Amino Acid Metabolism Practice Questions

Q: Creatinine is synthesized from which amino acid?

Arginine. **Explanation:** Creatinine is a breakdown product of **Creatine phosphate**, which serves as a high-energy reservoir in muscle. The synthesis of creatine is a multi-organ process involving three specific amino acids: **Arginine, Glycine, and Methionine (as S-adenosylmethionine/SAM).** 1. **Step 1 (Kidney):** Arginine and Glycine combine to form Guanidinoacetate (GAA) via the enzyme *Amidinotransferase*. 2. **Step 2 (Liver):** GAA is methylated by SAM to form Creatine. 3. **Step 3 (Muscle):** Creatine is phosphorylated to Creatine Phosphate. It then undergoes non-enzymatic, irreversible cyclization to form **Creatinine**, which is excreted by the kidneys. **Analysis of Incorrect Options:** * **B. Alanine:** A non-essential amino acid primarily involved in the glucose-alanine cycle for transporting nitrogen to the liver; it does not contribute to the guanidino group of creatine. * **C. Histidine:** A precursor for Histamine and a provider of one-carbon units (via FIGLU); it is not involved in creatine synthesis. * **D. Cysteine:** A sulfur-containing amino acid used for synthesizing Taurine and Glutathione; it does not play a role in the formation of the creatine backbone. **Clinical Pearls for NEET-PG:** * **GAMT Deficiency:** A defect in Guanidinoacetate methyltransferase leads to cerebral creatine deficiency, presenting with seizures and intellectual disability. * **Diagnostic Marker:** Serum creatinine is a reliable indicator of GFR (Glomerular Filtration Rate) because its production is constant and proportional to muscle mass. * **Memory Aid:** Remember **"GAM"** (Glycine, Arginine, Methionine) for Creatine synthesis.

Q: Which of the following transports nitrogen from muscle to the liver?

Alanine. **Explanation:** The correct answer is **Alanine**. This question tests the understanding of the **Glucose-Alanine Cycle (Cahill Cycle)**, a crucial metabolic pathway for nitrogen transport. **1. Why Alanine is Correct:** During periods of fasting or intense exercise, muscle proteins are broken down into amino acids. The amino groups are transferred to $\alpha$-ketoglutarate to form glutamate. In the muscle, the enzyme **ALT (Alanine Aminotransferase)** transfers this amino group from glutamate to pyruvate (a product of glycolysis), forming **Alanine**. Alanine is then released into the blood and transported to the liver. In the liver, it is converted back to pyruvate for gluconeogenesis, while the nitrogen enters the **Urea Cycle** for excretion. **2. Why Other Options are Incorrect:** * **Lactate:** Involved in the **Cori Cycle**. It transports carbon skeletons (energy) from muscle to liver during anaerobic glycolysis but does **not** carry nitrogen. * **Glutamine:** While glutamine is the primary transporter of nitrogen from **most peripheral tissues** (especially the brain) to the liver/kidney, **Alanine** is the specific and preferred transporter for **skeletal muscle**. * **Aspartate:** It is an intermediate in the urea cycle and malate-aspartate shuttle but does not function as a primary inter-organ nitrogen transporter. **Clinical Pearls for NEET-PG:** * **Primary Nitrogen Transporters:** Remember: **Alanine** = Muscle; **Glutamine** = General peripheral tissues/Brain. * **ALT (SGPT):** The key enzyme for the Cahill cycle; it requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Net Result:** The Glucose-Alanine cycle allows muscles to eliminate nitrogen safely while receiving glucose from the liver to sustain activity.

Q: At which level does tyrosine enter the citric acid cycle?

