Amino Acid Metabolism Practice Questions

Q: Increased 5-hydroxyindoleacetic acid in urine is seen in which of the following conditions?

Carcinoid syndrome. **Explanation:** **Correct Answer: C. Carcinoid syndrome** The underlying medical concept here is the metabolism of **Serotonin (5-HT)**. Carcinoid tumors, typically arising from enterochromaffin cells in the midgut, overproduce serotonin. In the body, serotonin is metabolized by the enzyme **Monoamine Oxidase (MAO)** into **5-hydroxyindoleacetic acid (5-HIAA)**, which is then excreted in the urine. Therefore, a 24-hour urinary 5-HIAA test is the gold-standard biochemical marker for diagnosing and monitoring Carcinoid syndrome. **Analysis of Incorrect Options:** * **A. Pheochromocytoma:** This is a tumor of the adrenal medulla characterized by excess catecholamines (Epinephrine/Norepinephrine). The urinary markers for this condition are **Vanillylmandellic acid (VMA)** and **Metanephrines**, not 5-HIAA. * **B. Ogilvie syndrome:** Also known as acute colonic pseudo-obstruction, this is a clinical condition involving massive dilation of the colon without mechanical obstruction. It has no specific biochemical association with serotonin metabolism. * **D. Tumor lysis syndrome:** This occurs due to rapid breakdown of cancer cells. It is characterized by hyperuricemia (increased **Uric acid**), hyperkalemia, hyperphosphatemia, and hypocalcemia. **High-Yield Clinical Pearls for NEET-PG:** * **Dietary Caution:** Patients must avoid serotonin-rich foods (bananas, walnuts, pineapples, avocados) for 72 hours before the 5-HIAA test to prevent false positives. * **Pellagra Connection:** In Carcinoid syndrome, up to 60% of dietary **Tryptophan** is diverted to serotonin synthesis (normally only 1%). This leads to a deficiency in Niacin (Vitamin B3) production, potentially causing **Pellagra** (Dermatitis, Diarrhea, Dementia). * **Clinical Triad:** Flushing, Diarrhea, and Right-sided heart failure (Tricuspid regurgitation/Pulmonary stenosis).

Q: A 9-year-old female child presented with polyuria, polydipsia, and metabolic acidosis. On slit lamp examination, crystal deposits are seen in the cornea. What is the diagnosis?

Cystinosis. ### Explanation **Correct Answer: B. Cystinosis** **Mechanism:** Cystinosis is an autosomal recessive lysosomal storage disorder caused by a mutation in the **CTNS gene**, which encodes **cystinosin**, a lysosomal membrane transporter. Defective transport leads to the accumulation of **cystine crystals** within lysosomes across various tissues. The clinical presentation in this case points to **Fanconi Syndrome** (polyuria, polydipsia, and metabolic acidosis due to proximal renal tubular dysfunction), which is the hallmark of infantile nephropathic cystinosis. The pathognomonic finding is the presence of **hexagonal cystine crystals in the cornea** on slit-lamp examination. --- ### Why the other options are incorrect: * **A. Cystinuria:** This is a defect in the renal tubular reabsorption of COLA amino acids (Cystine, Ornithine, Lysine, Arginine). It presents with **cystine stones (calculi)** in the urinary tract, not systemic crystal deposition or Fanconi syndrome. * **C. Cystathioninuria:** An autosomal recessive deficiency of cystathionase. It is generally considered a benign metabolic anomaly with no specific clinical symptoms like corneal crystals or polyuria. * **D. Homocystinuria:** Caused by cystathionine β-synthase deficiency. It presents with ectopia lentis (downward dislocation), intellectual disability, and thromboembolism, but **not** corneal crystal deposition. --- ### NEET-PG High-Yield Pearls: * **Diagnosis:** Confirmed by measuring **intracellular cystine levels** in polymorphonuclear leukocytes. * **Treatment:** **Cysteamine** (a cystine-depleting agent) is the drug of choice. * **Renal Pathology:** Cystinosis is the most common cause of inherited Fanconi Syndrome in children. * **Biochemical Note:** Do not confuse *Cystine* (dimer of cysteine) with *Cysteine* (monomer). In cystinosis, it is the dimer that accumulates.

