Deck 30: Nitrogen: Amino Acid Metabolism

Question 1

You are examining a pediatric patient brought to you by his parents because they are disturbed by his progressive mood changes and apparent hightened anxiety. Physical examination shows pellegra-like skin eruptions and signs of cerebellar ataxia that include unsteady gait and uncoordinated eye movements. Serum and urinalysis show significant aminoacidemia and aciduria. These signs and symptoms are most likely due to which of the following disorders?

A) alkaptonuria
B) glycine decarboxylase deficiency (GCC deficiency)
C) Hartnup disorder
D) maple syrup urine disease
E) PKU

Answer 1

Hartnup disorder

Question 2

You are examining an infant brought to your office due to the onset of seizures. The parents report that their baby has also exhibited a progressive lack of energy, feeding difficulties, hypotonia, abnormal jerking movements, and difficulty breathing. These symptoms are suggestive of a defect in the processesof neurotransmission and thus, are most likely due to which of the following disorders?

A) alkaptonuria
B) glycine decarboxylase deficiency (GCC deficiency)
C) Hartnup disorder
D) maple syrup urine disease
E) PKU

Answer 2

glycine decarboxylase deficiency (GCC deficiency)

Question 3

A deficiency in one of the critical enzymes involved in the catabolism of the branched-chain amino acids results in neonatal vomiting, lethargy, and poor suckling behavior. Progressive neurological signs include decerebrate posturing. Which one of the following disorders corresponds to this defect?

A) alkaptonuria
B) homocystinuria
C) isovaleric acidemia
D) maple syrup urine disease
E) phenylketonuria (PKU)

Answer 3

maple syrup urine disease

Question 4

The parents of a 3-month-old infant rush their child to the ER following a seizure. Blood work indicates the child is hyperphenylalaninemic with serum phenylalanine measured at 650 μM. The attending physician suspects the infant has PKU and places the infant on a low-phenylalanine diet. Upon follow-up 3-days later, the level of serum phenylalanine is found to be reduced to 250 μM. However, while at home, the infant developed progressive movement disorders, difficulty swallowing, seizures, and elevated body temperature. A defect in which of the following enzymes would best explain the hyperphenylalaninemia and the lack of positive outcome on the low phenylalanine treatment?

A) dihydropteridine reductase
B) nitric oxide synthase
C) phenylalanine hydroxylase
D) tryptophan hydroxylase
E) tyrosine hydroxylase

Question 5

A 10-year-old girl is brought to her pediatrician for a routine examination. The child is thin and has long fingers and toes, dislocated lenses, and mental retardation. Enzyme studies show an inability to convert homocysteine to cystathionine by cystathionine b-synthase. Newborn screening would have been able to identify the metabolic disorder present in this girl through the measurement of an increased serum concentration of which of the following amino acids?

A) leucine
B) methionine
C) phenylalanine
D) serine
E) tyrosine

Question 6

A 2-year-old boy is hospitalized with chronic diarrhea. He exhibits considerable weight loss such that he is only about 70% of expected weight for age. His muscles show significant atrophy, but he still has normal subcutaneous adipose tissue. His abdomen is distended, and he has some areas of scaly peeling skin. These findings are most consistent with which of the following conditions?

A) hepatic cirrhosis
B) infectious hepatitis
C) kwashiorkor
D) marasmus
E) nephrosis

Question 7

A mother has brought her 3-month-old baby to the pediatrician and indicates that the infant is lethargic and has poor suckling and seems uninterested in eating. In addition, the mother notes that the baby's diapers often smell like burnt sugar. This infant likely has a defect in which of the following enzymes?

A) branched-chain α-keto acid dehydrogenase
B) cystathionine synthase
C) glycine cleavage complex (GCC)
D) homogentisate oxidase
E) phenylalanine hydroxylase

Question 8

Infants exhibiting profound metabolic ketoacidosis, muscular hypotonia, developmental retardation, and who have very large accumulations of methylmalonic acid in their blood and urine suffer from a disorder known as methylmalonic acidemia. This disorder results from a defect in which of the following enzymes?

A) homogentisic acid oxidase
B) a-keto acid dehydrogenase
C) methylmalonyl-CoA mutase
D) phenylalanine hydroxylase
E) tyrosine aminotransferase

Question 9

A 5-month-old infant who has been experiencing lethargy, recurrent vomiting, respiratory distress, and muscular hypotonia is brought to the emergency room near-comatose. Lab results indicate severe metabolic ketoacidosis associated with an extreme accumulation of methylmalonic acid in the blood and urine. Negative findings for pernicious anemia or other hematologic or neurologic symptoms of cobalamin deficiency indicate that the infant is suffering a defect in which of the following enzymes?

