Amino Acid Metabolism Practice Questions

Q: Which of the following statements is true regarding the management of Phenylketonuria?

Limiting the substrate for the deficient enzyme. **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**. This enzyme normally converts the amino acid Phenylalanine into Tyrosine. **1. Why Option A is Correct:** In PKU, the metabolic block leads to the toxic accumulation of **Phenylalanine (the substrate)** in the blood and brain. High levels of phenylalanine are neurotoxic and lead to intellectual disability. Therefore, the primary management strategy is **limiting the substrate** by providing a diet low in Phenylalanine. This prevents the buildup of toxic metabolites like phenylpyruvate and phenyllactate. **2. Analysis of Incorrect Options:** * **Option B:** While Tyrosine becomes an "essential" amino acid in PKU and is supplemented, the statement "supplementing missing amino acids" is too vague. The hallmark of management is restriction, not just supplementation. * **Option C:** Although research is ongoing, **gene therapy** is not yet a standard or "successfully usable" clinical treatment for PKU in routine practice. * **Option D:** While regular cow's milk is high in phenylalanine and must be restricted, the child **cannot completely avoid milk**. Infants require specialized Phenylalanine-free medical formulas to ensure they receive adequate protein for growth. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** Due to phenylacetic acid in sweat and urine. * **Hypopigmentation:** Phenylalanine inhibits Tyrosinase, leading to fair skin and blue eyes. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high phenylalanine levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus. * **Sapropterin:** A synthetic form of BH4 (cofactor) used in BH4-responsive variants of PKU.

Q: Which of the following is a semi-essential amino acid?

Histidine. **Explanation:** Amino acids are classified based on their nutritional requirement into essential, non-essential, and semi-essential categories. **Why Histidine is the correct answer:** **Histidine** (along with Arginine) is classified as a **semi-essential (or conditionally essential)** amino acid. While the adult body can synthesize sufficient amounts to maintain nitrogen balance, the rate of synthesis is inadequate during periods of rapid growth, such as infancy, childhood, pregnancy, or recovery from major trauma. Therefore, dietary intake is mandatory during these specific physiological states to meet the increased demand. **Analysis of Incorrect Options:** * **A. Tyrosine:** This is a **non-essential** amino acid because it can be synthesized in the body from the essential amino acid Phenylalanine (via phenylalanine hydroxylase). * **C. Alanine:** This is a **non-essential** amino acid. It is easily synthesized via the transamination of pyruvate, a product of glycolysis. * **D. Isoleucine:** This is a strictly **essential** amino acid. It cannot be synthesized by the human body and must be obtained entirely through the diet. **High-Yield NEET-PG Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, **Histidine**, **Arginine**, Leucine, Lysine). Note that **H** and **A** are the semi-essential ones. * **Histidine Precursor:** It is the precursor for **Histamine** (via decarboxylation), a key mediator in allergic reactions and gastric acid secretion. * **Clinical Correlation:** Histidine levels are used in the **FIGLU excretion test** to diagnose Folic Acid deficiency.

Q: A 25-year-old male presented with complaints of bilateral knee, hip joints, and lower spine pain. On examination, the joints were erythematous and tender. Dark black spots were noted on the sclera and ear cartilage. X-rays of the affected joints were taken. Which of the following drugs has been approved for this condition?

Nitisinone. ### Explanation **Diagnosis: Alkaptonuria** The clinical triad of **large joint arthritis** (ochronotic arthropathy), **dark pigmentation of the sclera and ear cartilage** (ochronosis), and the history of dark spots suggests Alkaptonuria. This is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, leading to the accumulation of **Homogentisic Acid (HGA)**. HGA oxidizes to form a melanin-like polymer that deposits in connective tissues. **1. Why Nitisinone is correct:** **Nitisinone (Orfadin)** is a competitive inhibitor of the enzyme **4-hydroxyphenylpyruvate dioxygenase**. By inhibiting this upstream enzyme in the tyrosine catabolic pathway, it prevents the formation and subsequent accumulation of Homogentisic Acid. It was recently approved (by the EMA/FDA) as the first disease-modifying treatment for Alkaptonuria to slow the progression of ochronosis and arthropathy. **2. Why the other options are incorrect:** * **Ceftriaxone:** A third-generation cephalosporin used for bacterial infections (e.g., septic arthritis or Lyme disease), which does not fit the chronic pigmentary presentation. * **Nicotinamide:** Vitamin B3, used to treat Pellagra (Dermatitis, Diarrhea, Dementia). * **Bortezomib:** A proteasome inhibitor used in the treatment of Multiple Myeloma. **3. High-Yield Clinical Pearls for NEET-PG:** * **Classic Sign:** Urine turns black upon standing or alkalinization (due to oxidation of HGA). * **Biochemical Pathway:** Phenylalanine → Tyrosine → 4-Hydroxyphenylpyruvate → **Homogentisic Acid** →(Homogentisate Oxidase)→ Maleylacetoacetate. * **Radiology:** X-rays typically show **intervertebral disc calcification** and narrowing (wafer-like calcification), which is a hallmark of Alkaptonuria. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine intake.

