Amino Acid Metabolism Practice Questions

Q: In which of the following conditions is there an increased level of ammonia in the blood?

Ornithine transcarbamoylase deficiency. **Explanation:** **1. Why Option A is Correct:** Hyperammonemia (increased blood ammonia) occurs when there is a defect in the **Urea Cycle**, the primary metabolic pathway for detoxifying ammonia into urea in the liver. **Ornithine Transcarbamoylase (OTC) deficiency** is the most common urea cycle disorder. It is an X-linked recessive condition where the enzyme responsible for combining carbamoyl phosphate and ornithine to form citrulline is deficient. This leads to a massive backup of ammonia, resulting in neurotoxicity, cerebral edema, and lethargy. **2. Why Other Options are Incorrect:** * **B. Galactosaemia:** This is a disorder of carbohydrate metabolism (deficiency of GALT enzyme). It presents with jaundice, hepatomegaly, and cataracts, but not primary hyperammonemia. * **C. Histidinaemia:** This is a benign disorder of histidine metabolism (histidase deficiency). While histidine levels rise, the urea cycle remains intact, so ammonia levels are normal. * **D. Phenylketonuria (PKU):** This is a disorder of aromatic amino acid metabolism (Phenylalanine hydroxylase deficiency). It leads to the accumulation of phenylalanine and phenylketones, causing intellectual disability and a "mousy odor," but does not impair ammonia detoxification. **3. NEET-PG High-Yield Pearls:** * **OTC Deficiency Unique Marker:** It is the only urea cycle defect that shows **increased Orotic Acid** in urine (due to the accumulation of carbamoyl phosphate, which enters the pyrimidine synthesis pathway). * **Inheritance:** All urea cycle disorders are Autosomal Recessive **except** OTC deficiency, which is **X-linked Recessive**. * **Management:** Acute hyperammonemia is treated with hemodialysis and ammonia scavengers like **Sodium Benzoate** or **Sodium Phenylbutyrate**.

Q: Essential atrophy of the choroid is due to an inborn error of metabolism of which amino acid?

Ornithine. **Explanation:** **Gyrate Atrophy of the Choroid and Retina** (also known as essential atrophy of the choroid) 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 and proline. A deficiency leads to a 10- to 20-fold increase in plasma **ornithine** levels (hyperornithinemia), which is toxic to the retinal pigment epithelium (RPE) and choroid. **Why the other options are incorrect:** * **Cystine & Cysteine:** These are sulfur-containing amino acids. Their metabolic derangement is associated with **Cystinosis**, which presents with pathognomonic needle-shaped crystals in the cornea and conjunctiva, not primary chorioretinal atrophy. * **Arginine:** While ornithine is a metabolic product of arginine (via the urea cycle), the primary defect in Gyrate Atrophy specifically involves the inability to process ornithine. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Patients typically present in the first decade with night blindness (nyctalopia) and constricted visual fields. * **Fundus Appearance:** Characteristic "scalloped," well-defined areas of chorioretinal atrophy starting in the mid-periphery and progressing centrally. * **Associated Features:** High myopia, posterior subcapsular cataracts, and cystoid macular edema (CME). * **Treatment:** A **Vitamin B6 (Pyridoxine)** trial is essential, as a subset of patients are "B6-responsive" and show a reduction in ornithine levels. Dietary restriction of arginine is also recommended.

Q: In all of the following enzyme deficiencies, hyperammonemia is a common feature, EXCEPT:

