Amino Acid Metabolism Practice Questions

Q: Homocystinuria is due to abnormal metabolism of

Methionine. **Explanation:** **Homocystinuria** is a group of inherited metabolic disorders characterized by the accumulation of homocysteine in the blood and urine. The most common form (Classical Homocystinuria) is caused by a deficiency of the enzyme **Cystathionine β-synthase (CBS)**. **Why Methionine is the correct answer:** Homocysteine is an intermediate in the metabolism of **Methionine**, a sulfur-containing amino acid. In the normal pathway, Methionine is converted to S-adenosylmethionine (SAM), then to S-adenosylhomocysteine (SAH), and finally to Homocysteine. Under normal conditions, Homocysteine is then converted to Cystathionine by CBS (requiring Vitamin B6). A defect in this pathway leads to a "backlog," causing elevated levels of both Homocysteine and its precursor, Methionine. **Why the other options are incorrect:** * **Valine and Leucine:** These are **Branched-Chain Amino Acids (BCAAs)**. Abnormal metabolism of these leads to **Maple Syrup Urine Disease (MSUD)**, not Homocystinuria. * **Tryptophan:** This is an aromatic amino acid. Defects in its metabolism or transport are associated with **Hartnup disease** or Pellagra-like symptoms, but not Homocystinuria. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** 1. Ectopia lentis (downward subluxation of lens), 2. Intellectual disability, 3. Thromboembolic episodes (major cause of early death). * **Skeletal features:** Marfanoid habitus (tall stature, long limbs). * **Treatment:** High doses of **Vitamin B6 (Pyridoxine)** are effective in ~50% of cases (B6-responsive). Other treatments include a low-methionine diet and Cysteine supplementation. * **Diagnosis:** Positive **Cyanide-nitroprusside test** (detects sulfhydryl groups in urine).

Q: Tryptophan deficiency leads to which of the following conditions?

Hartnup disease. **Explanation:** **Correct Answer: C. Hartnup disease** Hartnup disease is an autosomal recessive disorder caused by a mutation in the **SLC6A19 gene**, which encodes a neutral amino acid transporter in the proximal renal tubules and intestinal mucosa. This leads to the malabsorption and excessive urinary loss of neutral amino acids, most significantly **Tryptophan**. Since Tryptophan is a precursor for **Niacin (Vitamin B3)**, its deficiency results in pellagra-like symptoms, including a photosensitive dermatitis, cerebellar ataxia, and aminoaciduria. **Analysis of Incorrect Options:** * **A. Her’s Disease:** This is Glycogen Storage Disease Type VI, caused by a deficiency in **liver glycogen phosphorylase**, leading to hepatomegaly and mild hypoglycemia. It is unrelated to amino acid metabolism. * **B. Lesch-Nyhan Syndrome:** An X-linked recessive disorder caused by a deficiency of **HGPRT**, an enzyme in the purine salvage pathway. It presents with hyperuricemia, intellectual disability, and self-mutilation. * **D. Alkaptonuria:** A disorder of tyrosine metabolism caused by a deficiency of **homogentisate oxidase**. It is characterized by the accumulation of homogentisic acid, leading to dark urine (on standing) and ochronosis. **High-Yield Clinical Pearls for NEET-PG:** * **The 3 D’s of Pellagra:** Dermatitis, Diarrhea, and Dementia are seen in Hartnup disease due to secondary Niacin deficiency. * **Diagnosis:** Characterized by "neutral aminoaciduria" (valine, leucine, isoleucine, phenylalanine, tryptophan). * **Treatment:** High-protein diet and nicotinic acid (Niacin) supplementation. * **Blue Diaper Syndrome:** A related condition where bacterial breakdown of unabsorbed tryptophan in the gut leads to indicanuria, turning the diaper blue.

Q: Which of the following statements is FALSE about alkaptonuria?

