Essential vs. non-essential amino acids US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Essential vs. non-essential amino acids. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Essential vs. non-essential amino acids US Medical PG Question 1: 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
Essential vs. non-essential amino acids 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.
Essential vs. non-essential amino acids US Medical PG Question 2: 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?
- A. Aspartame
- B. Niacin
- C. Homogentisic Acid
- D. Tyrosine (Correct Answer)
- E. Leucine
Essential vs. non-essential amino acids Explanation: ***Tyrosine***
- A deficit of **phenylalanine hydroxylase** prevents the conversion of phenylalanine to tyrosine, making **tyrosine** an essential amino acid that must be supplemented.
- Dietary restriction of **phenylalanine** is also crucial to prevent the accumulation of toxic byproducts that can cause severe neurological damage.
*Aspartame*
- **Aspartame** is an artificial sweetener that contains **phenylalanine**, which would be harmful for a child with phenylalanine hydroxylase deficiency.
- Consumption of aspartame would increase the body's phenylalanine load, exacerbating the metabolic disorder.
*Niacin*
- **Niacin** (vitamin B3) is a vitamin and its supplementation is not related to the phenylalanine hydroxylase pathway or its deficiency.
- Deficiency of niacin is associated with **pellagra**, characterized by dermatitis, diarrhea, and dementia.
*Homogentisic Acid*
- **Homogentisic acid** is an intermediate in the metabolism of tyrosine, and its accumulation is characteristic of **alkaptonuria**, a different metabolic disorder.
- It is not a therapeutic supplement for phenylalanine hydroxylase deficiency.
*Leucine*
- **Leucine** is a branched-chain amino acid, and its metabolism is unrelated to phenylalanine hydroxylase deficiency.
- Supplemental leucine is not required in this condition and would not address the metabolic defect.
Essential vs. non-essential amino acids US Medical PG Question 3: A student is experimenting with the effects of nitric oxide in the body. He used a variety of amino acid isolates and measured the resulting nitric oxide levels and the physiological effects on the body. The amino acids function as substrates for nitric oxide synthase. After supplement administration, blood vessels dilated, and the systemic blood pressure decreased. Which of the following amino acids was used in this study?
- A. Histidine
- B. Tyrosine
- C. Methionine
- D. Arginine (Correct Answer)
- E. Leucine
Essential vs. non-essential amino acids Explanation: ***Arginine***
- **Arginine** is the direct precursor to **nitric oxide (NO)** through the action of **nitric oxide synthase (NOS)**.
- The production of NO leads to **vasodilation** and a subsequent decrease in **systemic blood pressure**, which aligns with the observed effects.
*Histidine*
- **Histidine** is a precursor for **histamine**, which can cause vasodilation, but it is not the direct substrate for **nitric oxide synthase**.
- Its primary role in NO synthesis is indirect, unlike arginine.
*Tyrosine*
- **Tyrosine** is a precursor for **catecholamines** like dopamine, norepinephrine, and epinephrine, which are involved in various physiological responses but not directly in **nitric oxide synthesis**.
- While catecholamines can affect blood pressure, their synthesis does not involve **nitric oxide synthase (NOS)** as a substrate.
*Methionine*
- **Methionine** is an essential amino acid primarily involved in **methylation reactions** and the synthesis of other sulfur-containing compounds.
- It does not directly serve as a substrate for **nitric oxide synthase** in the production of nitric oxide.
*Leucine*
- **Leucine** is a branched-chain amino acid (BCAA) primarily involved in **protein synthesis** and muscle metabolism.
- It does not serve as a substrate for **nitric oxide synthase** to produce nitric oxide.
Essential vs. non-essential amino acids US Medical PG Question 4: 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?
- A. Isoleucine
- B. Valine
- C. Arginine (Correct Answer)
- D. Leucine
- E. Methionine
Essential vs. non-essential amino acids Explanation: ***Arginine***
- The symptoms of **seizures, developmental delay, hypotonia, and elevated lactate/pyruvate** levels, along with a family history of early deaths, are highly suggestive of a **mitochondrial disorder**, specifically **pyruvate dehydrogenase complex (PDC) deficiency**.
