Amino Acid Metabolism Practice Questions

Q: A 2-year-old child with intellectual disability presents with blue eyes, blonde hair, fair skin, and a peculiar body odor. What is the most likely diagnosis?

Phenylketonuria. **Explanation:** **Phenylketonuria (PKU)** is the correct diagnosis based on the classic clinical triad of intellectual disability, pigmentary dilution, and a characteristic odor. 1. **Why PKU is correct:** PKU is most commonly caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts phenylalanine to tyrosine. * **Intellectual Disability:** Accumulation of phenylalanine and its metabolites (phenylpyruvate, phenyllactate) is neurotoxic. * **Fair Skin, Blue Eyes, Blonde Hair:** Tyrosine is a precursor for **melanin**. Reduced tyrosine levels lead to hypopigmentation. * **Mousy/Musty Odor:** This is due to the excretion of **phenylacetic acid** in sweat and urine. 2. **Why other options are incorrect:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in Branched-Chain Alpha-Keto Acid Dehydrogenase. It presents with a **burnt sugar/maple syrup odor**, not fair skin or mousy odor. * **Isovaleric Aciduria:** A disorder of leucine metabolism characterized by a distinct **"sweaty feet" odor**. * **Canavan Disease:** A leukodystrophy presenting with macrocephaly, hypotonia, and developmental delay, but it lacks the specific pigmentary changes and mousy odor of PKU. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** Diet low in phenylalanine; **Tyrosine becomes an essential amino acid** for these patients. * **Maternal PKU:** If a mother with PKU doesn't maintain a strict diet during pregnancy, the fetus may develop microcephaly and congenital heart defects (even if the fetus doesn't have the genotype).

Q: Maple syrup urine disease can be caused by a problem with the metabolism of which particular compound?

Valine. **Explanation:** **Maple Syrup Urine Disease (MSUD)** is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This enzyme is responsible for the oxidative decarboxylation of the three branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. 1. **Why Valine is correct:** Since the BCKAD complex is defective, the body cannot metabolize Valine (along with Leucine and Isoleucine). This leads to a toxic buildup of these amino acids and their corresponding alpha-keto acids in the blood and urine. 2. **Why other options are incorrect:** * **Glycogen:** Disorders of glycogen metabolism are called Glycogen Storage Diseases (e.g., Von Gierke’s), which typically present with hypoglycemia and hepatomegaly. * **Collagen:** Defects in collagen lead to structural disorders like Scurvy (Vitamin C deficiency) or Osteogenesis Imperfecta, not metabolic ketoacidosis. * **Dopamine:** Dopamine is a catecholamine derived from Tyrosine. Abnormalities here are linked to Parkinson’s disease or phenylketonuria (indirectly), but not MSUD. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember **"I Love Vermont"** (Isoleucine, Leucine, Valine) for the BCAAs involved. * **Clinical Presentation:** Infants present with poor feeding, vomiting, and a characteristic **burnt sugar/maple syrup odor** in the urine. * **Diagnosis:** Elevated levels of BCAAs in plasma and **Alloisoleucine** (pathognomonic). * **Cofactor:** The BCKAD complex requires **Thiamine (Vitamin B1)**. Some patients respond to high-dose Thiamine supplementation (Thiamine-responsive MSUD).

Q: Carbamoyl phosphate synthetase I acts on which substrates?

CO2 and ammonia. **Explanation:** **Carbamoyl Phosphate Synthetase I (CPS-I)** is the rate-limiting enzyme of the **Urea Cycle**, occurring within the mitochondrial matrix of hepatocytes. Its primary function is to fix nitrogen for excretion. **Why C is correct:** CPS-I catalyzes the condensation of **Ammonia ($NH_3$)** and **Bicarbonate ($HCO_3^-$, derived from $CO_2$)** to form Carbamoyl Phosphate. This reaction requires the consumption of **2 ATP** molecules and is uniquely dependent on **N-acetylglutamate (NAG)** as an essential allosteric activator. **Why the other options are incorrect:** * **A. Ornithine:** This is a substrate for *Ornithine Transcarbamoylase (OTC)*, which combines ornithine with carbamoyl phosphate to form Citrulline. * **B. Carbamoyl phosphate:** This is the **product** of the CPS-I reaction, not the substrate. * **D. Arginine:** This is an intermediate of the urea cycle. It is cleaved by *Arginase* to produce urea and regenerate ornithine. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** CPS-I is **Mitochondrial** (Urea cycle), whereas CPS-II is **Cytosolic** (Pyrimidine synthesis). * **Activator:** NAG is the "obligatory activator" of CPS-I. A deficiency in NAG synthase mimics CPS-I deficiency. * **Clinical Correlation:** CPS-I deficiency is the most severe urea cycle disorder, presenting with **Type I Hyperammonemia**, lethargy, and seizures shortly after birth. Unlike OTC deficiency, there is **no orotic aciduria** in CPS-I deficiency. * **Nitrogen Sources:** The two nitrogens in urea come from **Free Ammonia** (via CPS-I) and **Aspartate**.

