Amino Acid Metabolism Practice Questions

Q: Tryptophan is glucogenic and ketogenic by producing which of the following?

Acetyl CoA and alanine. **Explanation:** Tryptophan is one of the few amino acids that is both **glucogenic and ketogenic**. Its complex catabolic pathway (the Kynurenine-Anthranilate pathway) leads to the cleavage of the molecule into two distinct fragments: 1. **Ketogenic component:** Tryptophan is converted into **Acetoacetyl-CoA**, which then forms **Acetyl-CoA**. This enters the ketogenic pool. 2. **Glucogenic component:** During the pathway, the side chain of tryptophan is released as **Alanine**. Alanine is a premier glucogenic amino acid as it can be transaminated to pyruvate, a direct precursor for gluconeogenesis. **Analysis of Incorrect Options:** * **Option B (Acetoacetate and Fumarate):** This describes the catabolism of **Phenylalanine and Tyrosine**. While they are also both glucogenic and ketogenic, they yield fumarate (glucogenic) and acetoacetate (ketogenic). * **Option C & D (Arginine):** Arginine is a purely glucogenic amino acid that enters the TCA cycle via α-ketoglutarate. It is not a product of tryptophan catabolism. **High-Yield NEET-PG Pearls:** * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **P**hil **T**ries **T**o **I**solate). * **Clinical Correlation:** Tryptophan is the precursor for **Niacin (Vitamin B3)**, **Serotonin**, and **Melatonin**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) leading to Pellagra-like symptoms because Tryptophan is unavailable for Niacin synthesis.

Q: Which amino acid is regarded as the collector of amino groups?

Glutamate. **Explanation:** **Glutamate** is known as the "collector of amino groups" because of its central role in **transamination** and **deamination** reactions. During protein catabolism, most amino acids transfer their $\alpha$-amino group to $\alpha$-ketoglutarate (via aminotransferases), resulting in the formation of Glutamate. This process effectively funnels nitrogen from various amino acids into a single pool. Glutamate then undergoes **oxidative deamination** by the enzyme *Glutamate Dehydrogenase* in the mitochondria, releasing the amino group as free ammonia ($NH_3$) for the Urea Cycle. **Analysis of Incorrect Options:** * **Aspartate:** While it is an important nitrogen donor in the Urea Cycle (providing the second nitrogen atom to form Argininosuccinate), it is not the primary collector from all other amino acids. * **Arginine:** This is a semi-essential amino acid and an intermediate of the Urea Cycle. Its primary role is as a precursor for Nitric Oxide (NO) and Urea, not as a nitrogen collector. * **Cysteine:** This is a sulfur-containing amino acid involved in glutathione synthesis and disulfide bond formation; it does not play a central role in general amino group collection. **High-Yield Facts for NEET-PG:** * **Glutamine** is the major transport form of ammonia from peripheral tissues (like muscle and brain) to the liver. * **Alanine** is the primary transporter of amino groups from the muscle to the liver (Glucose-Alanine Cycle). * **Glutamate Dehydrogenase** is the only enzyme that can use either $NAD^+$ or $NADP^+$ as a coenzyme. * **$\alpha$-Ketoglutarate** is the universal acceptor of amino groups during transamination.

Q: A 2-month-old infant presented with hepatic crisis, peripheral neuropathy, and Fanconi-like syndrome. Laboratory investigations showed elevated transaminases, low coagulation factors, and elevated succinylacetone in serum and urine. What is the most likely diagnosis?

