketosis) or fatty acid oxidation defects (which lack ketosis).

Correlate the acute presentation with the patient's developmental history.

The inability to pull to stand or use a pincer grasp at

12

months indicates baseline developmental delay. The acute crisis was likely precipitated by the increased catabolic stress of a minor viral illness.

Inborn errors of metabolism often present insidiously with developmental delay and hypotonia, interrupted by severe acute decompensations during periods of metabolic stress (e.g., infections, fasting).

Anahtar Kavram

Organic acidemias typically present with an elevated anion gap metabolic acidosis, hyperammonemia, hypoglycemia, and ketonuria, often triggered by catabolic stress in an infant with baseline developmental delay.

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

Sık Yapılan Hatalar

Konular

Bu konudan daha fazla soru