Amino Acid Metabolism Practice Questions

Q: Which of the following is NOT a feature of Hartnup's disease?

Mental retardation. **Explanation:** **Hartnup’s disease** is an autosomal recessive disorder characterized by a defect in the **SLC6A19 transporter**, which is responsible for the absorption of neutral amino acids (especially **Tryptophan**) in the renal proximal tubules and the small intestine. **Why "Mental Retardation" is the correct answer:** While patients may experience transient neurological symptoms during flares, **permanent mental retardation is NOT a characteristic feature** of Hartnup’s disease. This distinguishes it from other metabolic disorders like Phenylketonuria (PKU). Most patients lead a normal life with proper nutrition. **Analysis of other options:** * **Cutaneous photosensitivity (B):** Tryptophan is a precursor for **Niacin (Vitamin B3)**. Deficiency leads to "Pellagra-like" skin rashes, specifically a scaly, erythematous eruption on sun-exposed areas. * **Cerebellar ataxia (A):** Intermittent neurological symptoms, such as unsteady gait and incoordination, occur during periods of severe tryptophan depletion or metabolic stress. * **Psychological disturbance (D):** Patients often present with emotional lability, irritability, or even delirium during acute exacerbations. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmark:** Neutral aminoaciduria (specifically excluding proline, hydroxyproline, and arginine). * **The 3 D's:** Symptoms mimic Pellagra (Dermatitis, Diarrhea, Dementia/Depression), but the cause is amino acid transport failure, not primary dietary niacin deficiency. * **Diagnosis:** High levels of neutral amino acids in urine (Chromatography). * **Treatment:** High-protein diet and **Nicotinamide** supplementation. * **Indicanuria:** Unabsorbed tryptophan in the gut is converted by bacteria into indoles, which are excreted in urine (blue diaper syndrome variant).

Q: Which condition is characterized by the smell of sweaty feet?

Glutaric acidemia. ### Explanation The correct answer is **Glutaric acidemia (specifically Type II)**. #### 1. Why Glutaric Acidemia is Correct Glutaric acidemia Type II (Multiple Acyl-CoA Dehydrogenase Deficiency) is a metabolic disorder where the body cannot properly process certain fats and proteins. This leads to the accumulation of organic acids, specifically **isovaleric and glutaric acids**. The characteristic **"sweaty feet" odor** is primarily attributed to the accumulation of **isovaleric acid**. Note: While Isovaleric acidemia is the classic cause of this odor, Glutaric acidemia Type II presents similarly due to the shared metabolic pathway disruption. #### 2. Why Other Options are Incorrect * **A. Maple syrup urine disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. It is characterized by a **burnt sugar or maple syrup** odor in the urine. * **B. Phenylketonuria (PKU):** Caused by a deficiency of Phenylalanine Hydroxylase. It is characterized by a **mousy or musty** odor. * **C. Homocystinuria:** A defect in cystathionine beta-synthase. It does not typically present with a specific diagnostic odor but is associated with marfanoid habitus and ectopia lentis. #### 3. High-Yield Clinical Pearls for NEET-PG Memorizing characteristic odors is essential for metabolic biochemistry questions: * **Sweaty Feet:** Isovaleric acidemia, Glutaric acidemia Type II. * **Mousy/Musty:** Phenylketonuria (PKU). * **Maple Syrup/Burnt Sugar:** MSUD. * **Rotten Fish:** Trimethylaminuria. * **Cabbage-like/Rancid Butter:** Tyrosinemia Type I. * **Swimming Pool odor:** Hawkinsinuria. * **Boiled Cabbage:** Hypermethioninemia.

Q: Urea is formed in:

Liver. **Explanation:** The **Liver** is the primary site for the urea cycle (Ornithine cycle) because it is the only organ that expresses all the necessary enzymes in significant quantities. Specifically, the enzyme **Arginase**, which catalyzes the final step of cleaving Arginine into Urea and Ornithine, is almost exclusively found in the liver. This process is vital for detoxifying ammonia—a neurotoxic byproduct of protein catabolism—by converting it into water-soluble urea for excretion. **Why other options are incorrect:** * **Brain:** While the brain produces ammonia, it lacks the full complement of urea cycle enzymes. It detoxifies ammonia primarily by converting it to **Glutamine** via Glutamine Synthetase. * **Kidney:** The kidney is responsible for the **excretion** of urea, not its synthesis. While it can synthesize small amounts of Arginine, it lacks sufficient Arginase to complete the urea cycle. * **Intestine:** The intestine participates in nitrogen metabolism (producing ammonia via bacterial action), but it does not possess the enzymatic machinery to produce urea. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **Subcellular location:** The cycle is "split"; the first two steps occur in the **Mitochondria**, while the remaining steps occur in the **Cytosol**. * **Hyperammonemia:** Deficiency of any urea cycle enzyme leads to ammonia toxicity. **Ornithine Transcarbamoylase (OTC) deficiency** is the most common urea cycle disorder and is X-linked recessive. * **BUN (Blood Urea Nitrogen):** Levels rise in renal failure (Azotemia) but decrease in severe liver disease due to impaired synthesis.

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.

Question 1

Which of the following is NOT a feature of Hartnup's disease?

Accepted Answer

Mental retardation

Answer

Cerebellar ataxia

Answer

Cutaneous photosensitivity

Answer

Psychological disturbance

Question 2

Which condition is characterized by the smell of sweaty feet?

Accepted Answer

Glutaric acidemia

Answer

Maple syrup urine disease (MSUD)

Answer

Phenylketonuria

Answer

Homocystinuria

Question 3

Urea is formed in:

Accepted Answer

Liver

Answer

Brain

Answer

Kidney

Answer

Intestine

Question 4

Homocystinuria is due to abnormal metabolism of

Accepted Answer

Methionine

Answer

Valine

Answer

Tryptophan

Answer

Leucine

Question 5

Tryptophan deficiency leads to which of the following conditions?

Accepted Answer

Hartnup disease

Answer

Her's disease

Answer

Lesch Nyhan syndrome

Answer

Alkaptonuria

Question 6

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 7

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 8

Niacin is synthesized from which amino acid?

Accepted Answer

Tryptophan

Answer

Arginine

Answer

Histidine

Answer

Tyrosine

Question 9

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 10

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

Accepted Answer

Homogentisic acid

Answer

Phenylalanine

Answer

Ornithine

Answer

Cystine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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