Q: Cabbage-like odor is found in the body fluids in which of the following metabolic disorders?

Tyrosinemia. ### Explanation The correct answer is **Tyrosinemia (specifically Type I)**. **1. Why Tyrosinemia is correct:** Tyrosinemia Type I (Hepatorenal Tyrosinemia) is caused by a deficiency of the enzyme **Fumarylacetoacetate hydrolase (FAH)**. This leads to the accumulation of fumarylacetoacetate and its conversion into **succinylacetone**. The characteristic **cabbage-like or boiled vegetable odor** in body fluids (urine and sweat) is attributed to the accumulation of methionine metabolites and sulfur-containing compounds that occur secondary to liver damage in these patients. **2. Why the other options are incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine hydroxylase deficiency. It is characterized by a **mousy or musty odor** due to the accumulation of phenylacetic acid. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex. It results in a **burnt sugar or maple syrup odor** due to the accumulation of isoleucine. * **Isovaleric Acidemia:** Caused by Isovaleryl-CoA dehydrogenase deficiency. It presents with a characteristic **"sweaty feet" or "cheesy" odor** due to the accumulation of isovaleric acid. **3. Clinical Pearls for NEET-PG:** * **Tyrosinemia Type I:** High risk of early-onset **Hepatocellular Carcinoma (HCC)** and renal rickets. * **Diagnostic Marker:** Succinylacetone in urine is pathognomonic. * **Treatment:** Nitisinone (NTBC), which inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent the formation of toxic metabolites. **Summary of Odors in Metabolic Disorders:** * **Mousy:** PKU * **Cabbage-like:** Tyrosinemia * **Maple Syrup:** MSUD * **Sweaty Feet:** Isovaleric acidemia / Glutaric acidemia Type II * **Fishy:** Trimethylaminuria * **Swimming Pool:** Hawkinsinuria

Q: Which of the following amino acids enters the TCA cycle at the succinyl CoA step?

Methionine. ### Explanation The entry of amino acids into the TCA cycle depends on their carbon skeleton degradation pathway. **Methionine** is a sulfur-containing amino acid that follows a specific catabolic route to enter the TCA cycle at the **Succinyl CoA** step. **Why Methionine is Correct:** Methionine is converted into **S-adenosylmethionine (SAM)**, which then forms **Homocysteine**. Homocysteine combines with serine to form cystathionine, eventually yielding **Propionyl CoA**. Propionyl CoA undergoes carboxylation (Biotin-dependent) and isomerization (Vitamin B12-dependent) to become **Succinyl CoA**. Other amino acids following this "VOMIT" mnemonic (Valine, Odd-chain fatty acids, Methionine, Isoleucine, Threonine) also enter at this step. **Analysis of Incorrect Options:** * **A. Tryptophan:** This is both glucogenic and ketogenic. It enters the TCA cycle via **Pyruvate** and **Acetoacetyl CoA**. * **B. Tyrosine:** This is also both glucogenic and ketogenic. Its carbon skeleton is degraded to **Fumarate** and **Acetoacetate**. * **C. Glycine:** Glycine is primarily converted to **Pyruvate** (via serine) or undergoes the glycine cleavage system to CO₂ and NH₄⁺. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin B12 Deficiency:** Leads to the accumulation of **Methylmalonic acid (MMA)** because the conversion of Methylmalonyl CoA to Succinyl CoA is blocked. This is a key diagnostic marker to differentiate B12 deficiency from Folate deficiency. * **Homocystinuria:** Most commonly due to a deficiency in **Cystathionine β-synthase**, an enzyme in the methionine catabolic pathway. * **Ketogenic vs. Glucogenic:** Leucine and Lysine are the only *purely* ketogenic amino acids. Methionine is *purely* glucogenic.