Genitourinary system is not involved.. **Explanation:** Alkaptonuria is an autosomal recessive disorder of tyrosine metabolism caused by a deficiency of the enzyme **homogentisic acid oxidase**. This deficiency leads to the accumulation of homogentisic acid (HGA) in the blood and tissues. **Why Option A is the Correct (False) Statement:** The statement "Genitourinary system is not involved" is **false** because the genitourinary system is frequently affected. Excess HGA is excreted in the urine, and its oxidation products can lead to the formation of **dark-colored renal stones (calculi)** and prostatic stones in males. Therefore, the GU system is indeed involved. **Analysis of Other Options:** * **Option B:** This is a true statement. The primary biochemical defect is the lack of homogentisic acid oxidase, which normally converts HGA into maleylacetoacetate. * **Option C:** This is a true statement. When urine containing HGA is left standing or exposed to an alkaline environment, it undergoes oxidation and polymerization to form a melanin-like pigment, turning the **urine black**. * **Option D:** This is a true statement. Chronic accumulation of HGA leads to **Ochronosis** (pigment deposition in connective tissues). This causes severe arthritis and **calcification of intervertebral discs**, leading to a characteristic "bamboo spine" appearance on X-rays. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** Homogentisic aciduria (black urine), Ochronosis (blue-black pigmentation of ear cartilage/sclera), and Arthritis. * **Diagnosis:** Ferric chloride test (urine turns transiently deep blue) or Silver Nitrate test. * **Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and **Nitisinone**, which inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase to reduce HGA production.

Q: Which of the following is FALSE regarding Hartnup disease?

Mental retardation is the common presentation. **Explanation:** **Hartnup disease** is an autosomal recessive disorder caused by a mutation in the **SLC6A19 gene**, which encodes a sodium-dependent **neutral amino acid transporter** located in the proximal renal tubules and the intestinal mucosa. 1. **Why Option B is False (The Correct Answer):** While neurological symptoms can occur during acute flares, **mental retardation is NOT a common or characteristic presentation** of Hartnup disease. Most affected individuals lead normal lives without cognitive impairment. If neurological symptoms appear, they usually manifest as intermittent cerebellar ataxia or emotional lability during periods of severe nutritional deficiency. 2. **Why other options are True:** * **Option A:** The primary pathology is a defect in the transport of **neutral amino acids** (e.g., Tryptophan, Alanine, Serine) in the gut and kidneys. * **Option C:** Interestingly, **most children remain asymptomatic** because dietary protein intake in modern diets is usually sufficient to compensate for the transport losses. * **Option D:** **Photosensitivity** is a hallmark. Tryptophan is a precursor for **Niacin (Vitamin B3)**. Malabsorption of tryptophan leads to niacin deficiency, resulting in a **pellagra-like skin rash** (dermatitis in sun-exposed areas). **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Marker:** Diagnosis is confirmed by **neutral aminoaciduria** (detected via chromatography). * **The "3 Ds" of Pellagra:** Dermatitis, Diarrhea, and Dementia (though in Hartnup, the full triad is rare). * **Treatment:** High-protein diet and **Nicotinamide (Vitamin B3)** supplementation. * **Differential Diagnosis:** Always differentiate from dietary Pellagra; Hartnup will show amino acids in the urine, whereas dietary deficiency will not.

Q: Niacin is synthesized from which amino acid?

Tryptophan. **Explanation:** **Tryptophan** is the correct answer because it serves as the precursor for the endogenous synthesis of **Niacin (Vitamin B3)** via the **Kynurenine pathway**. In humans, approximately **60 mg of Tryptophan** is required to synthesize **1 mg of Niacin**. This process is clinically significant because it requires **Vitamin B6 (Pyridoxine)** as a cofactor for the enzyme kynureninase; a deficiency in B6 can lead to secondary niacin deficiency. **Analysis of Incorrect Options:** * **Arginine (A):** This amino acid is the precursor for **Nitric Oxide (NO)**, Creatine, and Urea. * **Histidine (B):** It undergoes decarboxylation to form **Histamine**, a key mediator in allergic reactions and gastric acid secretion. * **Tyrosine (D):** It is the precursor for **Catecholamines** (Dopamine, Epinephrine, Norepinephrine), **Thyroid hormones** (T3, T4), and **Melanin**. **High-Yield Clinical Pearls for NEET-PG:** * **Hartnup Disease:** A genetic defect in the transport of neutral amino acids (including Tryptophan) in the gut and kidneys. It manifests with **Pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia) due to decreased niacin synthesis. * **Carcinoid Syndrome:** Large amounts of Tryptophan are diverted to produce **Serotonin**, leading to a deficiency in Niacin synthesis and subsequent Pellagra. * **Corn-based diets:** Populations relying solely on maize (corn) are at risk for Pellagra because corn is low in Tryptophan and its Niacin is in a bound, unabsorbable form (Niacytin).

