Amino Acid Metabolism Practice Questions

Q: Deficiency of which enzyme leads to Tyrosinemia type II?

Tyrosine transaminase. **Explanation:** Tyrosinemia Type II (also known as **Richner-Hanhart Syndrome**) is caused by a deficiency of the enzyme **Tyrosine transaminase** (specifically, hepatic tyrosine aminotransferase). This enzyme is responsible for the first step of tyrosine catabolism, converting tyrosine into p-hydroxyphenylpyruvate. Its deficiency leads to significant hypertyrosinemia, which results in characteristic clinical findings: **palmoplantar keratosis** (painful skin lesions) and **corneal ulcers/dystrophy**. **Analysis of Incorrect Options:** * **A. Fumarylacetoacetate hydrolase:** Deficiency of this enzyme leads to **Tyrosinemia Type I** (Hepatorenal Tyrosinemia). This is the most severe form, characterized by cabbage-like odor, liver failure, and renal tubular acidosis (Fanconi syndrome). * **B. Phenylalanine hydroxylase:** Deficiency leads to **Phenylketonuria (PKU)**. This enzyme normally converts phenylalanine to tyrosine; its absence causes intellectual disability and a "mousy" body odor. * **D. Tyrosinase:** Deficiency of this copper-containing enzyme leads to **Oculocutaneous Albinism**, as it is essential for converting tyrosine into melanin. **High-Yield Clinical Pearls for NEET-PG:** * **Tyrosinemia Type III:** Caused by a deficiency of **p-hydroxyphenylpyruvate dioxygenase**. * **Alkaptonuria:** Caused by a deficiency of **Homogentisate oxidase** (look for black urine on standing and ochronosis). * **Dietary Management:** For Tyrosinemia Type II, the treatment involves a diet low in both **Phenylalanine and Tyrosine**. * **Diagnostic Marker:** Elevated serum tyrosine levels without the presence of succinylacetone (which is specific to Type I).

Q: Phenylketonuria (PKU) is a congenital amino acid metabolic disorder. In one of the rare variants of PKU, dihydropterin synthesis is affected. Which enzyme is deficient in this variant?

Dihydropterin reductase. **Explanation:** **Phenylketonuria (PKU)** is primarily caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts Phenylalanine to Tyrosine. However, this reaction requires a vital co-factor: **Tetrahydrobiopterin (BH4)**. **Why Option C is correct:** In the "classic" form of PKU, the PAH enzyme itself is defective. However, in **rare/malignant variants**, the defect lies in the metabolism of the co-factor BH4. BH4 must be regenerated from Dihydrobiopterin (BH2) by the enzyme **Dihydropterin reductase**. If this enzyme is deficient, BH4 levels fall, leading to a secondary failure of Phenylalanine Hydroxylase activity. These variants are clinically more severe because BH4 is also a required co-factor for the synthesis of neurotransmitters like Dopamine and Serotonin. **Why other options are incorrect:** * **Option A (Histidine decarboxylase):** This enzyme converts Histidine to Histamine; it is unrelated to Phenylalanine metabolism. * **Option B (Phenylalanine hydroxylase):** Deficiency of this enzyme causes "Classic PKU," not the rare variant involving dihydropterin synthesis. * **Option D (Tyrosine deficiency):** While Tyrosine becomes an essential amino acid in PKU patients, its deficiency is a *consequence* of the metabolic block, not the *enzymatic cause* of the disorder. **High-Yield Clinical Pearls for NEET-PG:** * **Mousiness/Musty Odor:** Characteristic of PKU due to Phenylacetate in sweat and urine. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening of PKU. * **Malignant PKU:** Unlike classic PKU, these variants do not respond to simple dietary restriction of Phenylalanine because neurotransmitter synthesis remains impaired. Treatment requires BH4 supplementation and neurotransmitter precursors (L-Dopa, 5-HTP).

Q: Which group contains non-essential amino acids?

