Amino acid metabolism and disorders Practice Questions

Question 1

An 8-day-old boy is brought to the physician by his mother because of vomiting and poor feeding. The pregnancy was uncomplicated, and he was born at full term. He appears pale and lethargic. Physical examination shows diffusely increased muscle tone. His urine is noted to have a sweet odor. This patient's symptoms are most likely caused by the accumulation of which of the following?

Accepted Answer

Isoleucine

Answer

Phenylalanine

Answer

Homogentisic acid

Answer

Homocysteine

Answer

Phytanic acid

Question 2

An investigator is studying severely ill patients who experience hypoglycemia and ketonuria during times of fasting. The investigator determines that during these episodes, amino acids liberated from muscle proteins are metabolized to serve as substrates for gluconeogenesis. Nitrogen from this process is transported to the liver primarily in the form of which of the following molecules?

Accepted Answer

Alanine

Answer

Glutamate

Answer

α-ketoglutarate

Answer

Arginine

Answer

Pyruvate

Question 3

A 6-month-old boy is brought to a pediatrician by his parents for his first visit after they adopt him from a European country. His parents are concerned about the boy’s short episodes of shaking of his arms and legs; they believe it might be epilepsy. They also note that the child is less responsive than other children of his age. The family is unable to provide any vaccination, birth, or family history. His pulse is 130/min, respiratory rate is 28/min, and blood pressure is 90/50 mm Hg. The boy has a light skin tone and emits a noticeable musty body odor. Which of the following should be supplemented in this patient’s diet?

Accepted Answer

Tyrosine

Answer

Isoleucine

Answer

Leucine

Answer

Phenylalanine

Answer

Histidine

Question 4

A 30-year-old African American G1P0 mother gives birth to a male infant at 33 weeks' gestation. The mother had no prenatal care and took no prenatal vitamins. The child’s postnatal period was complicated by neonatal sepsis due to group B Streptococcus. He required a two week stay in the neonatal intensive care unit to receive antibiotics, cardiopulmonary support, and intravenous nutrition. He eventually recovered and was discharged. At a normal follow-up visit to the pediatrician’s office one month later, the mother asks about the child’s skin color and hair color. On examination, the child has white hair and diffusely pale skin. The child’s irises appear translucent. Further questioning of the mother reveals that there is a distant family history of blindness. This child most likely has a defect in an enzyme involved in the metabolism of which of the following molecules?

Accepted Answer

DOPA

Answer

Leucine

Answer

Homocysteine

Answer

Phenylalanine

Answer

Homogentisic acid

Question 5

A 4-day-old male newborn delivered at 39 weeks' gestation is evaluated because of poor feeding, recurrent vomiting, and lethargy. Physical examination shows tachypnea with subcostal retractions. An enzyme assay performed on a liver biopsy specimen shows decreased activity of carbamoyl phosphate synthetase I. This enzyme plays an important role in the breakdown and excretion of amino groups that result from protein digestion. Which of the following is an immediate substrate for the synthesis of the molecule needed for the excretion of amino groups?

Accepted Answer

Aspartate

Answer

N-acetylglutamate

Answer

Homocysteine

Answer

Phenylalanine

Answer

Valine

Question 6

A 29-year-old female presents to the family physician concerned over the blue-black discoloration of her sclera and skin. She notes that at the end of her day, there are black stains in her underwear. The incomplete breakdown of which of the following amino acids is responsible for this presentation?

Accepted Answer

Tyrosine

Answer

Leucine

Answer

Valine

Answer

Methionine

Answer

Isoleucine

Question 7

You are counseling a mother whose newborn has just screened positive for a deficit of phenylalanine hydroxylase enzyme. You inform her that her child will require dietary supplementation of which of the following?

Accepted Answer

Tyrosine

Answer

Aspartame

Answer

Niacin

Answer

Homogentisic Acid

Answer

Leucine

Question 8

A 9-month-old infant presents to your office for a check-up. Exam reveals developmental delay, microcephaly, and a mousy odor to his breath. You should be concerned that the infant may have which of the following?

Accepted Answer

Deficit of phenylalanine hydroxylase activity

Answer

Excess tetrahydrobiopterin cofactor

Answer

Deficit of porphobilinogen deaminase activity

Answer

Deficit of tyrosine hydroxylase activity

Answer

Excess phenylalanine hydroxylase activity

Question 9

A 6-day-old female newborn is brought to the physician because of yellowish discoloration of her eyes and body, vomiting, and poor feeding for 3 days. She has had diarrhea for the past 2 days. She was born at 38 weeks' gestation and the antenatal period was uncomplicated. She appears lethargic. Vital signs are within normal limits. Examination shows jaundice of the skin and conjunctivae. Bilateral cataracts are present. The abdomen is soft and nontender. The liver is palpated 4-cm below the right costal margin; there is no splenomegaly. Muscle tone is decreased in all extremities. Serum glucose concentration is 37 mg/dL. Which of the following is the most appropriate recommendation to prevent long-term complications of this illness?

Accepted Answer

Stop milk feeds

Answer

Phototherapy

Answer

Thiamine therapy

Answer

Levothyroxine therapy

Answer

Frequent glucose feeds

Question 10

A medical student is studying digestive enzymes at the brush border of the duodenum. He isolates and inactivates an enzyme in the brush border that has a high affinity for the pancreatic proenzyme trypsinogen. When the enzyme is inactivated, trypsinogen is no longer converted to its active form. Which of the following is the most likely underlying mechanism of this enzyme?

Accepted Answer

Cleavage of a propeptide from an N-terminus

Answer

Attachment of a carbohydrate to a side chain

Answer

Phosphorylation of an amino acid side chain

Answer

Carboxylation of a glutamate residue

Answer

Conjugation of ubiquitin to lysine residue

Amino acid metabolism and disorders

Study Resources

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Essential vs. non-essential amino acids

Amino acid degradation pathways

Glucogenic and ketogenic amino acids

Aromatic amino acid metabolism

Branched-chain amino acid metabolism

Sulfur-containing amino acid metabolism

One-carbon metabolism

Amino acid transport disorders

Disorders of aromatic amino acids (PKU, alkaptonuria)

Disorders of branched-chain amino acids (MSUD)

Urea cycle disorders

Homocystinuria and methionine metabolism disorders

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