Tyrosinemia. **Explanation:** The clinical presentation and laboratory findings point directly to **Tyrosinemia Type I** (Hepatorenal Tyrosinemia). **Why Tyrosinemia Type I is correct:** Tyrosinemia Type I is caused by a deficiency of the enzyme **Fumarylacetoacetate Hydrolase (FAH)**, the final step in the tyrosine degradation pathway. This deficiency leads to the accumulation of fumarylacetoacetate, which is diverted into the formation of **Succinylacetone**. * **Succinylacetone** is the pathognomonic marker for this condition; it is toxic to the liver (causing cirrhosis and hepatic crisis) and the kidneys (causing **Fanconi-like syndrome** with renal tubular acidosis). * It also inhibits heme synthesis, leading to porphyria-like neurological crises (**peripheral neuropathy**). **Why other options are incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. Presents with intellectual disability, "mousy" odor, and hypopigmentation, but not hepatic failure or succinylacetone elevation. * **Homocystinuria:** Caused by Cystathionine $\beta$-synthase deficiency. Characterized by ectopia lentis, marfanoid habitus, and thromboembolism. * **Hawkinsinuria:** A rare defect in the 4-hydroxyphenylpyruvate dioxygenase enzyme. It presents with failure to thrive and metabolic acidosis in infancy but lacks the severe hepatic crisis and succinylacetone marker. **NEET-PG High-Yield Pearls:** * **Diagnostic Marker:** Succinylacetone in urine/blood (Most specific). * **Treatment of Choice:** **Nitisinone (NTBC)**, which inhibits 4-hydroxyphenylpyruvate dioxygenase, preventing the formation of toxic metabolites. * **Long-term Risk:** Patients have a very high risk of developing **Hepatocellular Carcinoma (HCC)**. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine.

Q: A 30-year-old chronically malnourished male presented with weakness, lassitude, anorexia, poor appetite, abdominal discomfort, and diarrhea. He also exhibits apathy, lethargy, and mild disorientation. On examination, glossitis and dermatitis in sun-exposed areas are noted. What is the chromosome on which the gene for the metabolic disorder that mimics these findings is present?

5p. **Explanation:** The clinical presentation of dermatitis (in sun-exposed areas), diarrhea, and dementia (apathy/disorientation), combined with glossitis, is the classic triad of **Pellagra**, caused by a deficiency of **Niacin (Vitamin B3)**. The question asks for the chromosome of a metabolic disorder that *mimics* these findings. This refers to **Hartnup disease**, an autosomal recessive disorder characterized by a defect in the transport of neutral amino acids (including **Tryptophan**) in the renal tubules and intestines. Since Tryptophan is a precursor for Niacin synthesis (60 mg Tryptophan = 1 mg Niacin), its deficiency leads to pellagra-like symptoms. The gene responsible for Hartnup disease is **SLC6A19**, which is located on **Chromosome 5p**. **Analysis of Options:** * **A. 5p (Correct):** Location of the SLC6A19 gene. Mutations here cause Hartnup disease, mimicking Niacin deficiency. * **B. 21p:** Associated with Down Syndrome (Trisomy 21) and certain amyloid proteins, but not neutral amino acid transport. * **C. 6p:** Contains the HLA complex (Major Histocompatibility Complex), linked to various autoimmune diseases but not pellagra-like symptoms. * **D. 8p:** Associated with conditions like Spherocytosis or certain neurodevelopmental disorders, but irrelevant to Tryptophan metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Pellagra 4 D’s:** Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Hartnup Disease Diagnosis:** High levels of neutral amino acids in urine (**Neutral Aminoaciduria**) but normal levels in blood. * **Niacin Synthesis:** Requires **Vitamin B6 (Pyridoxine)** as a cofactor (Kynureninase enzyme). Deficiency of B6 can also cause secondary pellagra. * **Dietary Link:** Pellagra is common in populations consuming **Maize (Corn)** as a staple, as the niacin in maize is bound (niacytin) and it is deficient in Tryptophan.

Q: A 15-year-old girl is seen by a dermatologist for removal of multiple squamous cell carcinomas of the skin. The patient has nearly white hair, pink irises, very pale skin, and a history of burning easily when exposed to the sun. This patient's condition is caused by a disorder involving which of the following substances?

