Urea cycle Practice Questions

Question 1

A 2-day-old male infant is brought to the emergency department by ambulance after his parents noticed that he was convulsing and unresponsive. He was born at home and appeared well initially; however, within 24 hours he became increasingly irritable and lethargic. Furthermore, he stopped feeding and began to experience worsening tachypnea. This continued for about 6 hours, at which point his parents noticed the convulsions and called for an ambulance. Laboratories are obtained with the following results:

Orotic acid: 9.2 mmol/mol creatinine (normal: 1.4-5.3 mmol/mol creatinine)
Ammonia: 135 µmol/L (normal: < 50 µmol/L)
Citrulline: 2 µmol/L (normal: 10-45 µmol/L)

Which of the following treatments would most likely be beneficial to this patient?

Accepted Answer

Benzoate administration

Answer

Galactose avoidance

Answer

Aspartame avoidance

Answer

Uridine administration

Answer

Fructose avoidance

Question 2

A 36-year-old man is brought to the emergency department by his girlfriend because of increasing confusion for the past 6 hours. He drinks large amounts of alcohol daily and occasionally uses illicit drugs. He is lethargic and oriented only to person. Physical examination shows jaundice, hepatomegaly, and scattered petechiae over the trunk and back. Neurologic examination shows normal, reactive pupils and a flapping tremor when the wrists are extended. A drug with which of the following mechanism of action would be most appropriate for this patient's condition?

Accepted Answer

Excretion of NH4

Answer

Production of NH3

Answer

Activation of GABA receptors

Answer

Excretion of free iron

Answer

Inhibition of D2 receptors

Question 3

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?

Accepted Answer

Carbamoyl phosphate synthetase I

Answer

Argininosuccinate lyase

Answer

Argininosuccinate synthetase

Answer

Ornithine transcarbamylase

Answer

Arginase

Question 4

A 24-year-old man is running a marathon. Upon reaching the finish line, his serum lactate levels were measured and were significantly increased as compared to his baseline. Which of the following pathways converts the lactate produced by muscles into glucose and transports it back to the muscles?

Accepted Answer

Cori cycle

Answer

Citric acid cycle

Answer

Glycolysis

Answer

Glycogenesis

Answer

Pentose phosphate pathway

Question 5

A 65-year old man comes to the emergency department because of altered mental status for 1 day. He has had headaches, severe nausea, vomiting, and diarrhea for 2 days. He has a history of hypertension, insomnia, and bipolar disorder. His medications include lisinopril, fluoxetine, atorvastatin, lithium, olanzapine, and alprazolam. His temperature is 37.2 °C (99.0 °F), pulse is 90/min, respirations are 22/min, and blood pressure is 102/68 mm Hg. He is somnolent and confused. His mucous membranes are dry. Neurological examination shows dysarthria, decreased muscle strength throughout, and a coarse tremor of the hands bilaterally. The remainder of the examination shows no abnormalities. In addition to IV hydration and electrolyte supplementation, which of the following is the next best step in management?

Accepted Answer

Hemodialysis

Answer

Bowel irrigation

Answer

Intravenous diazepam

Answer

Oral cyproheptadine

Answer

Intravenous dantrolene

Question 6

A 2-month-old boy is brought to his pediatrician’s office to be evaluated for new onset seizures and poor weight gain. The patient’s father says he is unable to track with his eyes and is unresponsive to verbal stimuli. The patient is hypotonic on physical exam. Further studies show elevated serum lactate levels and elevated levels of alanine and pyruvate. Family history reveals that several distant family members suffered from neurological diseases and died of unknown causes at a young age. Which of the following amino acids should be increased in this patient’s diet?

Accepted Answer

Arginine

Answer

Isoleucine

Answer

Valine

Answer

Leucine

Answer

Methionine

Question 7

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?

Accepted Answer

Carbamoyl phosphate synthetase I

Answer

Ornithine translocase

Answer

Argininosuccinase

Answer

Argininosuccinate synthetase

Answer

Arginase

Question 8

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?

Accepted Answer

Ornithine transcarbamylase

Answer

Branched-chain alpha-ketoacid dehydrogenase

Answer

Propionyl-CoA carboxylase

Answer

Homogentisic acid dioxygenase

Answer

Cystathionine beta-synthase

Question 9

A 3-week old boy is brought to the physician for the evaluation of lethargy, recurrent vomiting, and poor weight gain since birth. Physical examination shows decreased skin turgor and a bulging frontal fontanelle. Serum studies show an ammonia concentration of 170 μmol/L (N < 30) and low serum citrulline levels. The oral intake of which of the following nutrients should be restricted in this patient?

Accepted Answer

Protein

Answer

Gluten

Answer

Lactose

Answer

Fructose

Answer

Vitamin A

Question 10

A 4-year-old girl is brought to the physician by her mother because of fatigue and generalized weakness for 4 months. Examination shows decreased muscle tone. Her fasting serum glucose concentration is 41 mg/dL. The physician suspects a defect in one of the enzymes involved in the carnitine shuttle. Increased serum concentration of which of the following should most raise suspicion of a different diagnosis?

Accepted Answer

Ammonia

Answer

Creatine kinase

Answer

Alanine aminotransferase

Answer

Uric acid

Answer

β-hydroxybutyrate

Urea cycle

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Overview and purpose of urea cycle

Urea cycle reactions and enzymes

Nitrogen sources and transport forms

Compartmentalization between mitochondria and cytosol

Energy requirements of urea cycle

Regulation of urea cycle

Integration with TCA cycle

Hyperammonemia causes and effects

Urea cycle disorders

Alternative nitrogen excretion pathways

Diagnostic markers of urea cycle function

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About Urea cycle for USMLE

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