Ornithine amino transferase. **Explanation:** The primary mechanism for ammonia detoxification in humans is the **Urea Cycle**, which occurs in the liver. Any enzyme deficiency directly involved in this cycle leads to the accumulation of ammonia, resulting in **hyperammonemia**. **1. Why Ornithine Amino Transferase (OAT) is the correct answer:** OAT is an enzyme involved in the interconversion of ornithine and glutamate/proline. It is **not** a direct component of the urea cycle. A deficiency in OAT leads to **Gyrate Atrophy of the choroid and retina**, characterized by progressive vision loss. While it affects ornithine levels, it does not impair the liver's overall capacity to detoxify ammonia; hence, hyperammonemia is not a feature. **2. Why the other options are incorrect:** * **CPS-I (Option B):** This is the rate-limiting step of the urea cycle. Deficiency causes Type I Hyperammonemia (the most severe form). * **OTC (Option C):** This is the most common urea cycle disorder (X-linked). Deficiency leads to Type II Hyperammonemia and is distinguished by elevated **Orotic acid** levels. * **Argininosuccinate Synthetase (Option A):** Deficiency leads to **Citrullinemia Type I**, characterized by a massive buildup of citrulline and secondary hyperammonemia. **High-Yield Clinical Pearls for NEET-PG:** * **Most common Urea Cycle Disorder:** Ornithine Transcarbamoylase (OTC) deficiency. * **Only X-linked Urea Cycle Disorder:** OTC deficiency (all others are Autosomal Recessive). * **Management of Hyperammonemia:** Low protein diet, Sodium benzoate/phenylbutyrate (ammonia scavengers), and Lactulose. * **Gyrate Atrophy:** Remember the triad of OAT deficiency, high ornithine levels, and circular "punched-out" retinal lesions.

Q: Which of the following is NOT a component of the urea cycle?

Urease. The urea cycle (Ornithine cycle) is the primary pathway for the detoxification of ammonia into urea in the liver. **Why Urease is the Correct Answer:** **Urease** is an enzyme that catalyzes the hydrolysis of urea into ammonia and carbon dioxide. It is **not** part of the human urea cycle. Instead, it is produced by certain bacteria (e.g., *H. pylori*, *Proteus mirabilis*) and plants. In humans, bacterial urease in the gut can contribute to hyperammonemia in patients with liver failure by converting urea back into ammonia, which then enters the portal circulation. **Analysis of Incorrect Options:** * **Arginase:** This is the final enzyme of the cycle. It cleaves Arginine into Urea and Ornithine. It is primarily found in the liver. * **Ornithine Transcarbamoylase (OTC):** This mitochondrial enzyme catalyzes the reaction between Carbamoyl Phosphate and Ornithine to form Citrulline. It is the most common site of inherited urea cycle defects. * **Arginosuccinase (Arginosuccinate Lyase):** This enzyme cleaves Arginosuccinate into Arginine and Fumarate. The fumarate produced links the urea cycle to the TCA cycle (the "Krebs Bicycle"). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential activator. * **Subcellular location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Most common deficiency:** OTC deficiency (X-linked recessive), characterized by orotic aciduria and hyperammonemia.

Q: Albinism is a genetic disease that results in incomplete metabolism of which amino acid?

Tyrosine. **Explanation:** **Correct Option: C. Tyrosine** Albinism (specifically Oculocutaneous Albinism) is primarily caused by a deficiency of the enzyme **Tyrosinase**. In the melanocytes, Tyrosinase is the rate-limiting enzyme responsible for converting **Tyrosine** into DOPA and subsequently into DOPAquinone, which eventually forms **melanin** (the pigment for skin, hair, and eyes). A genetic defect in this pathway leads to a lack of melanin production, resulting in the characteristic hypopigmentation seen in albinism. **Incorrect Options:** * **A. Histidine:** Metabolism of histidine leads to the formation of Histamine (via decarboxylation) or FIGLU (formiminoglutamate). Defects in this pathway cause Histidinemia, not pigment disorders. * **B. Cystine:** Cystine is involved in the transport disorder Cystinuria (renal stones) or the lysosomal storage disease Cystinosis. While cysteine is a component of pheomelanin, the primary metabolic block in albinism is at the Tyrosine level. * **D. Alanine:** Alanine is a non-essential amino acid primarily involved in glucose-alanine cycle (gluconeogenesis) and transamination reactions. It has no role in melanin synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Most common forms of Albinism are **Autosomal Recessive**. * **Key Enzyme:** Tyrosinase is a **copper-containing enzyme**. * **Clinical Risk:** Patients with albinism have a significantly increased risk of **Squamous Cell Carcinoma** and Basal Cell Carcinoma due to lack of photoprotective melanin. * **Differential:** Do not confuse Albinism (total lack of pigment) with **Vitiligo** (autoimmune destruction of melanocytes) or **Phenylketonuria** (where hypopigmentation occurs because high Phenylalanine inhibits Tyrosinase).

