Amino acid metabolism and disorders US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Amino acid metabolism and disorders. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Amino acid metabolism and disorders US Medical PG 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?
- A. Phenylalanine
- B. Homogentisic acid
- C. Isoleucine (Correct Answer)
- D. Homocysteine
- E. Phytanic acid
Amino acid metabolism and disorders Explanation: ***Isoleucine***
- The combination of **vomiting**, poor feeding, lethargy, **increased muscle tone**, and a **sweet-smelling urine** (often described as maple syrup odor) in a neonate strongly points to **Maple Syrup Urine Disease (MSUD)**.
- MSUD is caused by a defect in the **branched-chain alpha-keto acid dehydrogenase complex**, leading to the accumulation of branched-chain amino acids (leucine, isoleucine, and valine) and their corresponding alpha-keto acids.
*Phenylalanine*
- Accumulation of **phenylalanine** is characteristic of **Phenylketonuria (PKU)**, which typically presents with intellectual disability, seizures, and an eczematous rash if untreated, but not a maple syrup odor in urine or acute neonatal crisis.
- The urine odor in PKU is often described as **mousy** or musty, distinctly different from a sweet or maple syrup odor.
*Homogentisic acid*
- Accumulation of **homogentisic acid** is seen in **Alkaptonuria**, an inborn error of metabolism that primarily causes dark urine upon standing (due to oxidation of homogentisic acid), ochronosis (blue-black pigmentation of cartilage and connective tissue later in life), and arthritis.
- It does not present with acute neonatal symptoms such as vomiting, lethargy, or a sweet urine odor.
*Homocysteine*
- Elevated levels of **homocysteine** are found in **Homocystinuria**, which can lead to intellectual disability, developmental delay, dislocation of the ocular lens (ectopia lentis), Marfanoid habitus, and thromboembolic events.
- It does not typically present in the neonatal period with a sweet urine odor or acute neurological symptoms like increased muscle tone.
*Phytanic acid*
- Accumulation of **phytanic acid** is characteristic of **Refsum disease**, a rare peroxisomal disorder that causes progressive neurological symptoms such as retinitis pigmentosa, peripheral neuropathy, ataxia, and deafness in childhood or adulthood.
- It does not present in the neonatal period with the described acute symptoms or unique urine odor.
Amino acid metabolism and disorders US Medical PG 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?
- A. Glutamate
- B. α-ketoglutarate
- C. Alanine (Correct Answer)
- D. Arginine
- E. Pyruvate
Amino acid metabolism and disorders Explanation: ***Alanine***
- During prolonged fasting, **muscle proteins are catabolized** to provide amino acids for gluconeogenesis in the liver.
- **Alanine** is the primary amino acid released from muscle into the bloodstream to transport nitrogen to the liver through the **glucose-alanine cycle (Cahill cycle)**.
- In this cycle, pyruvate in muscle accepts an amino group from glutamate to form alanine, which is then transported to the liver, where it is deaminated back to pyruvate (for gluconeogenesis) and ammonia (for the urea cycle).
- **Glutamine** also serves as an important nitrogen transporter, particularly to the kidneys and intestines.
*Glutamate*
- **Glutamate** is an important amino acid in nitrogen metabolism within tissues, but it is not the primary form in which nitrogen is transported from muscle to the liver in significant quantities.
- While glutamate participates in transamination reactions within muscle, its efflux from muscle into the blood is less prominent than alanine for inter-organ nitrogen transport.
*α-ketoglutarate*
- **α-ketoglutarate** is a key intermediate in the **Krebs cycle** and accepts an amino group to form glutamate.
- It is an alpha-keto acid, not an amino acid, and therefore does not directly transport nitrogen in the form of an amino group to the liver via the bloodstream.
*Arginine*
- **Arginine** is primarily involved in the **urea cycle** within the liver, where it helps in the detoxification of ammonia, but it is not a major transporter of nitrogen from peripheral tissues to the liver for gluconeogenesis.
- Its role is mainly within the liver for urea synthesis, not for inter-organ nitrogen transport in this context.
*Pyruvate*
- **Pyruvate** is a keto acid that can be converted to alanine via transamination.
- While pyruvate is a precursor to alanine and a substrate for gluconeogenesis, it transports carbon skeletons and not nitrogen itself; **alanine is the actual nitrogen carrier** in this cycle.
Amino acid metabolism and disorders US Medical PG 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?
- A. Isoleucine
- B. Leucine
- C. Tyrosine (Correct Answer)
- D. Phenylalanine
- E. Histidine
Amino acid metabolism and disorders Explanation: ***Tyrosine***
- The patient's presentation with **seizures**, **developmental delay** (less responsive), **light skin tone**, and a **musty body odor** is highly suggestive of **phenylketonuria (PKU)**.
