A 2-day-old male is seen in the newborn nursery for repeated emesis and lethargy. He was born at 39 weeks to a 24-year-old mother following an uncomplicated pregnancy and birth. He has been breastfeeding every 2 hours and has 10 wet diapers per day. His father has a history of beta-thalassemia minor. Laboratory results are as follows: Hemoglobin: 12 g/dL Platelet count: 200,000/mm³ Mean corpuscular volume: 95 µm³ Reticulocyte count: 0.5% Leukocyte count: 5,000/mm³ with normal differential Serum: Na+: 134 mEq/L Cl-: 100 mEq/L K+: 3.3 mEq/L HCO3-: 24 mEq/L Urea nitrogen: 1 mg/dL Creatinine: 0.6 mg/dL Ammonia: 150 µmol/L (normal: 50-80 µmol/L) Which of the following is the most likely diagnosis?

Ornithine transcarbamylase deficiency

A 2-week-old boy presents to the emergency department because of unusual irritability and lethargy. The patient is admitted to the pediatric intensive care unit and minutes later develops metabolic encephalopathy. This progressed to a coma, followed by death before any laboratory tests are completed. The infant was born at home via vaginal delivery at 39 weeks' of gestation. His mother says that the symptoms started since the infant was 4-days-old, but since he only seemed ‘tired’, she decided not to seek medical attention. Further testing during autopsy shows hyperammonemia, low citrulline, and increased orotic acid. Which of the following enzymes is most likely deficient in this patient?

Branched-chain alpha-ketoacid dehydrogenase

A newborn boy develops projectile vomiting 48 hours after delivery. He is found to be lethargic, with poor muscle tone, and is hyperventilating. Within hours, he suffers important neurological deterioration, leading to seizures, coma, and, ultimately, death. An autopsy is performed and the pathology team makes a diagnosis of a rare genetic disorder that leads to low levels of N-acetylglutamate. Which of the following enzymes would be secondarily affected by this process?

Carbamoyl phosphate synthetase I

A 4-day-old boy is brought to the physician because of somnolence, poor feeding, and vomiting after his first few breast feedings. He appears lethargic. His respiratory rate is 73/min. Serum ammonia is markedly increased. Genetic analysis shows deficiency in N-acetylglutamate synthase. The activity of which of the following enzymes is most likely directly affected by this genetic defect?

Carbamoyl phosphate synthetase I

Urea cycle disorders for USMLE Step 3: High-Yield Biochemistry Lessons

Urea Cycle 101 - The Body's Ammonia Detox

Function: Converts neurotoxic ammonia (NH₃) from amino acid catabolism into water-soluble urea for renal excretion.
Location: Liver (Mitochondria & Cytosol).
Rate-Limiting Enzyme: Carbamoyl Phosphate Synthetase I (CPS I).
- Allosterically activated by N-acetylglutamate (NAGS).
Mnemonic (Intermediates): 📌 Ordinarily, Careless Crappers Are Also Frivolous About Urination.

⭐ The first two enzymes (CPS I, OTC) are mitochondrial; the rest are cytosolic. This compartmentalization is a key concept for understanding urea cycle disorders.

Enzyme Deficiencies - The Usual Suspects

Ornithine Transcarbamylase (OTC) Deficiency
- Most common; X-linked recessive inheritance.
- Labs: ↑ Orotic acid in blood & urine, ↑↑ ammonia.
- Excess carbamoyl phosphate is shunted to pyrimidine synthesis.
Carbamoyl Phosphate Synthetase I (CPS I) Deficiency
- Autosomal recessive.
- Labs: ↑↑ Ammonia, but no orotic aciduria.
Argininosuccinate Synthetase Deficiency (Citrullinemia Type I)
- Labs: ↑↑ Citrulline.
Argininosuccinate Lyase Deficiency (Argininosuccinic Aciduria)
- Labs: ↑ Argininosuccinic acid.
Arginase Deficiency
- Presents later (age 1-3) with spastic diplegia, growth delay.
- Labs: ↑ Arginine; milder hyperammonemia.

⭐ OTC deficiency is the most common urea cycle disorder. Its X-linked inheritance and associated orotic aciduria are classic distinguishing features on exams.

📌 Mnemonic: Ordinarily, Careless Crappers Are Also Frivolous About Urination (Ornithine, Carbamoyl phosphate, Citrulline, Aspartate, Argininosuccinate, Fumarate, Arginine, Urea).

Urea Cycle Pathway with Enzymes

Clinical Features & Diagnosis - Ammonia Alert

Presentation: Often neonatal encephalopathy (lethargy, poor feeding, vomiting, seizures, hypotonia/hypertonia) progressing to coma. Can present later with ataxia, confusion, or psychosis triggered by catabolic stress (illness, high protein intake).
Hallmark Lab: ↑↑ Plasma ammonia.
- Neonates: >150 µmol/L
- Older individuals: >100 µmol/L
Initial Labs: Respiratory alkalosis (hyperventilation), normal anion gap, normal glucose.

⭐ Ornithine Transcarbamylase (OTC) Deficiency is the most common UCD and is X-linked. All other UCDs are autosomal recessive.

Management - Damage Control

Goal: Rapidly ↓ plasma ammonia to prevent irreversible neurological damage.
Acute Hyperammonemic Crisis: Medical emergency!

Nitrogen Scavengers:
- IV Sodium Benzoate: Combines with glycine → hippurate (renally excreted).
- IV Sodium Phenylacetate: Combines with glutamine → phenylacetylglutamine (renally excreted).
- IV Arginine: Replenishes urea cycle intermediate (essential in ASS/ASL deficiency).

⭐ Goal: Lower ammonia by 50% in the first 4-6 hours and normalize within 24 hours.

Nitrogen Scavenger Mechanisms in Urea Cycle Disorders

High‑Yield Points - ⚡ Biggest Takeaways

Hyperammonemia is the central feature, causing encephalopathy, seizures, and cerebral edema.

Most present in infancy with poor feeding, vomiting, and lethargy progressing to coma.

Ornithine transcarbamylase (OTC) deficiency, the most common disorder, is X-linked recessive.

Elevated urinary orotic acid is a key finding in OTC deficiency.

All other urea cycle disorders are inherited in an autosomal recessive pattern.

Acute management involves halting protein intake and using ammonia scavengers.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play

Urea cycle disorders