Q: Development of pellagra-like skin lesions in carcinoid syndrome is primarily due to:

Inadequate conversion of tryptophan to niacin. ### Explanation **Underlying Medical Concept:** In a healthy individual, approximately **1% of dietary Tryptophan** is used to synthesize Serotonin (5-HT), while the majority is utilized for protein synthesis and the production of **Niacin (Vitamin B3)** via the Kynurenine pathway. In **Carcinoid Syndrome** (typically originating from neuroendocrine tumors of the midgut), the tumor cells become metabolically hyperactive. They divert up to **60% of the body's Tryptophan** into the synthesis of Serotonin. This massive "biochemical shift" creates a profound systemic deficiency of Tryptophan available for the Kynurenine pathway. Since Tryptophan is a precursor for Niacin, this diversion leads to secondary Niacin deficiency, manifesting clinically as **Pellagra** (Dermatitis, Diarrhea, Dementia). **Analysis of Options:** * **Option A (Correct):** The diversion of the precursor (Tryptophan) to the Serotonin pathway directly limits the endogenous synthesis of Niacin. * **Option B:** While the symptoms resemble dietary deficiency, the root cause in carcinoid syndrome is metabolic diversion, not a lack of intake. * **Option C:** The rash is not a direct effect of serotonin on the skin; it is a result of the underlying vitamin deficiency. * **Option D:** While carcinoid syndrome is a paraneoplastic process, the specific mechanism for the *pellagra-like lesions* is the biochemical depletion of Tryptophan. **High-Yield Clinical Pearls for NEET-PG:** * **The 60mg Rule:** 60 mg of Tryptophan yields 1 mg of Niacin. * **Diagnostic Marker:** Elevated urinary **5-HIAA** (5-Hydroxyindoleacetic acid), the breakdown product of serotonin. * **Other causes of Pellagra:** Hartnup disease (impaired Tryptophan absorption) and Isoniazid (INH) therapy (depletes Vitamin B6, a cofactor for Niacin synthesis). * **Classic Triad of Carcinoid:** Flushing, Diarrhea, and Right-sided heart failure (Tricuspid regurgitation).

Q: Hartnup disease can present with which of the following clinical features?

Pellagra-like symptoms. **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. This defect leads to the impaired intestinal absorption and renal tubular reabsorption of neutral amino acids, most significantly **Tryptophan**. 1. **Why Option A is Correct:** Tryptophan is a vital precursor for the endogenous synthesis of **Niacin (Vitamin B3)**. In Hartnup disease, the deficiency of Tryptophan results in a secondary deficiency of Niacin. This manifests clinically as **Pellagra-like symptoms**, characterized by the "3 Ds": Dermatitis (photosensitive scaly rash), Diarrhea, and Dementia (ataxia and neurological symptoms). 2. **Why Other Options are Incorrect:** * **B. Nephrolithiasis:** This is characteristic of **Cystinuria**, where there is a defect in the transport of COAL (Cystine, Ornithine, Arginine, Lysine), leading to cystine stones. * **C. Protein intolerance:** This is typically seen in **Urea Cycle Disorders** (e.g., OTC deficiency), where protein ingestion leads to hyperammonemia. * **D. Microcephaly:** This is a common feature of **Maternal PKU** or certain TORCH infections, but not a primary feature of neutral amino acid transport defects. **Clinical Pearls for NEET-PG:** * **Diagnosis:** Confirmed by detecting **neutral aminoaciduria** (specifically Tryptophan) in the urine using paper chromatography. * **Treatment:** High-protein diet and **Nicotinamide (Niacin) supplementation**. * **Distinction:** Unlike dietary Pellagra, Hartnup disease will show high levels of neutral amino acids in the urine.

Q: Maple syrup urine disease is characterized by all except?

Hypopigmentation. **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 leads to the accumulation of branched-chain amino acids (BCAAs)—**Leucine, Isoleucine, and Valine**—and their corresponding alpha-keto acids in the blood and urine. **Why Hypopigmentation is the Correct Answer:** Hypopigmentation is **not** a feature of MSUD. It is a classic clinical finding in **Phenylketonuria (PKU)**. In PKU, the deficiency of Phenylalanine Hydroxylase leads to decreased Tyrosine production; since Tyrosine is a precursor for melanin, patients exhibit fair skin and light-colored hair. MSUD does not involve the melanin pathway. **Analysis of Other Options:** * **Hypotonia & Hypertonia:** MSUD typically presents in the first week of life with progressive neurological deterioration. Infants often exhibit alternating bouts of **hypotonia** (floppiness) and **hypertonia** (rigidity/opisthotonus). * **Pancreatitis:** Acute and recurrent pancreatitis is a recognized, though less common, systemic complication of MSUD, likely due to metabolic derangements and toxic accumulation of keto acids. **High-Yield Clinical Pearls for NEET-PG:** * **Odor:** Urine has a characteristic **burnt sugar/maple syrup** smell due to the accumulation of **S-isoleucine**. * **Diagnosis:** Elevated levels of BCAAs in plasma and the presence of **alloisoleucine** (pathognomonic). * **Screening:** Positive **2,4-Dinitrophenylhydrazine (DNPH)** test in urine. * **Treatment:** Dietary restriction of BCAAs and, in some cases, **Thiamine (Vitamin B1)** supplementation (as B1 is a cofactor for the BCKAD complex).

Q: Urea, creatinine, and nitric oxide are synthesized from which amino acid?