Fumarate. **Explanation:** Tyrosine is a **glucogenic and ketogenic** amino acid. Its catabolism follows a specific pathway where it is first converted to homogentisate and eventually cleaved into two fragments: **Fumarate** and **Acetoacetate**. 1. **Why Fumarate is correct:** The final step of tyrosine (and phenylalanine) degradation involves the enzyme *fumarylacetoacetate hydrolase*, which splits fumarylacetoacetate into fumarate and acetoacetate. Fumarate directly enters the Citric Acid Cycle (TCA), making tyrosine glucogenic, while acetoacetate enters the ketone body pool, making it ketogenic. 2. **Why other options are incorrect:** * **Succinyl CoA:** This is the entry point for "VOMIT" amino acids (Valine, Odd-chain fatty acids, Methionine, Isoleucine, and Threonine). * **Pyruvate:** This is the entry point for 3-carbon amino acids like Alanine, Glycine, Serine, and Cysteine. * **Alpha-ketoglutarate:** This is the entry point for 5-carbon amino acids that form glutamate, such as Glutamine, Proline, Arginine, and Histidine. **Clinical Pearls for NEET-PG:** * **Alkaptonuria:** Caused by a deficiency of *homogentisate oxidase*, leading to dark urine (on standing) and ochronosis. * **Tyrosinemia Type I:** Caused by a deficiency of *fumarylacetoacetate hydrolase* (the final enzyme). It is the most severe form, characterized by a "cabbage-like" odor and liver failure. * **Phenylketonuria (PKU):** Tyrosine becomes an **essential** amino acid in PKU patients because they cannot convert phenylalanine to tyrosine due to *phenylalanine hydroxylase* deficiency.

Q: What is a potential consequence for a child born to a mother with phenylketonuria?

Microcephaly, mental retardation, congenital heart disease. **Explanation:** This question addresses **Maternal Phenylketonuria (PKU) Syndrome**, a condition where a mother with PKU has poorly controlled phenylalanine levels during pregnancy. Phenylalanine is a potent teratogen that crosses the placenta via active transport, leading to fetal levels significantly higher than maternal levels. **1. Why Option A is Correct:** High maternal phenylalanine levels disrupt fetal organogenesis and brain development. The classic triad of Maternal PKU Syndrome includes: * **Microcephaly:** Due to the neurotoxic effects of phenylalanine on the developing fetal brain. * **Mental Retardation (Intellectual Disability):** Even if the infant does not have the PKU genotype, the intrauterine exposure causes permanent cognitive impairment. * **Congenital Heart Disease (CHD):** Most commonly Fallot’s Tetralogy or VSD. * **Intrauterine Growth Restriction (IUGR):** Often seen alongside these features. **2. Why Other Options are Incorrect:** * **Cataracts (Options B, C, D):** Cataracts are characteristic of **Congenital Rubella Syndrome** or **Galactosemia**, not PKU. * **Hydrocephalus (Option C):** PKU causes a small brain (microcephaly), whereas hydrocephalus (enlarged ventricles) is more typical of infections like **Congenital Toxoplasmosis**. * **Renal Dysplasia (Option D):** This is not a recognized feature of Maternal PKU Syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Preconception Counseling:** The goal is to maintain maternal phenylalanine levels between **2–6 mg/dL** for at least 3 months before conception and throughout pregnancy. * **Dietary Management:** A strict low-phenylalanine diet (avoiding high-protein foods and aspartame) is the mainstay of treatment. * **Genetics:** The fetus is usually a heterozygote (carrier) and does not have the disease itself; the damage is purely due to the **teratogenic intrauterine environment**.

Q: An infant presents with a history of vomiting and poor feeding. The urine has a burnt sugar odor. Lab examination reveals elevated levels of leucine, isoleucine, and valine. What is the most likely diagnosis?