Q: Homogentisate oxidase deficiency causes which of the following conditions?

Alkaptonuria. ### Explanation **Correct Answer: B. Alkaptonuria** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase (Homogentisate oxidase)**. This enzyme is essential in the catabolic pathway of phenylalanine and tyrosine, where it converts homogentisic acid (HGA) into maleylacetoacetate. When deficient, HGA accumulates in the body and is excreted in the urine. Upon exposure to air, HGA undergoes oxidation and polymerization to form a melanin-like pigment, causing the urine to turn **black**. **Analysis of Incorrect Options:** * **A. Phenylketonuria (PKU):** Caused by a deficiency of **Phenylalanine hydroxylase** (or rarely, Dihydrobiopterin reductase), leading to the accumulation of phenylalanine. * **C. Methylmalonic aciduria:** Results from a deficiency of **Methylmalonyl-CoA mutase** or a defect in Vitamin B12 metabolism, leading to the accumulation of methylmalonic acid. * **D. Albinism:** Most commonly caused by a deficiency of the enzyme **Tyrosinase**, which is required for the conversion of tyrosine to melanin. **High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The deposition of dark pigment in connective tissues (cartilage, sclera, skin) is a hallmark of Alkaptonuria, typically appearing in the 3rd or 4th decade of life. * **Ochronotic Arthritis:** Long-term accumulation of HGA in large joints (spine, hips, knees) leads to severe degenerative arthritis. * **Diagnostic Test:** Ferric chloride test (turns deep blue/green) or silver nitrate test. * **Management:** Dietary restriction of phenylalanine and tyrosine; **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase) is used to reduce HGA production.

Q: An infant is brought to a hospital because her wet diapers turn black when they are exposed to air. Physical examination is normal. Urine is positive both for reducing substance and when tested with ferric chloride. This disorder is caused by a deficiency of which of the following enzymes?

Homogentisic acid oxidase. **Explanation:** The clinical presentation of an infant whose diapers turn black upon exposure to air is the classic hallmark of **Alkaptonuria**. This autosomal recessive disorder is caused by a deficiency of **Homogentisic acid oxidase**, an enzyme in the phenylalanine and tyrosine catabolic pathway. **1. Why the Correct Answer is Right:** In Alkaptonuria, the deficiency of Homogentisic acid oxidase leads to the accumulation of **Homogentisic acid (HGA)**. When urine containing HGA is exposed to air, it undergoes oxidation and polymerization to form a brownish-black pigment called alkapton. HGA is a reducing agent, explaining the positive Benedict’s test (reducing substance), and it reacts with ferric chloride to produce a transient deep blue color. **2. Analysis of Incorrect Options:** * **B. Phenylalanine hydroxylase:** Deficiency causes **Phenylketonuria (PKU)**. Clinical features include intellectual disability, "mousy" body odor, and hypopigmentation, but not blackening of urine. * **C. L-histidine ammonialyase (Histidase):** Deficiency leads to **Histidinemia**. While it may show a positive ferric chloride test, it does not cause urine to darken. * **D. Ketoacid decarboxylase:** Deficiency of the Branched-chain alpha-keto acid dehydrogenase complex causes **Maple Syrup Urine Disease (MSUD)**, characterized by urine with a burnt-sugar odor and severe neurological distress. **Clinical Pearls for NEET-PG:** * **Ochronosis:** In adults, the pigment deposits in connective tissues (cartilage, sclera), leading to bluish-black discoloration. * **Arthritis:** Long-term accumulation leads to "Ochronotic arthritis," typically affecting large joints and the spine. * **Diagnosis:** Confirmed by detecting HGA in urine via Gas Chromatography-Mass Spectrometry (GC-MS). * **Dietary Management:** Restriction of Phenylalanine and Tyrosine; Nitisinone may be used to inhibit HGA production.

Q: Which of the following are considered conditionally essential amino acids?