Acidic Amino Acids. ### Explanation **1. Why Acidic Amino Acids are Correct:** Non-essential amino acids are those that the human body can synthesize de novo from metabolic intermediates (like those in the TCA cycle) and do not strictly require dietary intake. The **Acidic Amino Acids**—**Aspartic acid (Aspartate)** and **Glutamic acid (Glutamate)**—are classic examples. * **Glutamate** is synthesized via the reductive amination of $\alpha$-ketoglutarate. * **Aspartate** is synthesized via the transamination of oxaloacetate. Since both precursors are readily available in the TCA cycle, these amino acids are non-essential. **2. Analysis of Incorrect Options:** * **Branched Chain Amino Acids (BCAAs):** This group includes **Leucine, Isoleucine, and Valine**. All three are **Essential** amino acids. They must be obtained from the diet and are primarily metabolized in the muscle. * **Basic Amino Acids:** This group includes **Lysine, Arginine, and Histidine**. * **Lysine** is strictly essential. * **Arginine and Histidine** are considered semi-essential (required during periods of rapid growth or stress). * **Aromatic Amino Acids:** This group includes **Phenylalanine, Tyrosine, and Tryptophan**. * **Phenylalanine and Tryptophan** are strictly essential. * **Tyrosine** is non-essential only because it is synthesized from Phenylalanine. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (The "Aromatics + Isoleucine"). * **Clinical Correlation:** Defective metabolism of BCAAs leads to **Maple Syrup Urine Disease (MSUD)** due to a deficiency in the Branched-chain $\alpha$-keto acid dehydrogenase complex.

Q: Which of the following vitamins is used in the treatment of Alkaptonuria?

Vitamin C. ### Explanation **Correct Option: B (Vitamin C)** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate oxidase**, leading to the accumulation of Homogentisic Acid (HGA). Vitamin C (Ascorbic Acid) is used in management for two primary reasons: 1. **Inhibition of Oxidation:** It prevents the oxidation of Homogentisic acid into **Benzoquinone acetate**, the pigmented polymer responsible for tissue damage. 2. **Reduction of Ochronosis:** By inhibiting this polymerization, it slows down the deposition of dark pigments in connective tissues (ochronosis), thereby delaying the progression of ochronotic arthritis. **Why Other Options are Incorrect:** * **Vitamin A:** Primarily involved in vision (rhodopsin synthesis) and epithelial integrity; it has no role in the tyrosine metabolic pathway. * **Vitamin D:** Essential for calcium and phosphate homeostasis; its deficiency leads to Rickets/Osteomalacia, but it does not affect HGA metabolism. * **Vitamin K:** Acts as a co-factor for the gamma-carboxylation of clotting factors (II, VII, IX, X); it does not influence the catabolism of aromatic amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Homogentisic aciduria (urine turns black on standing/alkalinization), Ochronosis (bluish-black pigmentation of cartilage/sclera), and Arthritis (large joints). * **Diagnosis:** Ferric chloride test (transient deep blue color) and Silver nitrate test. * **Definitive Treatment:** **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase), which reduces the production of HGA. * **Dietary Advice:** Restriction of Phenylalanine and Tyrosine.

Q: The estimation of 3-methylhistidine in urine is used to study:

Skeletal muscle turnover. **Explanation:** **3-methylhistidine (3-MH)** is a unique amino acid derivative formed by the post-translational methylation of specific histidine residues in **actin and myosin**, the primary contractile proteins of skeletal muscle. **Why Option C is correct:** When muscle proteins are degraded (catabolized), 3-methylhistidine is released into the bloodstream. Unlike other amino acids, 3-MH cannot be re-utilized for protein synthesis or further metabolized; it is excreted unchanged in the urine. Therefore, the urinary excretion rate of 3-methylhistidine serves as a specific and sensitive **biochemical marker for skeletal muscle protein turnover** and myofibrillar protein breakdown. **Why other options are incorrect:** * **Option A (Folate status):** Folate status is typically assessed using serum folate levels or the **FIGLU (Formiminoglutamic acid) excretion test**, where FIGLU accumulates in the urine during folate deficiency. * **Option B (Renal disease):** While 3-MH is excreted by the kidneys, it is not a diagnostic marker for renal disease. Standard markers include Serum Creatinine, Urea, and Cystatin C. * **Option D (Protein absorption):** Protein absorption is generally assessed via fecal nitrogen studies or D-xylose tests (for general malabsorption), not by specific methylated amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** 3-MH is found exclusively in actin and myosin. * **Clinical Utility:** Elevated levels are seen in hypercatabolic states like severe trauma, sepsis, starvation, and muscular dystrophy. * **Dietary Note:** To ensure accuracy, patients should follow a **meat-free diet** for 3 days prior to the test, as exogenous 3-MH from consumed animal muscle can falsely elevate results.

