Amino Acid Metabolism Practice Questions

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.

Q: A 39-year-old male presents with chronic knee pain, headache, nausea, epigastric pain, and blurred vision. Over the next 48 hours, his condition progresses to confusion, slowed speech, and a decline in Glasgow Coma Scale score, requiring intubation. He also experiences seizure activity. CT brain shows cerebral edema. Laboratory findings reveal an ammonia level of 652 mmol/L (reference range, < 50 mmol/L), a profound elevation of urine orotic acid, a high plasma glutamine level, and a low arginine level. What is the pattern of inheritance of this disease?

X-linked disorder. The clinical presentation of hyperammonemia, encephalopathy (confusion, seizures, cerebral edema), and specific biochemical markers points toward **Ornithine Transcarbamylase (OTC) Deficiency**, the most common urea cycle disorder [1]. **1. Why the Correct Answer is Right:** The key diagnostic clue is the combination of **hyperammonemia** and **markedly elevated urine orotic acid**. In the urea cycle, when OTC is deficient, carbamoyl phosphate accumulates in the mitochondria and leaks into the cytoplasm. There, it enters the pyrimidine synthesis pathway, leading to the overproduction of orotic acid. * **Inheritance:** Unlike all other urea cycle enzyme deficiencies, which are autosomal recessive, **OTC deficiency is an X-linked recessive disorder**. While it primarily affects males, female carriers can be symptomatic due to skewed X-inactivation (Lyonization) [1]. **2. Why Incorrect Options are Wrong:** * **Autosomal Recessive:** This is the inheritance pattern for other urea cycle disorders like Citrullinemia (Argininosuccinate synthetase deficiency) or Argininosuccinic aciduria [2]. However, these would not typically present with the specific "high orotic acid + low arginine" profile seen here. * **Autosomal Dominant:** Urea cycle disorders are metabolic enzyme deficiencies; these typically require a loss of both alleles (recessive) or are X-linked. They do not follow dominant inheritance patterns. **3. Clinical Pearls for NEET-PG:** * **The "Orotic Acid" Rule:** High Ammonia + High Orotic Acid = OTC Deficiency. High Ammonia + Low Orotic Acid = Carbamoyl Phosphate Synthetase (CPS-1) Deficiency. * **Management:** Acute management involves stopping protein intake, administering IV glucose, and using nitrogen scavengers (Sodium benzoate/phenylacetate). Arginine supplementation is essential (except in Arginase deficiency). * **Triggers:** In adults, symptoms can be triggered by high-protein meals, GI bleeds, or postpartum stress.

Q: A patient is investigated for renal failure. Which of the following conditions is associated with boiled cabbage or rancid butter smelling urine in this patient?

Tyrosinemia Type 1. **Explanation:** **Tyrosinemia Type 1 (Hepatorenal Tyrosinemia)** is the correct answer. This autosomal recessive disorder is caused by a deficiency of the enzyme **Fumarylacetoacetate hydrolase (FAH)**, the final enzyme in the tyrosine degradation pathway. The deficiency leads to the accumulation of fumarylacetoacetate and maleylacetoacetate, which are converted into **succinylacetone**. Succinylacetone is pathognomonic for this condition and is highly toxic to the liver and kidneys, leading to acute liver failure and renal tubular dysfunction (Fanconi syndrome). The characteristic **"boiled cabbage" or "rancid butter" odor** is attributed to the accumulation of methionine metabolites or specific tyrosine breakdown products. **Analysis of Incorrect Options:** * **Phenylketonuria (PKU):** Associated with a **"mousy" or "musty" odor** due to the accumulation of phenylacetic acid. * **Diabetic Ketoacidosis (DKA):** Characterized by a **"fruity" or "sweet" odor** of the breath and urine due to the presence of acetone. * **Multiple Carboxylase Deficiency:** Often associated with a **"tom-cat urine" odor** due to the accumulation of 3-hydroxyisovaleric acid. **High-Yield Clinical Pearls for NEET-PG:** * **Tyrosinemia Type 1:** Most severe form; involves liver and kidney; marker is **Succinylacetone**; treatment is **Nitisinone (NTBC)**. * **Tyrosinemia Type 2 (Oculocutaneous):** Deficiency of Tyrosine Aminotransferase; presents with palmoplantar keratosis and corneal ulcers. * **Maple Syrup Urine Disease (MSUD):** "Burnt sugar" or "Maple syrup" odor. * **Isovaleric Acidemia:** "Sweaty feet" or "Cheesy" odor.

Question 1

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 2

Homogentisate oxidase deficiency causes which of the following conditions?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Methylmalonic aciduria

Answer

Albinism

Question 3

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 4

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 5

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 6

Serotonin is synthesized from which amino acid?

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Alanine

Answer

Glycine

Question 7

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

Question 8

A 39-year-old male presents with chronic knee pain, headache, nausea, epigastric pain, and blurred vision. Over the next 48 hours, his condition progresses to confusion, slowed speech, and a decline in Glasgow Coma Scale score, requiring intubation. He also experiences seizure activity. CT brain shows cerebral edema. Laboratory findings reveal an ammonia level of 652 mmol/L (reference range, < 50 mmol/L), a profound elevation of urine orotic acid, a high plasma glutamine level, and a low arginine level. What is the pattern of inheritance of this disease?

Accepted Answer

X-linked disorder

Answer

Autosomal dominant

Answer

Autosomal recessive

Answer

None of the above

Question 9

A patient is investigated for renal failure. Which of the following conditions is associated with boiled cabbage or rancid butter smelling urine in this patient?

Accepted Answer

Tyrosinemia Type 1

Answer

Phenylketonuria

Answer

Diabetic Ketoacidosis

Answer

Multiple carboxylase deficiency

Question 10

Glutamine is increased in CSF, blood, and urine in which defect?

Accepted Answer

CPS-I (Carbamoyl Phosphate Synthetase I)

Answer

Arginosuccinate synthetase

Answer

OTC (Ornithine Transcarbamylase)

Answer

Arginase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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