Aromatic amino acids. ### Explanation The clinical presentation of very pale skin, white hair, pink irises (transillumination), and early-onset squamous cell carcinomas (SCC) is diagnostic of **Oculocutaneous Albinism (OCA)**. **1. Why Aromatic Amino Acids is Correct:** OCA is most commonly caused by a deficiency or defect in the enzyme **tyrosinase**. Tyrosinase is the rate-limiting enzyme in melanogenesis, responsible for converting **Tyrosine** into DOPA and subsequently into melanin. Tyrosine is an **aromatic amino acid**. Without functional tyrosinase, melanocytes cannot produce melanin, leading to a total lack of pigment in the skin, hair, and eyes. This lack of protective melanin results in extreme photosensitivity and a high risk of UV-induced skin cancers (SCC and BCC) at a young age. **2. Why Incorrect Options are Wrong:** * **B. Branched-chain amino acids (Leucine, Isoleucine, Valine):** Defects here lead to **Maple Syrup Urine Disease (MSUD)**, characterized by neurological deterioration and a burnt-sugar odor in urine, not pigmentary disorders. * **C. Glycolipids:** Disorders of glycolipid metabolism (e.g., Gaucher or Fabry disease) are **Lysosomal Storage Diseases**, typically presenting with organomegaly and neurological or renal issues. * **D. Glycoproteins:** Defects in glycoprotein metabolism or transport (e.g., I-cell disease) lead to skeletal abnormalities and developmental delays. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Most forms of OCA are **Autosomal Recessive**. * **Melanocyte Count:** In Albinism, the **number of melanocytes is normal**, but melanin production is absent. (Contrast this with Vitiligo, where melanocytes are destroyed). * **Visual Acuity:** Patients often have nystagmus and reduced visual acuity due to foveal hypoplasia. * **Chediak-Higashi Syndrome:** A related condition featuring partial albinism, giant lysosomal granules, and recurrent infections.

Q: With ageing, a slight decrease in cognitive impairment is seen due to an increase in the level of which of the following?

Homocysteine. **Explanation:** **Correct Option: C. Homocysteine** The correct answer is **Homocysteine**. Elevated levels of homocysteine in the blood (**Hyperhomocysteinemia**) are strongly associated with cognitive decline, dementia, and Alzheimer’s disease in the elderly. Homocysteine is a sulfur-containing intermediate in methionine metabolism. It exerts neurotoxic effects through several mechanisms: 1. **Vascular Damage:** It promotes oxidative stress and endothelial dysfunction, leading to cerebral microvascular disease. 2. **Direct Neurotoxicity:** It acts as an agonist at NMDA receptors, leading to calcium influx and neuronal apoptosis. 3. **DNA Damage:** It interferes with methylation reactions essential for DNA repair in neurons. **Why other options are incorrect:** * **A. Methionine:** This is an essential amino acid and the precursor to homocysteine. While it is vital for protein synthesis, its direct elevation is not a primary biomarker for age-related cognitive impairment. * **B. Cysteine:** Homocysteine is converted to cysteine via the cystathionine $\beta$-synthase (CBS) pathway (transsulfuration). Cysteine is a precursor for glutathione (an antioxidant) and is generally considered neuroprotective rather than a cause of cognitive decline. * **D. Taurine:** Derived from cysteine, taurine acts as an inhibitory neurotransmitter and antioxidant. It is often studied for its potential neuroprotective benefits in aging. **NEET-PG High-Yield Pearls:** * **The Triple Deficiency:** Hyperhomocysteinemia is often caused by deficiencies in **Vitamin B12 (Cobalamin), B9 (Folate), and B6 (Pyridoxine)**. * **Key Enzymes:** Methionine Synthase (requires B12 and Folate) and Cystathionine $\beta$-synthase (requires B6). * **Clinical Link:** High homocysteine is also a significant independent risk factor for **atherosclerosis and coronary artery disease (CAD)**.

Q: Branched-chain ketoaciduria is a defect of catabolism of all the following amino acids except?