- **Arginine** supplementation is used in certain **mitochondrial disorders** to enhance **nitric oxide (NO) production**, which improves **endothelial function and tissue perfusion**. While the primary treatment for PDC deficiency is a **ketogenic diet** and cofactor supplementation (thiamine, lipoic acid), among the amino acid options listed, arginine has supportive evidence in mitochondrial cytopathies.
- Arginine may help counteract **endothelial dysfunction** and improve oxygen delivery to tissues affected by mitochondrial impairment.
*Isoleucine*
- **Isoleucine** is a **branched-chain amino acid (BCAA)** that is catabolized to produce acetyl-CoA and succinyl-CoA for energy.
- In **PDC deficiency**, the problem is the conversion of **pyruvate to acetyl-CoA**, not BCAA metabolism. Increasing BCAAs is not a therapeutic strategy for this condition.
- Excessive BCAA intake could potentially worsen metabolic imbalances without addressing the underlying enzymatic defect.
*Valine*
- **Valine** is another **branched-chain amino acid (BCAA)** with no specific role in managing PDC deficiency.
- Like isoleucine, valine supplementation does not address the **pyruvate dehydrogenase complex defect** and is not part of standard dietary management.
- BCAAs require careful monitoring in metabolic disorders and are not indicated for mitochondrial disorders like PDC deficiency.
*Leucine*
- **Leucine** is a **branched-chain amino acid (BCAA)** and potent activator of mTOR signaling for protein synthesis.
- Direct leucine supplementation is **not indicated for PDC deficiency** and does not address the metabolic block at the pyruvate dehydrogenase step.
- In disorders like **maple syrup urine disease (MSUD)**, BCAAs including leucine must be **restricted**, not supplemented.
*Methionine*
- **Methionine** is a sulfur-containing amino acid important for **methylation reactions** through S-adenosylmethionine (SAM) synthesis.
- There is **no established role** for methionine supplementation in **pyruvate dehydrogenase complex deficiency** or mitochondrial disorders presenting with lactic acidosis.
- Methionine metabolism is not directly related to the pathophysiology of PDC deficiency.
Essential vs. non-essential amino acids US Medical PG Question 5: 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?
- A. Excess tetrahydrobiopterin cofactor
- B. Deficit of porphobilinogen deaminase activity
- C. Deficit of tyrosine hydroxylase activity
- D. Excess phenylalanine hydroxylase activity
- E. Deficit of phenylalanine hydroxylase activity (Correct Answer)
Essential vs. non-essential amino acids Explanation: ***Deficit of phenylalanine hydroxylase activity***
- The combination of **developmental delay**, **microcephaly**, and a **mousy odor** is characteristic of **phenylketonuria (PKU)**.
- PKU is caused by a deficient **phenylalanine hydroxylase** enzyme, leading to a buildup of phenylalanine and its metabolites, which are toxic to the developing brain.
*Excess tetrahydrobiopterin cofactor*
- This condition (**BH4 excess**) is rare and does not typically present with the classic signs of PKU; rather, it often involves neurological symptoms due to other metabolic imbalances.
- An excess of the BH4 cofactor would theoretically enhance rather than inhibit phenylalanine hydroxylase activity, if the enzyme itself were functional.
*Deficit of porphobilinogen deaminase activity*
- A deficit in **porphobilinogen deaminase** is associated with **Acute Intermittent Porphyria (AIP)**, which presents with acute neurovisceral attacks.
- Symptoms of AIP include severe abdominal pain, psychiatric disturbances, and neurological deficits, but not developmental delay or a mousy odor.
*Deficit of tyrosine hydroxylase activity*
- A deficiency in **tyrosine hydroxylase** affects the synthesis of **dopamine** and other catecholamines, leading to neurological disorders, including **dystonia** and **Parkinsonian symptoms**.
- While it can cause developmental delay, it does not typically present with a mousy odor or microcephaly, and its primary symptoms relate to motor control.
*Excess phenylalanine hydroxylase activity*
- An **excess** of phenylalanine hydroxylase activity would lead to increased breakdown of phenylalanine, preventing its buildup.
- This would not cause the symptoms described; instead, it would likely result in lower-than-normal phenylalanine levels, which is generally not problematic.