A) malonyl-CoA decarboxylase
B) methionine synthase
C) methylmalonyl-CoA mutase
D) methylmalonyl-CoA racemase
E) propionyl-CoA carboxylase

Question 10

A 3-month-old infant experienced several severe seizures and was brought to the doctor for testing. The test results showed hyperphenylalaninemia and the doctor diagnosed PKU as the likely disease causing the seizures. The infant was placed on a low-phenylalanine diet but developed progressive neurological symptoms and died 6 months later. A defect in which of the following enzymes would best explain the observations and outcomes in this case

A) dihydropteridine reductase
B) nitric oxide synthase
C) phenylalanine hydroxylase
D) tryptophan hydroxylase
E) tyrosine hydroxylase

Question 11

The action of many vasodilators within the vasculature involves the influx of calcium ions. A defect in which of the following proteins would most likely result in reduced effectiveness of these types of drugs?

A) adenylate cyclase
B) calmodulin
C) eNOS (NOS-3)
D) cGMP phosphodiesterase
E) guanylate cyclase

Question 12

A 9-year-old boy is stung on the arm by a wasp and very rapidly develops redness and swelling at the site of the sting. Which of the following substances is most responsible for these early changes?

A) bradykinin
B) complement 3a
C) histamine
D) leukotriene B₄
E) thromboxane A₂

Question 13

A male infant is diagnosed with phenylketonuria (PKU) on newborn screening. Which of the following amino acids must be present in his diet to prevent a negative nitrogen balance?

A) alanine
B) cysteine
C) glutamine
D) serine
E) tyrosine

Question 14

A 5-year-old boy with severe mental retardation is brought to the physician because of a 4-week history of progressively frequent generalized tonic-clonic seizures. There is no available record of newborn screening. He has blond hair and blue eyes. Physical examination shows patches of itchy, dry, thickened skin on the face, neck, and inner creases of the elbows and knees. Serum studies for amino acid concentrations are pending. Urine studies show increased concentrations of phenylpyruvic and phenyllactic acids. The activity of which of the following enzymes is most likely defective in this patient?

A) fumarylacetoacetate hydrolase
B) p-hydroxyphenylpyruvate dehydrogenase
C) phenylalanine hydroxylase
D) phenylalanine transaminase
E) tyrosine transaminase

Question 15

A 5-year-old girl with moderate mental retardation is brought to the physician by her mother who is concerned that her daughter has been having difficulty with her vision for weeks. Physical examination shows a fair complexion and malar erythema. Ophthalmologic examination shows dislocated lenses bilaterally. She has long, slender hands and fingers. Blood tests reveal her cystathionine concentration is below normal. This patient is most likely to have an increased urine concentration of which of the following amino acids?

A) alanine
B) arginine
C) cysteine
D) leucine
E) methionine

Question 16

A 22-year-old man comes to the physician 1 week after an episode of severe right flank pain that resolved with the passing of a renal calculus. His 25-year-old brother had similar symptoms 1 year ago. This patient most likely has an increased urine concentration of which of the following amino acids?

A) argininosuccinate and citrulline
B) glycine and alanine
C) lysine and cystine
D) methionine and homocystine
E) phenylalanine and tyrosine

Question 17

In using dietary restriction of phenylalanine to manage a 2-month-old girl who has phenylketonuria, which of the following should be added to the diet?

A) aspartate
B) cytosine
C) serine
D) tryptophan
E) tyrosine

Question 18

The primary source of urinary ammonia (NH₃ or NH₄⁺) is which of the following?

A) catabolism of glutamine within renal cells
B) catabolism of polyamines within renal cells
C) filtered ammonia
D) filtered glutamine
E) synthesis of alanine within renal cells

Question 19

Which of the following is the amino acid in proteins that is most sensitive to air oxidation?

A) arginine
B) cysteine
C) histidine
D) methionine
E) serine

Question 20

The g-amide of which of the following amino acids is a source of nitrogen in many biosynthetic reaction?

A) aspartate
B) γ-aminobutyric acid (GABA)
C) glutamine
D) histidine
E) lysine

Question 21

A 6-day-old newborn, who is being breast-fed, is brought to the ER because he is unresponsive. He has been vomiting and is lethargic for the past 2 days. Examination shows an obtunded infant with hypertonia and muscle rigidity. A peculiar odor of burnt sugar is noted in his diaper. The plasma concentration of which of the following amino acids is most likely to be increased in this patient?

