Chapter·BiochemistryUrea cycle

Regulation of urea cycleDownloads

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1

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?

AGluten

BLactose

CFructose

DProtein

EVitamin A

2

A 49-year-old woman presents to the office for a follow-up visit. She was diagnosed with cirrhosis of the liver 1 year ago and is currently receiving symptomatic treatment along with complete abstinence from alcohol. She does not have any complaints. She has a 4-year history of gout, which has been asymptomatic during treatment with medication. She is currently prescribed spironolactone and probenecid. She follows a diet rich in protein. The physical examination reveals mild ascites with no palpable abdominal organs. A complete blood count is within normal limits, while a basic metabolic panel with renal function shows the following: Sodium 141 mEq/L Potassium 5.1 mEq/L Chloride 101 mEq/L Bicarbonate 22 mEq/L Albumin 3.4 mg/dL Urea nitrogen 4 mg/dL Creatinine 1.2 mg/dL Uric Acid 6.8 mg/dL Calcium 8.9 mg/dL Glucose 111 mg/dL Which of the following explains the blood urea nitrogen result?

ASpironolactone

BUse of probenecid

CIncrease in dietary protein

DThe urea value is within normal limits

ELiver disease

3

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?

ABranched-chain alpha-ketoacid dehydrogenase

BPropionyl-CoA carboxylase

CHomogentisic acid dioxygenase

DOrnithine transcarbamylase

ECystathionine beta-synthase

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