Essential vs. non-essential amino acids US Medical PG Question 6: A 3-month-old infant is brought to her pediatrician for a well-child visit. The infant was born to a 22-year-old mother via a spontaneous vaginal delivery at 38 weeks of gestation in her home. She moved to the United States approximately 3 weeks ago from a small village. She reports that her infant had 2 episodes of non-bloody and non-bilious vomiting. The infant's medical history includes eczema and 2 seizure episodes that resolved with benzodiazepines in the emergency department. Physical examination is notable for a musty body odor, eczema, and a fair skin complexion. Which of the following is the best next step in management?
- A. Dietary restriction (Correct Answer)
- B. Abdominal radiography
- C. Dermatology consult
- D. MRI of the brain
- E. Antiepileptic drug
Essential vs. non-essential amino acids Explanation: ***Dietary restriction***
- The infant's symptoms, including **eczema**, **seizures**, **fair skin**, and a distinctive "musty" odor, strongly suggest **phenylketonuria (PKU)**. PKU is an **autosomal recessive metabolic disorder** where the body cannot properly break down **phenylalanine**.
- The primary treatment for PKU is a **lifelong diet low in phenylalanine**. This involves restricting high-protein foods and using special medical formulas to provide adequate nutrition. Early and strict dietary management is crucial to prevent intellectual disability and other neurological complications.
*Abdominal radiography*
- While the infant experienced vomiting, the description of it being **non-bloody** and **non-bilious**, along with the absence of other gastrointestinal symptoms like distension, makes a significant abdominal pathology less likely than **metabolic derangement**.
- Abdominal radiography would be more appropriate for suspected **bowel obstruction** or perforation, for which there are no strong indications in this case.
*Dermatology consult*
- The presence of eczema is noted, but it is one of several symptoms pointing towards a **systemic metabolic disorder** rather than an isolated skin condition.
- Addressing the underlying metabolic cause (PKU) through dietary restriction will likely improve or resolve the eczema, making a consult for symptomatic treatment a secondary concern.
*MRI of the brain*
- The infant has experienced seizures, which often prompt neurological imaging. However, in the context of the other clinical findings (musty odor, fair skin, eczema), the seizures are highly suggestive of **metabolic encephalopathy** due to PKU.
- While an MRI might show changes related to chronic phenylalanine toxicity, the most urgent and effective intervention is to address the metabolic cause through **dietary restriction**.
*Antiepileptic drug*
- Starting an antiepileptic drug might control the seizures symptomatically, but it would **not address the underlying cause** of the seizures, which is the metabolic disturbance in PKU.
- **Untreated PKU** will lead to progressive neurological damage and intellectual disability even if seizures are controlled, highlighting the importance of root cause treatment.
Essential vs. non-essential amino acids US Medical PG Question 7: An investigator is studying nutritional deficiencies in humans. A group of healthy volunteers are started on a diet deficient in pantothenic acid. After 4 weeks, several of the volunteers develop irritability, abdominal cramps, and burning paresthesias of their feet. These symptoms are fully reversed after reintroduction of pantothenic acid to their diet. The function of which of the following enzymes was most likely impaired in the volunteers during the study?
- A. Gamma-glutamyl carboxylase
- B. Alpha-ketoglutarate dehydrogenase (Correct Answer)
- C. Dopamine beta-hydroxylase
- D. Methionine synthase
- E. Glutathione reductase
Essential vs. non-essential amino acids Explanation: ***Alpha-ketoglutarate dehydrogenase***
- **Pantothenic acid** (vitamin B5) is a precursor of **coenzyme A (CoA)**, which is essential for the function of alpha-ketoglutarate dehydrogenase in the **Krebs cycle**.
- Impairment of this enzyme, critical for energy production, can lead to widespread metabolic dysfunction, manifesting as neurological and gastrointestinal symptoms like **irritability, abdominal cramps**, and **burning paresthesias**, which are classic signs of pantothenic acid deficiency.
*Gamma-glutamyl carboxylase*
- This enzyme is involved in the post-translational modification of several proteins, including **clotting factors**, and requires **vitamin K** as a cofactor.
- Its deficiency leads to bleeding disorders, not the neurological and GI symptoms described.