Maple syrup urine disease. ### Explanation **Correct Option: D. Maple Syrup Urine Disease (MSUD)** The clinical presentation of poor feeding, vomiting, and a characteristic **burnt sugar (or maple syrup) odor** in the urine is pathognomonic for MSUD. This autosomal recessive disorder is caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This enzyme is responsible for the oxidative decarboxylation of branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. A defect leads to the accumulation of these amino acids and their corresponding alpha-keto acids in the blood and urine. **Why Incorrect Options are Wrong:** * **A. Phenylketonuria (PKU):** Caused by a deficiency of Phenylalanine Hydroxylase. It presents with a "mousy" or "musty" odor and intellectual disability, not a burnt sugar smell. * **B. Alkaptonuria:** Due to Homogentisate Oxidase deficiency. It is characterized by urine that turns black upon standing and ochronosis (pigmentation of connective tissue), typically presenting later in life. * **C. Tyrosinemia:** Type I is caused by Fumarylacetoacetate Hydrolase deficiency. It presents with liver failure, renal tubular dysfunction, and a "cabbage-like" or "boiled egg" odor. **High-Yield Clinical Pearls for NEET-PG:** * **The Odor:** Caused specifically by the accumulation of **S-isoleucine**. * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **L**ipoic acid, **C**oA (B5), **F**AD (B2), and **N**AD (B3). (*Mnemonic: **T**ender **L**oving **C**are **F**or **N**ancy*). * **Treatment:** Dietary restriction of BCAAs and, in some cases, high-dose **Thiamine** supplementation (Thiamine-responsive MSUD). * **Diagnosis:** Elevated levels of **Alloisoleucine** in the plasma is a highly specific diagnostic marker.

Q: Melatonin is synthesized from which amino acid?

Tryptophan. **Explanation:** **1. Why Tryptophan is Correct:** Melatonin, the hormone responsible for regulating the circadian rhythm (sleep-wake cycle), is synthesized primarily in the **pineal gland** from the essential amino acid **Tryptophan**. The metabolic pathway follows this sequence: * **Tryptophan** → 5-Hydroxytryptophan → **Serotonin** → N-Acetylserotonin → **Melatonin**. The conversion of Serotonin to Melatonin involves two key enzymes: *Serotonin N-acetyltransferase* (the rate-limiting step) and *Hydroxyindole-O-methyltransferase*. **2. Why Incorrect Options are Wrong:** * **Serotonin (Option B):** While Serotonin is the immediate chemical precursor to Melatonin, it is a **biogenic amine**, not an **amino acid**. The question specifically asks for the parent amino acid. * **Phenylalanine (Option C):** This is the precursor for Tyrosine, which leads to the synthesis of Catecholamines (Dopamine, Norepinephrine, Epinephrine), Thyroxine (T4), and Melanin. * **Histidine (Option D):** This amino acid undergoes decarboxylation to form **Histamine**, a mediator of allergic reactions and gastric acid secretion. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme:** Serotonin N-acetyltransferase (activity increases significantly in the dark). * **Cofactor:** Pyridoxal Phosphate (B6) is required for the decarboxylation step in this pathway. * **Tryptophan Derivatives:** Remember the "3 Ms and 1 S" – **M**elatonin, **M**elanin (via serotonin pathway in some contexts, but primarily Tyrosine), **M**agnesium (not related), and **S**erotonin. Also, Tryptophan is a precursor for **Niacin (Vitamin B3)**; 60 mg of Tryptophan yields 1 mg of Niacin. * **Hartnup Disease:** A defect in Tryptophan transport leading to pellagra-like symptoms due to Niacin deficiency.

Q: Albinism is due to deficiency of which enzyme?

Tyrosinase. **Explanation:** **Albinism** is a group of inherited disorders characterized by a lack of melanin pigment in the skin, hair, and eyes. The correct answer is **Tyrosinase** because it is the rate-limiting enzyme in the synthesis of melanin from the amino acid Tyrosine. Tyrosinase catalyzes the conversion of Tyrosine to DOPA (Dihydroxyphenylalanine) and subsequently to Dopaquinone. A deficiency in this enzyme leads to Oculocutaneous Albinism (OCA) Type 1. **Analysis of Incorrect Options:** * **A. Homogentisate oxidase:** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in dark urine (on standing) and ochronosis (pigmentation of connective tissues). * **C. Phenylalanine hydroxylase:** Deficiency causes **Phenylketonuria (PKU)**. While PKU patients may have fair skin due to decreased tyrosine (a melanin precursor), the primary pathology of Albinism specifically involves the melanin synthesis pathway itself. * **D. Tyrosine transaminase:** Deficiency leads to **Tyrosinemia Type II** (Richner-Hanhart syndrome), which presents with painful corneal erosions and palmoplantar keratoderma, not generalized albinism. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tyrosinase is the key regulatory enzyme for melanogenesis. * **Inheritance:** Most forms of Albinism are **Autosomal Recessive**. * **Clinical Risk:** Patients with albinism have a significantly increased risk of **Squamous Cell Carcinoma** and Basal Cell Carcinoma due to the lack of protective melanin against UV radiation. * **Differentiate:** Do not confuse Albinism (absent melanin) with Vitiligo (autoimmune destruction of melanocytes) or Piebaldism (defective migration of melanoblasts).

