Amino Acid Metabolism Practice Questions

Q: A neonate presents with seizures, low citrulline levels, hyperammonemia, normal orotic acid levels, and high glutamine. What is the most likely diagnosis?

Type I hyperammonemia. ***Type I hyperammonemia*** - Caused by a deficiency of **carbamoyl phosphate synthetase I (CPS I)**, which is the rate-limiting step of the urea cycle. - This block prevents the formation of carbamoyl phosphate, leading to **low orotic acid** and **low citrulline** levels, distinguishing it from OTC deficiency. *Type II hyperammonemia* - Caused by a deficiency of **ornithine transcarbamylase (OTC)**, the most common urea cycle disorder. - This condition is characterized by **elevated orotic acid** in blood and urine due to the accumulation of carbamoyl phosphate being shunted into the pyrimidine synthesis pathway. *Citrullinemia* - Results from a deficiency of **argininosuccinate synthetase**, preventing the condensation of citrulline and aspartate. - It classically presents with **markedly elevated citrulline levels** in the plasma, which contradicts the low citrulline finding in this patient. *Argininosuccinic aciduria* - Caused by a deficiency of **argininosuccinate lyase**, leading to the accumulation of argininosuccinate. - Patients typically show **elevated levels of argininosuccinate** and citrulline, rather than the low citrulline profile observed here.

Q: Which of the following amino acid classifications are correctly matched? 1. Uncharged polar AA - Cysteine 2. Nonpolar AA - Histidine 3. Basic AA - Proline 4. Basic AA - Lysine

A and D. ***A and D*** - **Cysteine** is correctly classified as an **uncharged polar amino acid** because it contains a sulfhydryl (-SH) group that is weakly polar. - **Lysine** is correctly classified as a **basic amino acid** due to the presence of an amino group in its side chain, which carries a positive charge at physiological pH. *A and B* - This option is incorrect because, while Cysteine is correctly identified, **Histidine** is classified as a **basic amino acid**, not a nonpolar one, due to its imidazole ring. *C and D* - This option is incorrect because **Proline** is classified as a **nonpolar amino acid**, not a basic one; its unique cyclic structure makes it rigid and hydrophobic. *B and C* - This option is incorrect because both classifications provided are wrong: **Histidine** is **basic** and **Proline** is **nonpolar**.

Q: Which coenzyme is essential for carrying out transamination reactions in amino acid metabolism?

Pyridoxal phosphate (PLP). ***Pyridoxal phosphate (PLP)*** - PLP is the active form of **Vitamin B6** and serves as the essential cofactor for all **aminotransferases** (transaminases) that catalyze the transfer of amino groups. - The aldehyde group of PLP forms a covalent **Schiff base** intermediate with the amino acid substrate, facilitating the migration of the amino group. *Thiamine pyrophosphate (TPP)* - TPP, derived from **Vitamin B1**, acts primarily as a coenzyme for **decarboxylation** reactions, such as those performed by pyruvate dehydrogenase. - It is not involved in the transfer of amino groups between amino acids and alpha-keto acids. *Nicotinamide adenine dinucleotide (NAD)* - NAD is a coenzyme derived from **niacin (Vitamin B3)** that functions primarily as an electron carrier in **redox reactions**. - While it participates in the oxidative deamination of glutamate by glutamate dehydrogenase, it is not the primary coenzyme for transamination. *Flavin adenine dinucleotide (FAD)* - FAD, derived from **riboflavin (Vitamin B2)**, typically functions in **oxidation-reduction** reactions involving flavoproteins. - It is not involved in the transfer of amino groups in classical transamination cycles.

Q: Blue diaper syndrome is associated with which of the following conditions?

Tryptophan malabsorption. **Explanation:** **Blue Diaper Syndrome** (Drummond’s Syndrome) is a rare, autosomal recessive metabolic disorder characterized by the **isolated malabsorption of Tryptophan** from the intestinal tract. **1. Why Option A is Correct:** In this condition, there is a defect in the transport system responsible for absorbing the amino acid Tryptophan. When Tryptophan remains unabsorbed in the gut, intestinal bacteria metabolize it into **Indole**. This indole is absorbed into the portal circulation and converted by the liver into **Indican** (Indoxyl sulfate). When Indican is excreted in the urine and exposed to air, it undergoes oxidation to form **Indigo Blue**, a water-insoluble pigment that stains the infant's diaper blue. **2. Why Other Options are Incorrect:** * **Option B:** Hyperabsorption of Tryptophan does not occur in this pathology; the issue is a transport deficiency leading to bacterial putrefaction. * **Options C & D:** Tyrosine metabolism defects are associated with conditions like Alkaptonuria (Black urine) or Tyrosinemia, but they do not produce the characteristic blue pigment associated with indole derivatives. **Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Infants present with digestive disturbances, irritability, and visual defects (e.g., cataracts, microphthalmos). * **Hypercalcemia:** Blue Diaper Syndrome is frequently associated with **familial hypercalcemia** and nephrocalcinosis, though the exact mechanism linking tryptophan transport to calcium levels remains unclear. * **Differential Diagnosis:** Do not confuse this with **Hartnup Disease**, which also involves Tryptophan malabsorption but affects other neutral amino acids and presents with pellagra-like skin rashes and ataxia.

Question 1

A neonate presents with seizures, low citrulline levels, hyperammonemia, normal orotic acid levels, and high glutamine. What is the most likely diagnosis?

Accepted Answer

Type I hyperammonemia

Answer

Type II hyperammonemia

Answer

Citrullinemia

Answer

Argininosuccinic aciduria

Question 2

Which of the following amino acid classifications are correctly matched?
1. Uncharged polar AA - Cysteine
2. Nonpolar AA - Histidine
3. Basic AA - Proline
4. Basic AA - Lysine

Accepted Answer

A and D

Answer

A and B

Answer

C and D

Answer

B and C

Question 3

Which coenzyme is essential for carrying out transamination reactions in amino acid
metabolism?

Accepted Answer

Pyridoxal phosphate (PLP)

Answer

Thiamine pyrophosphate (TPP)

Answer

Nicotinamide adenine dinucleotide (NAD)

Answer

Flavin adenine dinucleotide (FAD)

Question 4

A child presents with hypopigmented hair and mental retardation, and their urine has a mousy odor. Which enzyme deficiency is responsible?

Accepted Answer

Phenylalanine hydroxylase

Answer

Cystathionine synthase

Answer

Tyrosine hydroxylase

Answer

Tyrosine transaminase

Question 5

Blue diaper syndrome is associated with which of the following conditions?

Accepted Answer

Tryptophan malabsorption

Answer

Tryptophan hyperabsorption

Answer

Tyrosine malabsorption

Answer

Tyrosine hyperabsorption

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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