*Dopamine beta-hydroxylase*
- This enzyme converts **dopamine to norepinephrine** and requires vitamin C and copper.
- Its impairment can affect neurotransmitter synthesis but is not directly linked to pantothenic acid deficiency.
*Methionine synthase*
- This enzyme is crucial for the metabolism of **homocysteine** and requires **vitamin B12** and **folate** as cofactors.
- Its deficiency is associated with megaloblastic anemia and neurological symptoms, but not the specific presentation seen with pantothenic acid deficiency.
*Glutathione reductase*
- This enzyme is essential for maintaining the reduced state of **glutathione**, an antioxidant, and requires **riboflavin** (vitamin B2) in its coenzyme form, FAD.
- Dysfunction typically leads to oxidative stress, hemolytic anemia, and other symptoms different from those described.
Essential vs. non-essential amino acids US Medical PG Question 8: A 10-day-old infant with MSUD is admitted with lethargy, poor feeding, and plasma leucine of 2000 μmol/L (normal <200). Despite BCAA-free formula, the infant's condition worsens with encephalopathy. The neonatology team proposes hemodialysis, while the genetics team suggests continuing medical management with high-calorie IV fluids. The family is concerned about invasive procedures. Evaluate the optimal management strategy.
- A. High-dose thiamine trial before considering dialysis
- B. Continue medical management and reassess in 24 hours
- C. Exchange transfusion with albumin solutions
- D. Immediate hemodialysis to rapidly lower leucine levels (Correct Answer)
- E. Start peritoneal dialysis as less invasive alternative
Essential vs. non-essential amino acids Explanation: ***Immediate hemodialysis to rapidly lower leucine levels***
- **Extracorporeal detoxification** via hemodialysis is the most effective method to rapidly lower neurotoxic **leucine** levels when they exceed 1000 μmol/L and cause **encephalopathy**.
- Rapid clearance is critical to prevent **cerebral edema** and permanent **neurological sequelae** in neonates with acute MSUD crisis.
*Continue medical management and reassess in 24 hours*
- Delaying definitive treatment for 24 hours in a symptomatic infant with leucine at 2000 μmol/L is dangerous and increases the risk of **brain herniation**.
- Standard medical management including **BCAA-free formula** cannot clear existing toxic metabolites fast enough to reverse acute **metabolic encephalopathy**.
*Start peritoneal dialysis as less invasive alternative*
- **Peritoneal dialysis** is significantly slower and less efficient than hemodialysis at removing branched-chain amino acids like **leucine**.
- It is not recommended as first-line therapy in a critical **metabolic emergency** when hemodialysis or continuous venovenous hemodiafiltration (CVVH) is available.
*Exchange transfusion with albumin solutions*
- **Exchange transfusion** is largely ineffective for MSUD because it only removes toxins from the intravascular space and does not reach the larger **intracellular pool**.
- This method has been superseded by modern renal replacement therapies which offer superior **clearance rates** for small molecules.
*High-dose thiamine trial before considering dialysis*
- Only a small subset of MSUD patients are **thiamine-responsive**, and a trial should never delay **life-saving dialysis** in an unstable patient.
- **Thiamine** acts as a cofactor for the deficient enzyme complex but will not provide the immediate **detoxification** required for a level of 2000 μmol/L.
Essential vs. non-essential amino acids US Medical PG Question 9: A research study compares three siblings with homocystinuria: one responds to pyridoxine therapy, one responds to betaine, and one requires both treatments plus dietary restriction. All have elevated homocysteine but different methionine levels. Analyze which underlying molecular mechanism best explains the variable treatment responses among these siblings.
- A. Variable dietary methionine intake affecting phenotype expression
- B. Different epigenetic modifications of the CBS gene
- C. Varying levels of cystathionine β-synthase inhibitors
- D. Mutations in three different genes affecting homocysteine metabolism
- E. Different mutations in the same CBS gene causing varying residual enzyme activity (Correct Answer)
Essential vs. non-essential amino acids Explanation: ***Different mutations in the same CBS gene causing varying residual enzyme activity***
- **Homocystinuria** is most commonly caused by a deficiency in **Cystathionine β-synthase (CBS)**, and **allelic heterogeneity** (different mutations in the same gene) explains why siblings may have different phenotypes and treatment responses.