Q: Norepinephrine is metabolized by which of the following enzymes?

Both COMT and MAO. **Explanation:** Norepinephrine (NE), a key catecholamine neurotransmitter, is metabolized by two primary enzymes: **Monoamine Oxidase (MAO)** and **Catechol-O-Methyltransferase (COMT)** [1]. 1. **MAO (Monoamine Oxidase):** Located primarily on the outer mitochondrial membrane of neurons and other cells (liver, gut), MAO is responsible for the **oxidative deamination** of norepinephrine [1]. It acts mainly on the NE present within the nerve terminal. 2. **COMT (Catechol-O-Methyltransferase):** Found in the cytosol, particularly in the liver and kidneys, COMT performs **O-methylation**. It primarily metabolizes circulating catecholamines and those in the extraneuronal space. The final major metabolic end-product of norepinephrine and epinephrine metabolism is **Vanillylmandellic Acid (VMA)**, which is excreted in the urine [1]. **Analysis of Options:** * **Option A & B:** These are incorrect because they are incomplete. While both enzymes act on NE, they do not act in isolation; they work sequentially or independently to degrade catecholamines. * **Option D:** This is incorrect as NE is not significantly degraded by other pathways like hydrolysis; it relies almost entirely on MAO and COMT for termination of action (alongside reuptake) [1, 2]. **NEET-PG High-Yield Pearls:** * **Termination of Action:** The primary mechanism for terminating the action of NE at the synaptic cleft is **reuptake (Uptake-1)** into the presynaptic neuron, *not* enzymatic degradation [1, 2]. * **VMA Levels:** Urinary VMA is a critical diagnostic marker for **Pheochromocytoma** (a catecholamine-secreting tumor). * **MAO Subtypes:** MAO-A preferentially metabolizes NE, Serotonin, and Epinephrine, while MAO-B acts primarily on Dopamine. * **Metanephrines:** Intermediate metabolites (Normetanephrine and Metanephrine) are often more sensitive markers for diagnosing adrenergic tumors than VMA.

Q: Glycine is useful in the synthesis of all of the following except?

Spermine synthesis. **Explanation:** Glycine is the simplest non-essential amino acid and serves as a vital precursor for several specialized biomolecules. The correct answer is **Spermine synthesis** because polyamines like spermine and spermidine are synthesized from **methionine** (via S-adenosylmethionine) and **ornithine** (via putrescine), not glycine. **Why the other options are incorrect:** * **Purine Synthesis:** Glycine is a major contributor to the purine ring. It provides the entire **C4, C5, and N7** atoms of the purine nucleus. * **Creatine Synthesis:** Glycine is the first substrate in creatine synthesis. It reacts with Arginine to form guanidinoacetate (catalyzed by AGAT), which is later methylated to form creatine. * **Heme Synthesis:** Glycine is the fundamental building block of heme. It condenses with Succinyl CoA to form **$\delta$-aminolevulinic acid (ALA)**, the rate-limiting step catalyzed by ALA synthase. **High-Yield NEET-PG Clinical Pearls:** 1. **Glutathione:** Glycine is one of the three amino acids (along with Glutamate and Cysteine) required for the synthesis of the antioxidant glutathione. 2. **Conjugation:** Glycine is used for the conjugation of bile acids (Glycocholic acid) and the detoxification of benzoic acid to form **Hippuric acid**. 3. **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem. 4. **Non-Chiral:** It is the only amino acid that does not have an asymmetric carbon atom and is therefore not optically active.

