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 56-year-old male with a history of hepatitis C cirrhosis status post TIPS procedure is brought in by his wife to the emergency department because he has been acting disoriented, slurring his speech, and sleeping throughout the day. On arrival the patient is afebrile and his vital signs are pulse is 87/min, blood pressure is 137/93 mmHg, and respirations are 12/min with shallow breaths. Examination reveals a jaundiced male who appears older than stated age. Abdominal exam is positive for a fluid wave and shifting dullness to percussion. You note enlarged breasts, decreased facial hair, 3+ patellar reflexes bilaterally, and the following in the upper extremity (Image A). Paracentesis reveals ascitic fluid with neutrophil counts of < 100 cells/mcL. Serum creatinine is 1.0 and BUN is 15. Which of the following is the next best step in management?

Administer neomycin and glucose

IV albumin and antibiotic therapy with cefotaxime

Administer rifaximin and glucose

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

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 7-year-old boy is brought to the emergency department because of sudden-onset abdominal pain that began 1 hour ago. Three days ago, he was diagnosed with a urinary tract infection and was treated with nitrofurantoin. There is no personal history of serious illness. His parents emigrated from Kenya before he was born. Examination shows diffuse abdominal tenderness, mild splenomegaly, and scleral icterus. Laboratory studies show: Hemoglobin 9.8 g/dL Mean corpuscular volume 88 μm3 Reticulocyte count 3.1% Serum Bilirubin Total 3.8 mg/dL Direct 0.6 mg/dL Haptoglobin 16 mg/dL (N=41–165 mg/dL) Lactate dehydrogenase 179 U/L Which of the following is the most likely underlying cause of this patient's symptoms?

Enzyme deficiency in red blood cells

Defective red blood cell membrane proteins

Defect in orotic acid metabolism

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

Urea Cycle - The Body's Ammonia Detox

Function: Converts highly toxic ammonia ($NH_3$) into urea for excretion by the kidneys. Primary site: Liver (mitochondria & cytosol).
Key Steps: Ammonia is combined with $CO_2$ and aspartate to generate urea and fumarate (links to TCA cycle).
Rate-Limiting Enzyme: Carbamoyl Phosphate Synthetase I (CPSI).
- Activated by N-acetylglutamate (NAG).

Urea Cycle: Enzymes, Steps, and Cellular Location

⭐ Ornithine Transcarbamylase (OTC) Deficiency is the most common urea cycle disorder. It's X-linked recessive, leading to ↑ orotic acid in blood and urine.

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

Enzyme Deficiencies - When the Cycle Breaks

All are autosomal recessive, except for the X-linked Ornithine Transcarbamylase (OTC) deficiency. Core symptoms stem from hyperammonemia: lethargy, vomiting, seizures, and cerebral edema, typically presenting in neonates.

Enzyme Deficiency	Deficient Enzyme	Key Lab Findings	Clinical Pearl
Hyperammonemia Type I	Carbamoyl Phosphate Synthetase I (CPS I)	↑ NH₄⁺, ↓ Citrulline	No orotic aciduria.
OTC Deficiency	Ornithine Transcarbamylase	↑ NH₄⁺, ↑ Orotic Acid, ↓ Citrulline	Most common; X-linked. Orotic aciduria is the key differentiator.
Citrullinemia	Argininosuccinate Synthetase (ASS)	↑↑ Citrulline, ↑ NH₄⁺	Massive citrulline elevation.
Argininosuccinic Aciduria	Argininosuccinate Lyase (ASL)	↑ Argininosuccinic acid	May have trichorrhexis nodosa (brittle, coarse hair).
Argininemia	Arginase	↑ Arginine	Milder hyperammonemia; presents later with spastic diplegia, growth delay.

Metabolite accumulation in inborn errors of metabolism

⭐ In OTC deficiency, excess carbamoyl phosphate is shunted to the pyrimidine synthesis pathway, leading to a dramatic increase in urinary orotic acid-a key finding to differentiate it from CPS I deficiency.

Clinical Picture & Diagnosis - Spotting the Ammonia Crisis

Presentation: Acute neonatal encephalopathy (lethargy, vomiting, poor feeding, seizures), often after the first protein meal.
Key Labs: ↑↑ plasma ammonia (often >150 µmol/L), respiratory alkalosis (early hyperventilation), ↓ BUN.

⭐ Ornithine Transcarbamylase (OTC) deficiency is the most common urea cycle disorder and is inherited in an X-linked recessive pattern.

Urea Cycle Pathway and Enzyme Deficiencies

Treatment - Managing the Mayhem

Acute Crisis (Hyperammonemia):
- Halt all protein intake.
- Provide high calories (IV dextrose).
- Ammonia Scavengers: IV sodium phenylacetate & sodium benzoate.
- Consider hemodialysis for rapid ammonia reduction.
Chronic Management:
- Strict low-protein diet; essential amino acid supplements.
- Arginine or citrulline supplementation.
- Liver transplant is curative.

⭐ Avoid Valproate! This anti-seizure medication is contraindicated as it inhibits the urea cycle and can trigger life-threatening hyperammonemia.

Urea cycle and citrulline-NO cycle pathways

High‑Yield Points - ⚡ Biggest Takeaways

Urea cycle disorders cause hyperammonemia, a neurotoxin leading to encephalopathy.

Classic presentation: a neonate with lethargy, vomiting, and poor feeding after the first protein meal.

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

Lab findings universally show ↑ blood ammonia and ↑ glutamine.

OTC deficiency is unique for causing ↑ orotic acid in blood and urine.

Management involves a low-protein diet and ammonia-scavenging agents.

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