Q: OTC deficiency causes which of the following conditions?

Hyperammonemia type-2. **Explanation:** **Ornithine Transcarbamoylase (OTC) deficiency** is the most common urea cycle disorder. It is unique because it is the only urea cycle defect inherited as an **X-linked recessive** trait; all others are autosomal recessive. 1. **Why Hyperammonemia Type-2 is correct:** The urea cycle begins in the mitochondria. The second step involves the enzyme **OTC**, which catalyzes the reaction between Carbamoyl Phosphate and Ornithine to form Citrulline. A deficiency in OTC leads to an accumulation of Carbamoyl Phosphate and Ammonia, resulting in **Hyperammonemia Type-2**. Excess Carbamoyl Phosphate leaks into the cytosol and enters the pyrimidine synthesis pathway, leading to increased production of **Orotic acid** (a key diagnostic marker). 2. **Why other options are incorrect:** * **Hyperammonemia Type-1:** Caused by a deficiency of **Carbamoyl Phosphate Synthetase I (CPS-I)**. Unlike OTC deficiency, there is no orotic aciduria here. * **Citrullinemia:** Caused by a deficiency of **Argininosuccinate Synthetase**, leading to a massive buildup of Citrulline. * **Argininemia:** Caused by a deficiency of **Arginase**, the final enzyme of the cycle. It typically presents with spastic diplegia rather than acute neonatal hyperammonemia. **High-Yield Clinical Pearls for NEET-PG:** * **Orotic Aciduria:** Differentiates OTC deficiency (High orotic acid) from CPS-I deficiency (Low/Normal orotic acid). * **Management:** Treatment involves a low-protein diet and ammonia-scavenging drugs like **Sodium Benzoate** or **Phenylbutyrate**. * **Citrate/Arginine:** Citrulline levels are typically low in OTC deficiency.

Q: Calcification of intervertebral discs is characteristic of which condition?

Alkaptonuria. ### Explanation **Correct Answer: A. Alkaptonuria** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, leading to the accumulation of **Homogentisic Acid (HGA)**. The underlying pathology involves the oxidation of HGA into a melanin-like polymer called **alkapton**. This pigment deposits in connective tissues, a process known as **Ochronosis**. In the musculoskeletal system, these deposits weaken the collagen matrix of the intervertebral discs, leading to premature degeneration and **dystrophic calcification**. On X-ray, this presents as a classic "wafer-like" calcification of multiple intervertebral discs, often resulting in "bamboo spine" appearance similar to ankylosing spondylitis. **Why Incorrect Options are Wrong:** * **B. Phenylketonuria:** Caused by Phenylalanine Hydroxylase deficiency. It presents with intellectual disability, "mousy" odor, and hypopigmentation, but does not involve disc calcification. * **C. Gout:** Caused by monosodium urate crystal deposition. While it affects joints (Podagra), it typically involves peripheral joints and causes "punched-out" erosions rather than disc calcification. * **D. Rickets:** A defect in bone mineralization (Vitamin D deficiency). It leads to softening of bones (osteomalacia) and widening of growth plates, not pathological calcification of discs. **Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (blue-black pigmentation of sclera and ear cartilage), 3. Ochronotic arthritis (large joints and spine). * **Diagnostic Test:** Ferric Chloride test (turns deep blue/green) and Silver Nitrate test. * **Management:** High doses of Vitamin C (antioxidant) and **Nitisinone** (inhibits HGA production).