Q: Which of the following amino acids is NOT both glucogenic and ketogenic?

Leucine. ### Explanation Amino acids are classified based on the metabolic intermediates they produce during catabolism. They can be **glucogenic** (converted into glucose via gluconeogenesis), **ketogenic** (converted into ketone bodies or acetyl-CoA), or **both**. **1. Why Leucine is the Correct Answer:** Leucine is one of only two **purely ketogenic** amino acids (the other being Lysine). Its catabolism yields only Acetyl-CoA and Acetoacetate, which cannot be used for the net synthesis of glucose in humans. Therefore, it is NOT both glucogenic and ketogenic. **2. Analysis of Incorrect Options:** The amino acids that are **both glucogenic and ketogenic** can be remembered by the mnemonic **"PITTT"** (Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan). * **A. Phenylalanine:** It is both; it yields Fumarate (glucogenic) and Acetoacetate (ketogenic). * **B. Tyrosine:** It is both; as a derivative of phenylalanine, it also yields Fumarate and Acetoacetate. * **C. Tryptophan:** It is both; it yields Pyruvate (glucogenic) and Acetoacetate (ketogenic). **3. High-Yield Facts for NEET-PG:** * **Purely Ketogenic:** Leucine and Lysine (The "L" amino acids). * **Both Glucogenic & Ketogenic:** Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan. * **Purely Glucogenic:** All other 13 amino acids. * **Clinical Pearl:** In patients with **Maple Syrup Urine Disease (MSUD)**, there is a defect in the branched-chain alpha-keto acid dehydrogenase complex, leading to an accumulation of Leucine, Isoleucine, and Valine. Leucine is the most toxic of these to the CNS.

Q: Ingestion of l-tryptophan increases the levels of which neurotransmitter in the brain?

Serotonin. ### Explanation **Correct Option: A. Serotonin** Tryptophan is an essential amino acid that serves as the direct precursor for **Serotonin (5-hydroxytryptamine)**. The synthesis occurs in a two-step pathway: 1. **Tryptophan hydroxylase** (the rate-limiting enzyme) converts L-tryptophan to 5-hydroxytryptophan (5-HTP). This enzyme requires **Tetrahydrobiopterin (BH4)** as a cofactor. 2. **Aromatic L-amino acid decarboxylase** converts 5-HTP to Serotonin (requires Vitamin B6/Pyridoxine). Since the transport of tryptophan across the blood-brain barrier is a saturable process, increasing dietary intake of L-tryptophan directly raises brain serotonin levels. **Why Other Options are Incorrect:** * **B & C (Norepinephrine and Dopamine):** These are catecholamines derived from the amino acid **Tyrosine** (via the L-Dopa pathway). Tryptophan does not contribute to their synthesis. * **D (Acetylcholine):** This neurotransmitter is synthesized from **Choline and Acetyl-CoA** by the enzyme Choline acetyltransferase. It is not derived from an amino acid precursor like tryptophan. **High-Yield Clinical Pearls for NEET-PG:** * **Melatonin Connection:** Serotonin is further converted to Melatonin in the pineal gland; thus, Tryptophan is also the precursor for Melatonin. * **Niacin Synthesis:** About 60 mg of Tryptophan can produce 1 mg of **Niacin (Vitamin B3)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) leads to pellagra-like symptoms due to niacin deficiency. * **Carcinoid Syndrome:** In this condition, tumor cells divert up to 60% of tryptophan metabolism toward serotonin production, leading to secondary niacin deficiency (Pellagra).

Q: Which of the following best describes Tryptophan?