Q: A 7-month-old infant presents with a history of vomiting and failure to thrive. The patient improved with IV glucose. After one month, the infant returns with the same complaints. On evaluation, high glutamine and uracil levels are found. Which is the likely enzyme defect?

Ornithine transcarbamylase. ### Explanation The clinical presentation of vomiting, failure to thrive, and improvement with IV glucose (which reduces protein catabolism) suggests a **Urea Cycle Disorder (UCD)**. **Why Ornithine Transcarbamylase (OTC) Deficiency is correct:** OTC is the most common urea cycle defect and is X-linked recessive. In OTC deficiency, **Carbamoyl Phosphate** cannot be converted to Citrulline. As a result, Carbamoyl Phosphate accumulates in the mitochondria and leaks into the cytosol. Here, it enters the pyrimidine synthesis pathway, leading to the overproduction of **Orotic acid and Uracil**. High ammonia levels are shunted into **Glutamine** synthesis, explaining the elevated glutamine. **Why the other options are incorrect:** * **CPS1 Deficiency:** Like OTC deficiency, it presents with hyperammonemia and high glutamine. However, because CPS1 is upstream of Carbamoyl Phosphate production, there is **no increase** in uracil or orotic acid. * **Argininosuccinate Lyase Deficiency:** This leads to the accumulation of argininosuccinic acid. While it causes hyperammonemia, it does not typically present with the massive uracil elevation seen in OTC deficiency. * **Arginase Deficiency:** This is the least severe UCD. It presents later in childhood with spastic paraplegia rather than acute neonatal/infantile vomiting, and arginine levels would be significantly elevated. **High-Yield Clinical Pearls for NEET-PG:** * **OTC Deficiency** is the *only* X-linked Urea Cycle Disorder; all others are Autosomal Recessive. * **Orotic Aciduria + Hyperammonemia** = OTC Deficiency. * **Orotic Aciduria + Megaloblastic Anemia** (without hyperammonemia) = UMP Synthase deficiency. * **Glutamine** acts as a non-toxic storage form of ammonia; its elevation is a hallmark of most UCDs.

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: 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: 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).

Question 1

Deficiency of which enzyme leads to Tyrosinemia type II?

Accepted Answer

Tyrosine transaminase

Answer

Fumarylacetoacetate hydrolase

Answer

Phenylalanine hydroxylase

Answer

Tyrosinase

Question 2

Phenylketonuria (PKU) is a congenital amino acid metabolic disorder. In one of the rare variants of PKU, dihydropterin synthesis is affected. Which enzyme is deficient in this variant?

Accepted Answer

Dihydropterin reductase

Answer

Histidine decarboxylase

Answer

Phenylalanine hydroxylase

Answer

Tyrosine deficiency

Question 3

Which group contains non-essential amino acids?

Accepted Answer

Acidic Amino Acids

Answer

Branched Chain Amino Acids

Answer

Basic Amino Acids

Answer

Aromatic Amino Acids

Question 4

Which of the following vitamins is used in the treatment of Alkaptonuria?

Accepted Answer

Vitamin C

Answer

Vitamin A

Answer

Vitamin D

Answer

Vitamin K

Question 5

What is the most important amino acid transported from muscle to liver for gluconeogenesis?

Accepted Answer

Alanine

Answer

Methionine

Answer

Tryptophan

Answer

Arginine

Question 6

The estimation of 3-methylhistidine in urine is used to study:

Accepted Answer

Skeletal muscle turnover

Answer

Status of folate in the body

Answer

Renal disease

Answer

Protein absorption in the gut

Question 7

A 7-month-old infant presents with a history of vomiting and failure to thrive. The patient improved with IV glucose. After one month, the infant returns with the same complaints. On evaluation, high glutamine and uracil levels are found. Which is the likely enzyme defect?

Accepted Answer

Ornithine transcarbamylase

Answer

Carbamoyl phosphate synthetase 1 (CPS1)

Answer

Arginase

Answer

Argininosuccinate lyase

Question 8

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 9

Carbamoyl phosphate synthetase I acts on which substrates?

Accepted Answer

CO2 and ammonia

Answer

Ornithine

Answer

Carbamoyl phosphate

Answer

Arginine

Question 10

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

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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