Tyrosine and Cysteine. **Explanation:** **1. Why Tyrosine and Cysteine are correct:** Conditionally essential amino acids are those that can normally be synthesized by the body but must be provided in the diet under specific physiological or pathological conditions (e.g., prematurity, severe illness, or enzyme deficiencies). * **Tyrosine** is synthesized from **Phenylalanine**. * **Cysteine** is synthesized from **Methionine**. If the precursor essential amino acids (Phenylalanine or Methionine) are deficient, or if the metabolic pathway is impaired (e.g., Phenylketonuria), Tyrosine and Cysteine become "essential" and must be consumed through the diet. **2. Analysis of Incorrect Options:** * **Option B (Histidine and Arginine):** These are classified as **Semi-essential** amino acids. They are required in large amounts during periods of rapid growth (childhood and pregnancy) because endogenous synthesis is insufficient to meet the high demand. * **Option C (Leucine and Lysine):** These are **Purely Ketogenic** and **Strictly Essential** amino acids. They cannot be synthesized by the body at all. * **Option D (Phenylalanine and Tryptophan):** These are **Strictly Essential** aromatic amino acids. Phenylalanine is the precursor for Tyrosine. **3. NEET-PG Clinical Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phe, Val, Thr, Trp, Ile, Met, His, Arg, Leu, Lys). * **PKU Connection:** In Phenylketonuria (deficiency of Phenylalanine Hydroxylase), Tyrosine cannot be formed; thus, it becomes a mandatory dietary requirement (essential) for these patients. * **Cystinuria:** While Cysteine is conditionally essential, remember that *Cystine* (the dimer) is involved in the clinical pathology of renal stones.

Q: Transfer of an amino group from an amino acid to an alpha-keto acid is catalyzed by which of the following enzymes?

Transaminases. ### Explanation **Correct Option: A. Transaminases (Aminotransferases)** Transamination is the first step in the catabolism of most amino acids. It involves the transfer of an $\alpha$-amino group from an amino acid to an $\alpha$-keto acid (usually $\alpha$-ketoglutarate), resulting in the formation of a new amino acid (Glutamate) and a new $\alpha$-keto acid. This reaction is catalyzed by **Transaminases**. These enzymes require **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6, as a mandatory co-factor. **Why other options are incorrect:** * **B. Oxidases:** These enzymes catalyze oxidation-reduction reactions where oxygen serves as the electron acceptor (e.g., Amino acid oxidases), but they do not transfer amino groups between substrates. * **C. Transketolases:** These are enzymes of the Pentose Phosphate Pathway (HMP Shunt) that transfer two-carbon units between sugars. They require Thiamine Pyrophosphate (TPP) as a co-factor. * **D. Deaminases:** These enzymes catalyze **deamination**, which is the total removal of an amino group from an amino acid as free ammonia ($NH_3$), rather than its transfer to another keto acid. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Markers:** AST (SGOT) and ALT (SGPT) are transaminases used as sensitive markers of liver injury. ALT is more liver-specific, while AST is also found in cardiac and skeletal muscle. * **Exceptions:** All amino acids undergo transamination except **Lysine, Threonine, Proline, and Hydroxyproline**. * **The "Funnel" Effect:** Most amino acids transfer their amino group to $\alpha$-ketoglutarate to form **Glutamate**. Glutamate then undergoes oxidative deamination via *Glutamate Dehydrogenase* to release ammonia for the Urea Cycle. * **Co-factor Link:** Always associate Transaminases with **Vitamin B6 (PLP)**. A deficiency in B6 impairs amino acid metabolism.

Q: Serotonin is synthesized from which amino acid?

Tryptophan. **Explanation:** **Correct Answer: C. Tryptophan** Serotonin (5-hydroxytryptamine) is a key neurotransmitter synthesized from the essential amino acid **Tryptophan**. The synthesis occurs in a two-step pathway: 1. **Tryptophan hydroxylase** (the rate-limiting enzyme) converts Tryptophan to 5-hydroxytryptophan. This step requires **Tetrahydrobiopterin (BH4)** as a cofactor. 2. **Aromatic L-amino acid decarboxylase** converts 5-hydroxytryptophan to Serotonin (5-HT), requiring **Pyridoxal phosphate (Vitamin B6)**. **Analysis of Incorrect Options:** * **A. Tyrosine:** This is the precursor for Catecholamines (Dopamine, Norepinephrine, Epinephrine), Melanin, and Thyroid hormones (T3, T4). * **B. Alanine:** This is a non-essential amino acid primarily involved in the glucose-alanine cycle for transporting nitrogen to the liver; it does not serve as a precursor for serotonin. * **C. Glycine:** This is the simplest amino acid and serves as a precursor for Heme, Purines, Creatine, and Glutathione. **High-Yield Clinical Pearls for NEET-PG:** * **Melatonin Connection:** Serotonin is further converted into Melatonin in the pineal gland, making Tryptophan the ultimate precursor for both. * **Carcinoid Syndrome:** In patients with carcinoid tumors, up to 60% of dietary tryptophan is diverted to serotonin synthesis. This can lead to **Pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia) because there isn't enough tryptophan left to synthesize Niacin (Vitamin B3). * **Hartnup Disease:** A genetic defect in the transport of neutral amino acids (like Tryptophan) in the gut and kidneys, also resulting in Niacin deficiency.