Q: A patient with Gyrate Atrophy due to defective ornithine aminotransferase would be benefited by which dietary modification?

Arginine free diet. ### Explanation **Gyrate Atrophy** is an autosomal recessive dystrophy caused by a deficiency in the mitochondrial enzyme **Ornithine Aminotransferase (OAT)**. This enzyme is responsible for converting ornithine into glutamate or proline. #### 1. Why "Arginine-free diet" is correct: In OAT deficiency, ornithine cannot be metabolized, leading to **hyperornithinemia** (10–20 fold increase). Since **Arginine** is the direct metabolic precursor of ornithine in the urea cycle, restricting dietary arginine intake is the most effective way to lower systemic ornithine levels. Reducing ornithine levels can slow the progression of the characteristic "punched-out" chorioretinal atrophy. #### 2. Analysis of Incorrect Options: * **Option A (Ornithine-free diet):** Ornithine is not typically consumed directly in significant quantities from the diet; it is synthesized endogenously from arginine. Therefore, restricting the precursor (arginine) is the clinical standard. * **Option C & D (Vitamin Supplementation):** While **Pyridoxine (Vitamin B6)** is a cofactor for the OAT enzyme and a small subset of patients are "B6-responsive," Vitamin B1 and B12 have no therapeutic role in this specific condition. #### 3. High-Yield Clinical Pearls for NEET-PG: * **Clinical Presentation:** Patients present with night blindness (nyctalopia) and constricted visual fields in the first decade of life. * **Fundus Findings:** Characteristic well-defined, circular (gyrate) patches of chorioretinal atrophy starting in the mid-periphery and spreading centrally. * **Associated Findings:** Posterior subcapsular cataract and high myopia are common. * **Management Summary:** 1. Arginine-restricted diet. 2. Pyridoxine (B6) supplementation (to enhance residual enzyme activity). 3. Lysine supplementation (to compete with ornithine transport).

Question 1

Creatinine is synthesized from which amino acid?

Accepted Answer

Arginine

Answer

Alanine

Answer

Histidine

Answer

Cysteine

Question 2

Which of the following transports nitrogen from muscle to the liver?

Accepted Answer

Alanine

Answer

Lactate

Answer

Glutamine

Answer

Aspartate

Question 3

At which level does tyrosine enter the citric acid cycle?

Accepted Answer

Fumarate

Answer

Succinyl CoA

Answer

Pyruvate

Answer

alpha-ketoglutarate

Question 4

What is a potential consequence for a child born to a mother with phenylketonuria?

Accepted Answer

Microcephaly, mental retardation, congenital heart disease

Answer

Mental retardation, cataract, congenital heart disease

Answer

Hydrocephalus, cataract

Answer

Microcephaly, cataract, renal dysplasia

Question 5

An infant presents with a history of vomiting and poor feeding. The urine has a burnt sugar odor. Lab examination reveals elevated levels of leucine, isoleucine, and valine. What is the most likely diagnosis?

Accepted Answer

Maple syrup urine disease

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Tyrosinemia

Question 6

Melatonin is synthesized from which amino acid?

Accepted Answer

Tryptophan

Answer

Serotonin

Answer

Phenylalanine

Answer

Histidine

Question 7

Albinism is due to deficiency of which enzyme?

Accepted Answer

Tyrosinase

Answer

Homogentisate oxidase

Answer

Phenylalanine hydroxylase

Answer

Tyrosine transaminase

Question 8

OTC deficiency causes which of the following conditions?

Accepted Answer

Hyperammonemia type-2

Answer

Citrullinemia

Answer

Argininemia

Answer

Hyperammonemia type-1

Question 9

Calcification of intervertebral discs is characteristic of which condition?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Gout

Answer

Rickets

Question 10

A patient with Gyrate Atrophy due to defective ornithine aminotransferase would be benefited by which dietary modification?

Accepted Answer

Arginine free diet

Answer

Ornithine free diet

Answer

Pyridoxine and Vitamin B12 supplementation

Answer

Vitamin B1, B6, and B12 supplementation

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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