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 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

An investigator is studying nutritional deficiencies in humans. A group of healthy volunteers are started on a diet deficient in pantothenic acid. After 4 weeks, several of the volunteers develop irritability, abdominal cramps, and burning paresthesias of their feet. These symptoms are fully reversed after reintroduction of pantothenic acid to their diet. The function of which of the following enzymes was most likely impaired in the volunteers during the study?

Alpha-ketoglutarate dehydrogenase

Overview and purpose of urea cycle for USMLE Step 1: High-Yield Biochemistry Lessons

Urea Cycle - The Body's Detox Plan

Primary Goal: Converts highly toxic ammonia ($NH_3$), a byproduct of amino acid catabolism, into urea for safe excretion.
Location: Predominantly in the liver, bridging mitochondrial and cytosolic compartments.
Overall Reaction: $2 NH_3 + CO_2 + 3 ATP ightarrow Urea + 2 ADP + AMP + 4 P_i + H_2O$

Urea Cycle Overview with Mitochondrial and Cytosolic Stages

⭐ High-Yield: Hyperammonemia (↑ $NH_3$) is neurotoxic, depleting α-ketoglutarate in the brain, which inhibits the Krebs cycle. This can lead to tremor (asterixis), slurring of speech, and coma.

The Cycle's Blueprint - From Ammonia to Urea

Primary Goal: Convert highly toxic free ammonia ($NH_3$) into urea for excretion by the kidneys. Prevents neurotoxicity.
Location: Occurs in hepatocytes, spanning two cellular compartments:
- Mitochondria: First two steps.
- Cytosol: Remaining three steps.
Nitrogen Sources: The two nitrogen atoms in urea are sourced from:
- Free ammonia ($NH_3$)
- Aspartate

Key Steps & Mnemonic:
- 📌 Ordinarily, Careless Crappers Are Also Frivolous About Urination.
- (Ornithine, Carbamoyl Phosphate, Citrulline, Aspartate, Argininosuccinate, Fumarate, Arginine, Urea)

⭐ Exam Favorite: The rate-limiting enzyme is Carbamoyl Phosphate Synthetase I (CPS I). Its activity is critically dependent on N-acetylglutamate (NAGS), an allosteric activator.

Overall Stoichiometry:
- $2 NH_4^+ + HCO_3^- + 3 ATP + Aspartate ightarrow Urea + 2 ADP + 2 P_i + AMP + PP_i + Fumarate$

Cycle's Control Panel - Speed & Snafus

Primary Regulator: N-acetylglutamate (NAG) is the essential allosteric activator for the rate-limiting enzyme, Carbamoyl Phosphate Synthetase I (CPS I).
Feed-Forward Activation:
- ↑ Arginine levels activate N-acetylglutamate synthase (NAGS).
- NAGS synthesizes NAG from glutamate and acetyl-CoA.
Substrate Availability: High protein diet or prolonged fasting ↑ amino acid breakdown, boosting cycle flux.

⭐ Ornithine Transcarbamylase (OTC) Deficiency: The most common urea cycle disorder (X-linked). Excess carbamoyl phosphate is shunted to pyrimidine synthesis, causing ↑ orotic acid in blood & urine.

Urea Cycle with N-acetylglutamate Activation

High‑Yield Points - ⚡ Biggest Takeaways

The urea cycle is the body's primary mechanism for converting highly toxic ammonia (NH₃) into urea for excretion.

This crucial pathway occurs exclusively in the liver, with steps in both the mitochondria and cytosol.

The two nitrogen atoms in urea are sourced from free ammonia and aspartate.

The rate-limiting step is catalyzed by Carbamoyl Phosphate Synthetase I (CPS I).

N-acetylglutamate (NAG) is the essential allosteric activator of CPS I.

Inherited defects in the cycle lead to hyperammonemia, causing life-threatening encephalopathy.

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Overview and purpose of urea cycle