Q: A 15-year-old boy develops symptoms of renal colic. The stone eventually passes spontaneously, but it is not recovered. The urinalysis reveals hexagonal crystals, and a cyanide-nitroprusside test on the urine is positive. Which of the following is the most likely diagnosis?

Cystinuria. ### Explanation **1. Why Cystinuria is the Correct Answer:** Cystinuria is an autosomal recessive defect in the **COLA** transporter (Cystine, Ornithine, Lysine, and Arginine) in the proximal convoluted tubule and GI tract. Since cystine is poorly soluble in acidic urine, it precipitates to form stones. * **Pathognomonic Finding:** The presence of **hexagonal (six-sided), flat, colorless crystals** on urinalysis is diagnostic. * **Screening Test:** The **Cyanide-nitroprusside test** is the classic screening tool. Sodium cyanide reduces cystine to cysteine, which then reacts with nitroprusside to produce a characteristic **magenta/purple color**. **2. Analysis of Incorrect Options:** * **B. Thalassemia:** While chronic hemolysis can lead to gallstones (pigment stones), it does not cause renal stones with hexagonal crystals or a positive nitroprusside test. * **C. Hereditary Glycosuria:** This is a benign condition involving a defect in the SGLT2 transporter. It presents with glucose in the urine despite normal blood glucose levels, not nephrolithiasis. * **D. Primary Hyperoxaluria:** This leads to calcium oxalate stones. These crystals are typically shaped like **envelopes (dihydrate)** or **dumbbells (monohydrate)**, not hexagons. **3. High-Yield Clinical Pearls for NEET-PG:** * **Stone Radiopacity:** Cystine stones are **radio-opaque** (due to sulfur content) but less dense than calcium stones. They often appear "ground-glass" on X-ray. * **Management:** Treatment focuses on high fluid intake, **urinary alkalinization** (pH > 7.5) to increase solubility, and chelating agents like **D-penicillamine** or Tiopronin for refractory cases [1]. Around 90% of stones of less than 4 mm diameter pass spontaneously [1]. * **Mnemonic:** Remember **"COLA"** for the involved amino acids and **"Six-sided Cystine"** for the crystal shape.

Question 1

Which of the following statements is true regarding the management of Phenylketonuria?

Accepted Answer

Limiting the substrate for the deficient enzyme

Answer

Supplementing the missing amino acids through diet

Answer

Gene therapy is a successfully usable treatment

Answer

The child should avoid milk consumption

Question 2

Which of the following is a semi-essential amino acid?

Accepted Answer

Histidine

Answer

Tyrosine

Answer

Alanine

Answer

Isoleucine

Question 3

In the biosynthesis of urea, one nitrogen atom is derived from ammonia while the other nitrogen atom is from:

Accepted Answer

Aspartate

Answer

Glycine

Answer

Alanine

Answer

Glutamate

Question 4

A 25-year-old male presented with complaints of bilateral knee, hip joints, and lower spine pain. On examination, the joints were erythematous and tender. Dark black spots were noted on the sclera and ear cartilage. X-rays of the affected joints were taken. Which of the following drugs has been approved for this condition?

Accepted Answer

Nitisinone

Answer

Ceftriaxone

Answer

Nicotinamide

Answer

Bortezomib

Question 5

Homogentisate oxidase deficiency causes which of the following conditions?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Methylmalonic aciduria

Answer

Albinism

Question 6

An infant is brought to a hospital because her wet diapers turn black when they are exposed to air. Physical examination is normal. Urine is positive both for reducing substance and when tested with ferric chloride. This disorder is caused by a deficiency of which of the following enzymes?

Accepted Answer

Homogentisic acid oxidase

Answer

Phenylalanine hydroxylase

Answer

L-histidine ammonialyase

Answer

Ketoacid decarboxylase

Question 7

Which of the following are considered conditionally essential amino acids?

Accepted Answer

Tyrosine and Cysteine

Answer

Histidine and Arginine

Answer

Leucine and Lysine

Answer

Phenylalanine and Tryptophan

Question 8

Transfer of an amino group from an amino acid to an alpha-keto acid is catalyzed by which of the following enzymes?

Accepted Answer

Transaminases

Answer

Oxidases

Answer

Transketolases

Answer

Deaminases

Question 9

Serotonin is synthesized from which amino acid?

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Alanine

Answer

Glycine

Question 10

A 15-year-old boy develops symptoms of renal colic. The stone eventually passes spontaneously, but it is not recovered. The urinalysis reveals hexagonal crystals, and a cyanide-nitroprusside test on the urine is positive. Which of the following is the most likely diagnosis?

Accepted Answer

Cystinuria

Answer

Thalassemia

Answer

Hereditary glycosuria

Answer

Primary hyperoxaluria

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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