Q: Accumulation of homogentisic acid causes which condition?

Ochronosis. **Explanation:** The correct answer is **Ochronosis**. This condition is a hallmark clinical feature of **Alkaptonuria**, an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**. 1. **Why Ochronosis is correct:** In Alkaptonuria, the metabolic breakdown of Tyrosine and Phenylalanine is interrupted, leading to the accumulation of **Homogentisic Acid (HGA)**. When HGA is excreted in urine and exposed to air, it oxidizes to form a black pigment. Within the body, HGA undergoes oxidation and polymerization into a melanin-like substance that deposits in connective tissues (cartilage, sclera, and heart valves). This bluish-black discoloration of tissues is termed **Ochronosis**. 2. **Why other options are incorrect:** * **Tyrosinemia:** Refers to a group of disorders (Type I, II, III) caused by deficiencies in enzymes like Fumarylacetoacetate hydrolase or Tyrosine aminotransferase. While related to the same pathway, they lead to the accumulation of Tyrosine or Succinylacetone, not HGA. * **Albinism:** Caused by a deficiency of the enzyme **Tyrosinase**, leading to a failure in melanin synthesis from Tyrosine. It results in hypopigmentation, the opposite of ochronosis. * **Tyrosinosis:** An older term for Tyrosinemia Type III (deficiency of p-hydroxyphenylpyruvate hydroxylase). **NEET-PG Clinical Pearls:** * **Classic Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (pigmentation of ear cartilage/sclera), and 3. Ochronotic arthritis (large joint involvement). * **Diagnosis:** Ferric chloride test (turns urine deep blue/green) or Benedict’s test (gives a brown-black precipitate). * **Management:** Low protein diet and **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent HGA formation).

Question 1

A 6-day-old newborn infant develops lactosuria, seizures, and hypoglycemia. What is the likely diagnosis?

Accepted Answer

Aromatic amino aciduria

Answer

Phenylketonuria

Answer

Intrauterine infection

Answer

Tyrosinemia

Question 2

In which of the following conditions is there an increased level of ammonia in the blood?

Accepted Answer

Ornithine transcarbamoylase deficiency

Answer

Galactosaemia

Answer

Histidinaemia

Answer

Phenylketonuria

Question 3

Essential atrophy of the choroid is due to an inborn error of metabolism of which amino acid?

Accepted Answer

Ornithine

Answer

Cystine

Answer

Cysteine

Answer

Arginine

Question 4

In all of the following enzyme deficiencies, hyperammonemia is a common feature, EXCEPT:

Accepted Answer

Ornithine amino transferase

Answer

Argininosuccinate synthetase

Answer

Carbamoyl Phosphate synthetase I (CPS-I)

Answer

Ornithine transcarbamoylase (OTC)

Question 5

Which of the following is NOT a component of the urea cycle?

Accepted Answer

Urease

Answer

Arginase

Answer

Transcarbamoylase

Answer

Arginosuccinase

Question 6

Albinism is a genetic disease that results in incomplete metabolism of which amino acid?

Accepted Answer

Tyrosine

Answer

Histidine

Answer

Cystine

Answer

Alanine

Question 7

Homocysteine is not associated with which of the following?

Accepted Answer

Hearing loss

Answer

Coronary artery disease

Answer

Fracture

Answer

Neuropsychiatric manifestations

Question 8

Norepinephrine is metabolized by which of the following enzymes?

Accepted Answer

Both COMT and MAO

Answer

COMT

Answer

MAO

Answer

Neither COMT nor MAO

Question 9

Glycine is useful in the synthesis of all of the following except?

Accepted Answer

Spermine synthesis

Answer

Purine synthesis

Answer

Creatine synthesis

Answer

Heme synthesis

Question 10

Accumulation of homogentisic acid causes which condition?

Accepted Answer

Ochronosis

Answer

Tyrosinemia

Answer

Albinism

Answer

Tyrosinosis

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?