Arginine. **Explanation:** **Arginine** is a versatile, semi-essential amino acid that serves as a precursor for several biologically vital molecules. It is the correct answer because: 1. **Urea:** In the final step of the Urea Cycle, the enzyme **Arginase** cleaves Arginine into Urea and Ornithine. 2. **Nitric Oxide (NO):** Arginine is the sole precursor of NO, a potent vasodilator. The enzyme **Nitric Oxide Synthase (NOS)** converts L-arginine to L-citrulline and NO. 3. **Creatinine:** Arginine provides the amidino group to Glycine (forming Guanidinoacetate) in the first step of creatine synthesis, which eventually cyclizes to form creatinine. **Why other options are incorrect:** * **Alanine:** Primarily involved in the glucose-alanine cycle for transporting nitrogen to the liver; it is a major gluconeogenic amino acid. * **Aspartate:** While it provides the second nitrogen atom for the Urea Cycle (joining with Citrulline to form Argininosuccinate), it is not the direct precursor for NO or Creatinine. * **Glycine:** Although Glycine is required for **Creatine** and **Heme** synthesis, it does not contribute to the synthesis of Urea or Nitric Oxide. **High-Yield Clinical Pearls for NEET-PG:** * **Arginine** also stimulates the release of Growth Hormone, Insulin, and Glucagon. * **Creatine Synthesis:** Requires three amino acids: **G**lycine, **A**rginine, and **M**ethionine (as S-adenosylmethionine). Remember the mnemonic: **"GAM"**. * **Hyperargininemia:** A deficiency of Arginase leads to elevated arginine levels, causing spastic diplegia and developmental delay (unlike other urea cycle defects which present with severe neonatal hyperammonemia).

Q: A 40-year-old female presents with urine darkening on standing, joint pain and stiffness, and pigment deposition in joints. What is the probable diagnosis?

Alkaptonuria. **Explanation:** The clinical presentation of **urine darkening on standing**, joint stiffness, and pigment deposition (ochronosis) is pathognomonic for **Alkaptonuria**. **1. Why Alkaptonuria is Correct:** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**. This leads to the accumulation of **Homogentisic Acid (HGA)**. * **Urine Findings:** When urine is exposed to air (standing), HGA oxidizes into a melanin-like polymer, turning the urine black. * **Ochronosis:** HGA binds to connective tissues (cartilage, sclera, ears), causing bluish-black pigmentation. * **Arthropathy:** Long-term deposition in large joints (spine, hips, knees) leads to degenerative "Ochronotic arthritis," typically manifesting in the 4th decade of life. **2. Why Other Options are Incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. It presents with intellectual disability, "mousy" body odor, and hypopigmentation, not joint pigment deposition. * **Tyrosinemia:** Involves defects in the tyrosine catabolic pathway (e.g., Fumarylacetoacetate hydrolase). It primarily presents with liver failure, renal rickets (Fanconi syndrome), and a "cabbage-like" odor, but not black urine or ochronosis. **3. NEET-PG High-Yield Pearls:** * **Radiology:** Characterized by **intervertebral disc calcification** and narrowing at multiple levels (bamboo spine appearance, but unlike Ankylosing Spondylitis, it spares the sacroiliac joints). * **Diagnosis:** Confirmed by detecting Homogentisic acid in urine using **Gas Chromatography-Mass Spectrometry (GC-MS)**. * **Screening:** Benedict’s test is positive (reducing sugar), and Ferric Chloride test gives a transient deep blue/green color.

Q: A child presents with pellagra-like symptoms, aminoaciduria, and a family history of one affected sibling and three unaffected siblings, while the parents are normal. What is the diagnosis?

Hartnup disease. ### Explanation **Correct Option: D. Hartnup disease** **Underlying Concept:** Hartnup disease is an **autosomal recessive** disorder caused by a mutation in the **SLC6A19 gene**, which encodes a sodium-dependent neutral amino acid transporter in the proximal renal tubules and the small intestine. This leads to the malabsorption and excessive urinary loss of neutral amino acids, most notably **Tryptophan**. Tryptophan is a precursor for **Niacin (Vitamin B3)** synthesis. A deficiency in Tryptophan results in secondary Niacin deficiency, manifesting as **Pellagra-like symptoms** (the 3 Ds: Dermatitis, Diarrhea, and Dementia). The family history described (normal parents, affected siblings) is classic for an autosomal recessive inheritance pattern. **Why other options are incorrect:** * **A. Phenylketonuria (PKU):** Caused by a deficiency of Phenylalanine Hydroxylase. It presents with intellectual disability, "mousy" body odor, and hypopigmentation, not pellagra-like rashes. * **B. Alkaptonuria:** A deficiency of Homogentisate Oxidase. It is characterized by urine that turns black upon standing, ochronosis (bluish-black pigmentation of connective tissue), and arthritis. * **C. Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. It presents in neonates with poor feeding, seizures, and a characteristic "maple syrup" odor in the urine. **NEET-PG High-Yield Pearls:** * **Clinical Triad of Hartnup:** Photosensitive skin rash (Pellagra-like), cerebellar ataxia, and neutral aminoaciduria. * **Diagnosis:** Confirmed by detecting neutral amino acids (Alanine, Valine, Threonine, Isoleucine, Leucine, Phenylalanine, Tryptophan, Tyrosine, Histidine) in the urine via chromatography. * **Treatment:** High-protein diet and **Nicotinamide (Niacin)** supplementation. * **Key differentiator:** Unlike dietary Pellagra, Hartnup disease shows significant **aminoaciduria**.