A) cysteine
B) homocysteine
C) leucine
D) phenylalanine
E) tyrosine

Question 22

An 11-month-old boy is brought to the physician because of lethargy and irritability for the past 3 days, since a diagnosis of a middle ear infection. The physician prescribed an antibiotic to treat the ear infection. Laboratory studies show metabolic acidosis and an increased plasma valine concentration. Which of the following amino acid concentrations are most likely to be increased in this patient's blood if he has maple syrup urine disease?

A) arginine and lysine
B) asparagine and glutamine
C) homocysteine and cysteine
D) leucine and isoleucine
E) phenylalanine and tyrosine

Question 23

A 1-month-old male newborn is brought to the physician for a routine examination. His parents both have olive-colored skin, dark hair, and dark eyes. Physical examination shows hypopigmentation of the skin, light blonde hair, and translucent irises. The inherited disorder that causes this phenotypic expression is most likely due to a defect in the metabolism of which of the following?

A) epinephrine
B) phenylalanine
C) serotonin
D) tryptophan
E) tyrosine

Question 24

A 55-year-old man who has been suffering from chronic liver disease is diagnosed with elevated serum ammonia concentration. Failure of the liver to synthesize which of the following compounds most likely contributes to the increased serum ammonia concentration in this patient?

A) bilirubin
B) glutamine
C) heme
D) inosine
E) tyrosine

Question 25

A 5-day-old female newborn, who is breast-fed, is brought to the ER because of a 2-day history of poor feeding and vomiting. Physical examination shows lethargy, dehydration, hypertonicity, and rigidity of the extremities. There is a sweet odor of maple syrup in the newborn's diaper. Laboratory studies show metabolic acidosis, ketosis, and hypoglycemia. Following treatment of this patient's acute condition, long-term therapy is begun. This therapy is most likely to restrict which of the following substances?

A) galactose, fructose, and lactose
B) leucine, isoleucine, and valine
C) methionine, homocysteine, and cystathionine
D) ornithine, citrulline, and arginine
E) phenylalanine and tyrosine

Question 26

A 23-year-old woman is brought to the ER because of a 1-hour history of severe pain and coldness of her left leg. History taking indicates that she dropped out of high school in the 10th grade due to difficulty with comprehension. Intelligence testing demonstrated her IQ to be 80 at that time. Her parents and 2 older siblings have normal intelligence and have no history of these symptoms. Ophthalmologic examination shows a partially dislocated lens in the right eye. Physical examination shows mottling and loss of pulses in the left lower extremity and foot. Arteriography of the left lower extremity shows thrombosis of the femoral artery. This patient most likely has a metabolic disorder involving which of the following amino acid?

A) glycine
B) homocysteine
C) leucine
D) phenylalanine
E) tyrosine

Question 27

A 2-year-old girl is brought to the physician because of failure to thrive. She was diagnosed with phenylketonuria on a routine neonatal screening. Her weight and height are currently well below normal for her age. She is following a low-phenylalanine diet. Her plasma phenylalanine concentration is within the reference range. The most appropriate next step is measurement of the concentration of which of the following in the plasma?

A) arginine
B) histidine
C) lysine
D) tryptophan
E) tyrosine

Question 28

A 2-week-old girl of north European descent is found to have a blood methionine concentration of 1500 μM (normal <50) on newborn screening. Physical examination shows no abnormalities. Which of the following enzymes is most likely to be deficient in this patient's liver?

A) branched-chain a-keto acid dehydrogenase
B) cystathionine b-synthase
C) methionine synthase
D) phenylalanine hydroxylase
E) tyrosine aminotransferase

Question 29

During periods of drought, the leaves of desert trees often become a dietary staple for certain populations. Which of the following amino acids, present in sufficient quantity in the leaves, satisfies an essential nutrient requirement?

A) alanine
B) aspartate
C) glutamate
D) methionine
E) tyrosine

Question 30

A 7-month-old boy has failed to gain weight normally. His diet consists of a diluted milk formula. His weight and length are below normal for his age, and he appears emaciated. His temperature is below normal, and his pulse is slow. Which of the following is the most likely cause of his condition?

A) chronic renal failure
B) deficiency of growth hormone
C) hypothyroidism
D) kwashiorkor
E) marasmus