Non polar, essential, both glucogenic & ketogenic. ### Explanation **Tryptophan** is a unique and complex amino acid. To arrive at the correct answer, we evaluate it based on three criteria: 1. **Polarity:** Tryptophan contains an **indole ring** (a bicyclic structure). Despite having a nitrogen atom in the ring, the large hydrocarbon structure makes it hydrophobic and **non-polar**. 2. **Nutritional Status:** It is one of the 10 **essential amino acids** (PVT TIM HALL), meaning it cannot be synthesized by the body and must be obtained through diet. 3. **Metabolic Fate:** Tryptophan is **both glucogenic and ketogenic**. Its breakdown yields **Alanine** (which enters gluconeogenesis via pyruvate) and **Acetoacetyl-CoA** (which is ketogenic). #### Why other options are incorrect: * **Option A:** Incorrect because Tryptophan is not purely glucogenic; it is also ketogenic. * **Option B:** Incorrect because Tryptophan is classified as non-polar due to its bulky hydrophobic side chain. * **Option C:** Incorrect because Tryptophan is an essential amino acid, not non-essential. --- ### High-Yield Clinical Pearls for NEET-PG * **Precursor Role:** Tryptophan is the precursor for **Serotonin** (neurotransmitter), **Melatonin** (sleep hormone), and **Niacin** (Vitamin B3). * **Hartnup Disease:** A defect in the transport of neutral amino acids (including Tryptophan) in the gut and kidneys, leading to pellagra-like symptoms. * **Carcinoid Syndrome:** Tumors that divert Tryptophan metabolism toward Serotonin, potentially leading to a secondary Niacin deficiency. * **Largest Amino Acid:** Tryptophan has the highest molecular weight among the 20 standard amino acids. * **Absorbance:** It is responsible for the maximum UV light absorbance of proteins at **280 nm**.

Question 1

A 2-year-old child with intellectual disability presents with blue eyes, blonde hair, fair skin, and a peculiar body odor. What is the most likely diagnosis?

Accepted Answer

Phenylketonuria

Answer

Maple syrup urine disease

Answer

Isovaleric aciduria

Answer

Canavan disease

Question 2

Maple syrup urine disease can be caused by a problem with the metabolism of which particular compound?

Accepted Answer

Valine

Answer

Glycogen

Answer

Collagen

Answer

Dopamine

Question 3

Carbamoyl phosphate synthetase I acts on which substrates?

Accepted Answer

CO2 and ammonia

Answer

Ornithine

Answer

Carbamoyl phosphate

Answer

Arginine

Question 4

Which of the following statements is true regarding catecholamines?

Accepted Answer

Production terminates with dopamine in the brain but epinephrine in the adrenal gland

Answer

Production begins with the action of tyrosine hydroxylase on tyrosine

Answer

Are metabolized to both glucogenic and ketogenic fragments

Answer

All contain a methyl group donated by S-adenosylmethionine

Question 5

A 2-week-old neonate presents with complete hypotonia, convulsions, failure to thrive, and metabolic acidosis. The urine has a 'burnt sugar' odor, and a positive DNPH test. What is the enzyme deficiency in this metabolic disorder?

Accepted Answer

Branched chain keto acid dehydrogenase

Answer

Isovaleryl CoA dehydrogenase

Answer

Dihydrolipoamide dehydrogenase

Answer

Transacylase

Question 6

Cabbage-like odor is found in the body fluids in which of the following metabolic disorders?

Accepted Answer

Tyrosinemia

Answer

Phenylketonuria

Answer

Maple syrup urine disease

Answer

Isovaleric academia

Question 7

Which of the following amino acids enters the TCA cycle at the succinyl CoA step?

Accepted Answer

Methionine

Answer

Tryptophan

Answer

Tyrosine

Answer

Glycine

Question 8

Which of the following amino acids is NOT both glucogenic and ketogenic?

Accepted Answer

Leucine

Answer

Phenylalanine

Answer

Tyrosine

Answer

Tryptophan

Question 9

Ingestion of l-tryptophan increases the levels of which neurotransmitter in the brain?

Accepted Answer

Serotonin

Answer

Norepinephrine

Answer

Dopamine

Answer

Acetylcholine

Question 10

Which of the following best describes Tryptophan?

Accepted Answer

Non polar, essential, both glucogenic & ketogenic

Answer

Non polar, essential, and pure glucogenic

Answer

Polar, essential, and both glucogenic & ketogenic

Answer

Polar, non-essential, both glucogenic & ketogenic

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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