- In PKU, there is a deficiency in the enzyme **phenylalanine hydroxylase**, which converts **phenylalanine** to **tyrosine**. Therefore, **tyrosine** becomes an **essential amino acid** and must be supplemented in the diet.
*Isoleucine*
- **Isoleucine** is a **branched-chain amino acid** that is typically restricted, along with leucine and valine, in conditions like **maple syrup urine disease (MSUD)**, not PKU.
- Supplementation of isoleucine would be detrimental in MSUD and is not indicated for PKU.
*Leucine*
- Similar to isoleucine, **leucine** is a **branched-chain amino acid** whose metabolism is impaired in **MSUD**, not PKU.
- Supplementing leucine is not beneficial for PKU and would be harmful in MSUD.
*Phenylalanine*
- **Phenylalanine** is the amino acid that accumulates to toxic levels in **PKU** due to the enzyme deficiency.
- Therefore, phenylalanine must be **strictly restricted** in the patient's diet, not supplemented.
*Histidine*
- **Histidine** is an essential amino acid but is not directly involved in the metabolic pathway affected by PKU.
- There is no indication for histidine supplementation in the management of PKU.
Amino acid metabolism and disorders US Medical PG 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?
- A. Leucine
- B. Homocysteine
- C. Phenylalanine
- D. Homogentisic acid
- E. DOPA (Correct Answer)
Amino acid metabolism and disorders Explanation: ***DOPA***
- The symptoms of **white hair**, **pale skin**, and **translucent irises**, along with a family history of **blindness**, are classic signs of **oculocutaneous albinism**.
- **Oculocutaneous albinism** results from a defect in **tyrosinase**, an enzyme responsible for converting **DOPA** (3,4-dihydroxyphenylalanine) into **melanin**.
*Leucine*
- Defects in **leucine** metabolism are associated with **Maple Syrup Urine Disease**, characterized by sweet-smelling urine and neurological complications, not albinism.
- This condition involves impaired **branched-chain alpha-keto acid dehydrogenase** complex activity, leading to an accumulation of branched-chain amino acids.
*Homocysteine*
- Problems with **homocysteine** metabolism are seen in conditions like **homocystinuria**, which can cause developmental delay, skeletal abnormalities, and vascular issues, but not hypopigmentation.
- This is often due to deficiencies in **cystathionine beta-synthase** or other enzymes involved in folate and vitamin B12 metabolism.
*Phenylalanine*
- A defect in **phenylalanine** metabolism, specifically **phenylalanine hydroxylase**, leads to **phenylketonuria (PKU)**, which causes intellectual disability, seizures, and a musty odor, but not the specific ocular and cutaneous features described for albinism.
- While PKU can cause milder hypopigmentation due to reduced tyrosine availability, the described translucent irises and severe white hair are more indicative of albinism.
*Homogentisic acid*
- A defect in **homogentisic acid** metabolism causes **alkaptonuria**, characterized by dark urine upon standing, blue-black discoloration of cartilage (ochronosis), and early-onset arthritis, which are not present in this case.
- This condition results from a deficiency of **homogentisate 1,2-dioxygenase**.
Amino acid metabolism and disorders US Medical PG 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?
- A. N-acetylglutamate
- B. Homocysteine
- C. Phenylalanine
- D. Valine
- E. Aspartate (Correct Answer)
Amino acid metabolism and disorders Explanation: ***Aspartate***
- The question describes a case of **carbamoyl phosphate synthetase I (CPS I)** deficiency, which leads to **hyperammonemia** due to impaired urea cycle function.
- The urea cycle is responsible for excreting **amino groups** as urea; one of the key molecules directly incorporated into the urea molecule is **aspartate**, which donates an amino group to form **argininosuccinate**.
*N-acetylglutamate*
- **N-acetylglutamate** is an essential activator of **carbamoyl phosphate synthetase I (CPS I)**, the enzyme deficient in this patient.
- While crucial for the urea cycle's regulation, it is an **allosteric activator** rather than a direct substrate for the synthesis of molecules needed for amino group excretion.
*Homocysteine*
- **Homocysteine** is an intermediate in **methionine metabolism** and is primarily associated with cardiovascular disease and neurological issues when elevated.
- It plays no direct role as a substrate in the urea cycle for the excretion of amino groups.
*Phenylalanine*
- **Phenylalanine** is an **essential amino acid** that is a precursor to tyrosine and neurotransmitters.
- Its metabolism is separate from the urea cycle, and it is not a direct substrate for ammonia excretion in this pathway.
*Valine*
- **Valine** is a **branched-chain amino acid (BCAA)** primarily catabolized in muscles and used for energy.
- It is not a direct substrate in the urea cycle, which processes nitrogen from various amino acids into urea for excretion.
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