Q: Which of the following is found in urine in patients with Alkaptonuria?

Homogentisic acid. **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. It is caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, which is responsible for converting **Homogentisic acid (HGA)** into maleylacetoacetic acid. Due to this block, HGA accumulates in the blood and is excreted in large amounts in the urine. 1. **Why Homogentisic acid is correct:** When urine containing HGA is exposed to air or an alkaline environment, it undergoes oxidation and polymerization to form a brownish-black pigment (alkapton). This leads to the classic clinical sign: **urine that turns black on standing.** 2. **Why other options are incorrect:** * **Phenylalanine:** Elevated in Phenylketonuria (PKU) due to Phenylalanine hydroxylase deficiency. * **Ornithine & Cystine:** These, along with Lysine and Arginine (COLA), are found in the urine of patients with **Cystinuria**, a defect in renal tubular reabsorption of dibasic amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The deposition of black pigment in connective tissues (ear cartilage, sclera, and heart valves). * **Ochronotic Arthritis:** Large joint arthritis (especially the spine and hips) is a common late-stage complication. * **Diagnosis:** Confirmed by detecting HGA in urine using **Ferric Chloride test** (transient deep blue color) or Silver Nitrate test. * **Management:** Low protein diet (restricted Tyrosine/Phenylalanine) and **Nitisinone**, which inhibits 4-hydroxyphenylpyruvate dioxygenase, reducing HGA production.

Q: What is true regarding glutamine?

It is involved in ammonia transport.. **Explanation:** **1. Why Option A is Correct:** Glutamine is the **primary non-toxic carrier of ammonia** in the blood. In peripheral tissues (especially the brain and lungs), ammonia is combined with glutamate by the enzyme **glutamine synthetase** to form glutamine. This process effectively "sequesters" free ammonia, which is highly neurotoxic. Glutamine then travels through the circulation to the liver or kidneys, where the enzyme **glutaminase** releases the ammonia for urea synthesis or excretion, respectively. **2. Why the Other Options are Incorrect:** * **Option B:** While glutamine can be synthesized within the brain, it does **not** freely cross the blood-brain barrier (BBB) in significant amounts from the blood to the brain. Conversely, its precursor, glutamate, is also restricted. The brain relies on local synthesis to manage ammonia levels. * **Option C:** Glutamine is **not toxic**; it is a neutral, non-toxic amino acid. It is actually the mechanism used to *prevent* the toxicity associated with free ammonia (NH₃). * **Option D:** Glutamine is primarily stored and synthesized in **skeletal muscle**, not smooth muscle. Skeletal muscle is the largest producer of glutamine in the body. **Clinical Pearls for NEET-PG:** * **Hyperammonemia Connection:** In hepatic encephalopathy, high levels of ammonia lead to excessive glutamine synthesis in astrocytes. This causes an osmotic imbalance, leading to **astrocytic swelling and cerebral edema**. * **Renal Role:** In the kidneys, glutamine metabolism is crucial for **acid-base balance**; the ammonia released helps buffer hydrogen ions (forming $NH_4^+$) to be excreted in urine. * **Most Abundant:** Glutamine is the most abundant free amino acid in human blood.

Q: Which substance gets accumulated in abnormal amounts in Alkaptonuria?

Homogentisate. **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. It is caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, which is responsible for converting homogentisic acid (homogentisate) into maleylacetoacetate. Due to this metabolic block, **homogentisate** accumulates in the blood and is excreted in large amounts in the urine. **Why the other options are incorrect:** * **Acetoacetate:** This is a ketone body and a downstream product of tyrosine metabolism. In Alkaptonuria, the pathway is blocked upstream, preventing its formation from this specific pathway. * **Oxalate:** Accumulation of oxalate is characteristic of Primary Hyperoxaluria, not tyrosine metabolism disorders. * **Phenylacetic acid:** This metabolite accumulates in Phenylketonuria (PKU) due to the alternative pathway of phenylalanine metabolism when phenylalanine hydroxylase is deficient. **High-Yield Clinical Pearls for NEET-PG:** 1. **Triad of Alkaptonuria:** * **Homogentisic aciduria:** Urine turns black upon standing or alkalinization (due to oxidation of homogentisate to benzoquinone acetate). * **Ochronosis:** Bluish-black pigmentation of connective tissues (ear cartilage, sclera) due to polymer deposition. * **Ochronotic Arthritis:** Large joint arthritis caused by pigment deposition in the cartilage. 2. **Diagnosis:** Confirmed by detecting homogentisic acid in urine using **Ferric Chloride test** (transient deep blue color) or Gas Chromatography-Mass Spectrometry (GC-MS). 3. **Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and **Nitisinone**, which inhibits 4-hydroxyphenylpyruvate dioxygenase, reducing homogentisate production.