Q: A mousy odour in urine is characteristically seen in which of the following conditions?

Phenylketonuria. **Explanation:** **Phenylketonuria (PKU)** is the correct answer. This autosomal recessive disorder is caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)** or its cofactor **Tetrahydrobiopterin (BH4)**. This deficiency leads to the accumulation of Phenylalanine, which is alternatively metabolized into phenylketones (such as **phenylacetate**, phenylpyruvate, and phenyllactate). The **mousy or musty odor** characteristic of the urine in PKU patients is specifically attributed to **phenylacetate**. **Analysis of Incorrect Options:** * **A. Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. It results in urine that smells like **maple syrup or burnt sugar** due to the accumulation of isoleucine. * **C. Isovaleric Acidemia:** A defect in Isovaleryl-CoA dehydrogenase leads to the accumulation of isovaleric acid, which imparts a characteristic **"sweaty feet" or "cheesy" odor** to the urine. * **D. Cystinuria:** A transport defect of COLA (Cystine, Ornithine, Lysine, Arginine) in the proximal renal tubule. While it leads to hexagonal cystine crystals and stones, it typically presents with a **sulfurous (rotten egg)** smell, if any. **High-Yield Clinical Pearls for NEET-PG:** * **PKU Presentation:** Intellectual disability, hypopigmentation (fair skin/blue eyes due to decreased melanin), and seizures. * **Diagnostic Test:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Other Odors to Remember:** * **Tyrosinemia:** Boiled cabbage/Rancid butter. * **Trimethylaminuria:** Fishy odor. * **Multiple Carboxylase Deficiency:** Tomcat urine.

Question 1

Increased 5-hydroxyindoleacetic acid in urine is seen in which of the following conditions?

Accepted Answer

Carcinoid syndrome

Answer

Pheochromocytoma

Answer

Ogilvie syndrome

Answer

Tumor lysis syndrome

Question 2

A 9-year-old female child presented with polyuria, polydipsia, and metabolic acidosis. On slit lamp examination, crystal deposits are seen in the cornea. What is the diagnosis?

Accepted Answer

Cystinosis

Answer

Cystinuria

Answer

Cystothioninuria

Answer

Homocysteinuria

Question 3

A 25-year-old male presented with pigmentation of the nose and pinna. After voiding, his urine becomes dark. What spinal abnormality is most likely to be present?

Accepted Answer

Calcification of the disc

Answer

Atlantoaxial subluxation

Answer

Spondyloptosis

Answer

Basilar invagination

Question 4

Development of pellagra-like skin lesions in carcinoid syndrome is primarily due to:

Accepted Answer

Inadequate conversion of tryptophan to niacin

Answer

Dietary niacin deficiency

Answer

Serotonin-induced skin rash mimicking pellagra

Answer

Paraneoplastic manifestation

Question 5

Hartnup disease can present with which of the following clinical features?

Accepted Answer

Pellagra-like symptoms

Answer

Nephrolithiasis

Answer

Protein intolerance

Answer

Microcephaly

Question 6

Maple syrup urine disease is characterized by all except?

Accepted Answer

Hypopigmentation

Answer

Hypotonia

Answer

Hypertonia

Answer

Pancreatitis

Question 7

Urea, creatinine, and nitric oxide are synthesized from which amino acid?

Accepted Answer

Arginine

Answer

Alanine

Answer

Aspartate

Answer

Glycine

Question 8

A 40-year-old female presents with urine darkening on standing, joint pain and stiffness, and pigment deposition in joints. What is the probable diagnosis?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Tyrosinemia

Answer

Tyrosinemia

Question 9

A child presents with pellagra-like symptoms, aminoaciduria, and a family history of one affected sibling and three unaffected siblings, while the parents are normal. What is the diagnosis?

Accepted Answer

Hartnup disease

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Maple syrup urine disease

Question 10

A mousy odour in urine is characteristically seen in which of the following conditions?

Accepted Answer

Phenylketonuria

Answer

Maple syrup urine disease

Answer

Isovaleric acidemia

Answer

Cystinuria

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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