- Responses to **pyridoxine (B6)** occur when a mutation allows for residual enzyme activity or reduced cofactor affinity, whereas more severe mutations require **betaine** to drive the alternative **remethylation pathway**.
*Mutations in three different genes affecting homocysteine metabolism*
- While mutations in **MTHFR** or **methionine synthase** can cause homocystinuria, siblings typically inherit the same primary genetic defect from their parents in a **Mendelian** fashion.
- Differences in **methionine levels** (some high, some low) across three different genes would be extremely unlikely within a single nuclear family compared to **allelic variation** of the CBS gene.
*Variable dietary methionine intake affecting phenotype expression*
- Although **methionine restriction** is a treatment, the specific physiological ability to respond to **pyridoxine** is a fixed biochemical trait determined by the **enzyme's molecular structure**.
- Dietary intake alone cannot explain why one sibling’s enzyme function is restored by a **cofactor** while another's is not.
*Different epigenetic modifications of the CBS gene*
- **Epigenetic modifications** like methylation or histone acetylation usually affect the degree of gene expression rather than the specific **biochemical responsiveness** to a vitamin cofactor.
- Classic homocystinuria is an **autosomal recessive** disorder where clinical variability is well-documented to arise from **missense mutations** rather than imprinting or epigenetic silencing.
*Varying levels of cystathionine β-synthase inhibitors*
- There are no clinically recognized endogenous **competitive inhibitors** of CBS that would vary so significantly between siblings to dictate distinct treatment regimens.
- The treatment responses described (B6 vs. betaine) specifically target the **enzyme-cofactor interaction** or surrogate metabolic pathways, which points toward **primary structural defects** in the enzyme.
Essential vs. non-essential amino acids US Medical PG Question 10: A 4-year-old boy presents with dark urine that turns black upon standing. His parents report that his diapers developed dark stains since infancy. Physical examination is otherwise normal. Urine analysis shows elevated homogentisic acid. What metabolic consequence is this patient at risk for developing in adulthood?
- A. Progressive intellectual disability and seizures
- B. Cardiomyopathy with heart failure
- C. Cirrhosis with portal hypertension
- D. Ochronotic arthropathy affecting spine and large joints (Correct Answer)
- E. Chronic kidney disease from renal stone formation
Essential vs. non-essential amino acids Explanation: ***Ochronotic arthropathy affecting spine and large joints***
- This patient has **Alkaptonuria**, an autosomal recessive deficiency of **homogentisate oxidase** that causes **homogentisic acid** to accumulate and polymerize into a melanin-like pigment.
- These pigments deposit in connective tissues (ochronosis), leading to debilitating **ochronotic arthropathy** in adulthood, characterized by the calcification of **intervertebral discs** and damage to large joints.
*Chronic kidney disease from renal stone formation*
- While some metabolic disorders like primary hyperoxaluria lead to renal failure, Alkaptonuria is primarily associated with **nephrolithiasis** (kidney stones) rather than chronic parenchymal kidney disease.
- The dark urine itself is a byproduct of **homogentisic acid oxidation** and is not typically toxic to the renal tubule.
*Progressive intellectual disability and seizures*
- Neurological decline and seizures are characteristic of untreated **phenylketonuria (PKU)** or **maple syrup urine disease**, not Alkaptonuria.
- Patients with Alkaptonuria typically have **normal intelligence** and no increased risk for seizure disorders.
*Cardiomyopathy with heart failure*
- Alkaptonuria is associated with **valvular heart disease** (specifically aortic or mitral stenosis) due to pigment deposition and calcification, rather than primary cardiomyopathy.
- Heart failure in these patients is usually a secondary consequence of chronic **valvular calcification** rather than a direct metabolic effect on the myocardium.
*Cirrhosis with portal hypertension*
- Liver failure and cirrhosis are common in **tyrosinemia type I** due to the accumulation of toxic succinylacetone.
- **Alkaptonuria** involves a different step in the tyrosine degradation pathway (homogentisic acid) and does not result in **hepatotoxicity** or cirrhosis.
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