Q: Cysteine is formed from which amino acids?

Methionine and serine. ### Explanation **Correct Option: A (Methionine and Serine)** Cysteine is a non-essential amino acid synthesized via the **transsulfuration pathway**. This process involves a unique "collaboration" between two amino acids: 1. **Methionine** provides the **sulfur atom**. Methionine is first converted to S-adenosylmethionine (SAM) and then to Homocysteine. 2. **Serine** provides the **carbon skeleton**. Homocysteine condenses with Serine to form Cystathionine (catalyzed by *Cystathionine $\beta$-synthase*). 3. Cystathionine is then cleaved by *Cystathionase* to release **Cysteine** and $\alpha$-ketobutyrate. **Why Incorrect Options are Wrong:** * **B, C, & D:** While Glycine and Alanine are involved in various metabolic pathways (like heme synthesis or gluconeogenesis), they do not contribute to the carbon skeleton or the sulfur group required for cysteine synthesis. Serine is specifically required for the carbon backbone, and Methionine is the only source of the thiol (-SH) group. **High-Yield Clinical Pearls for NEET-PG:** * **Essentiality:** Cysteine is considered "semi-essential." If Methionine intake is inadequate, Cysteine becomes essential. * **Rate-Limiting Enzyme:** *Cystathionine $\beta$-synthase* (CBS) is the key enzyme. It requires **Vitamin B6 (Pyridoxine)** as a cofactor. * **Homocystinuria:** A deficiency in CBS leads to Homocystinuria (Type I). Clinical features include ectopia lentis (downward dislocation), intellectual disability, and premature arterial thrombosis. * **Cystinuria:** This is a transport defect (COAL: Cystine, Ornithine, Arginine, Lysine) in the renal tubules, leading to hexagonal cysteine stones, not a metabolic synthesis defect.

Question 1

Homocystinuria is due to abnormal metabolism of

Accepted Answer

Methionine

Answer

Valine

Answer

Tryptophan

Answer

Leucine

Question 2

Tryptophan deficiency leads to which of the following conditions?

Accepted Answer

Hartnup disease

Answer

Her's disease

Answer

Lesch Nyhan syndrome

Answer

Alkaptonuria

Question 3

Which of the following statements is FALSE about alkaptonuria?

Accepted Answer

Genitourinary system is not involved.

Answer

There is a deficiency of homogentisic acid oxidase.

Answer

The urine turns black.

Answer

Calcification occurs in vertebral bodies.

Question 4

Which of the following is FALSE regarding Hartnup disease?

Accepted Answer

Mental retardation is the common presentation

Answer

Defect in neutral amino acid transport

Answer

Most children are asymptomatic

Answer

Photosensitivity

Question 5

Niacin is synthesized from which amino acid?

Accepted Answer

Tryptophan

Answer

Arginine

Answer

Histidine

Answer

Tyrosine

Question 6

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

Accepted Answer

Aromatic amino aciduria

Answer

Phenylketonuria

Answer

Intrauterine infection

Answer

Tyrosinemia

Question 7

Which of the following is found in urine in patients with Alkaptonuria?

Accepted Answer

Homogentisic acid

Answer

Phenylalanine

Answer

Ornithine

Answer

Cystine

Question 8

What is true regarding glutamine?

Accepted Answer

It is involved in ammonia transport.

Answer

It can cross the blood-brain barrier.

Answer

It is a toxic substance in the body.

Answer

It is stored in smooth muscle.

Question 9

Which substance gets accumulated in abnormal amounts in Alkaptonuria?

Accepted Answer

Homogentisate

Answer

Acetoacetate

Answer

Oxalate

Answer

Phenylacetic acid

Question 10

Cysteine is formed from which amino acids?

Accepted Answer

Methionine and serine

Answer

Methionine and glycine

Answer

Alanine and glycine

Answer

Serine and glycine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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