Methionine. **Explanation:** **Branched-chain ketoaciduria**, commonly known as **Maple Syrup Urine Disease (MSUD)**, is caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This multi-enzyme complex is responsible for the oxidative decarboxylation of the keto-acid derivatives of the three branched-chain amino acids (BCAAs). 1. **Why Methionine is the correct answer:** Methionine is a sulfur-containing amino acid, not a branched-chain amino acid. Its catabolism involves conversion to S-adenosylmethionine (SAM) and eventually homocysteine and propionyl-CoA. It does not utilize the BCKAD complex; therefore, its metabolism is unaffected in MSUD. 2. **Why the other options are incorrect:** * **Leucine, Isoleucine, and Valine** are the three essential **Branched-Chain Amino Acids (BCAAs)**. * In MSUD, the first step of their catabolism (transamination) occurs normally, but the second step (oxidative decarboxylation) fails. This leads to a toxic buildup of these amino acids and their corresponding alpha-keto acids in the blood and urine. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember the BCAAs as **"LIV"** (Leucine, Isoleucine, Valine). * **Clinical Presentation:** Characterized by a distinctive **maple syrup or burnt sugar odor** in the urine (due to the keto-acid of Isoleucine). * **Diagnosis:** Elevated levels of BCAAs in plasma and **Alloisoleucine** (pathognomonic marker). * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**TLC For No Pants**). Some patients respond to high-dose Thiamine supplementation.

Q: Which of the following is NOT an essential amino acid?

Arginine. ### Explanation **Core Concept: Essential vs. Non-Essential Amino Acids** Amino acids are categorized based on the body's ability to synthesize them. **Essential amino acids** cannot be synthesized *de novo* by the body and must be obtained through the diet. **Non-essential amino acids** can be produced by the body from metabolic intermediates. **Why Arginine is the Correct Answer:** Arginine is classified as a **semi-essential (conditionally essential)** amino acid. While the urea cycle can produce arginine in adults, the rate of synthesis is insufficient to meet the high demands of growth in children or during periods of severe physiological stress (e.g., trauma or sepsis). However, in the context of a standard four-option MCQ where the other three are strictly essential, Arginine is the "least essential" and thus the correct choice for a non-essential/semi-essential category. **Analysis of Incorrect Options:** * **A. Methionine:** An essential sulfur-containing amino acid. It is the precursor for cysteine and serves as the primary methyl donor (via S-adenosylmethionine). * **C. Valine:** An essential branched-chain amino acid (BCAA). It is glucogenic and vital for muscle metabolism and tissue repair. * **D. Tryptophan:** An essential aromatic amino acid. It is a precursor for serotonin, melatonin, and niacin (Vitamin B3). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Essential Amino Acids:** **"PVT TIM HALL"** (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). * **Purely Ketogenic:** Leucine and Lysine (the only two that cannot produce glucose). * **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: **PITTT**). * **Semi-essential:** Arginine and Histidine (required during growth/positive nitrogen balance).

Q: Sweaty feet odor in body fluids occurs due to a deficiency of which of the following?

FAD linked dehydrogenase. ### Explanation The "sweaty feet odor" is a classic clinical sign of **Isovaleric Acidemia**, an autosomal recessive organic acidemia. **1. Why the Correct Answer is Right:** Isovaleric acidemia is caused by a deficiency of **Isovaleryl-CoA dehydrogenase**. This enzyme is an **FAD-linked dehydrogenase** responsible for the third step in the catabolism of the branched-chain amino acid **Leucine**. * When this enzyme is deficient, Isovaleryl-CoA accumulates and is hydrolyzed to **isovaleric acid**. * Isovaleric acid is a short-chain fatty acid that is volatile and possesses a pungent, characteristic odor described as **"sweaty feet"** or "cheese-like," which manifests in the patient's breath, urine, and sweat. **2. Why the Other Options are Wrong:** * **A. Biotin:** Biotin is a cofactor for carboxylase enzymes (e.g., Propionyl-CoA carboxylase). Deficiency leads to Propionic acidemia, which presents with metabolic acidosis but not a sweaty feet odor. * **C. Thiamin linked decarboxylase:** This refers to the **Branched-chain alpha-keto acid dehydrogenase (BCKDH)** complex. Deficiency causes **Maple Syrup Urine Disease (MSUD)**, characterized by a "burnt sugar" or "maple syrup" odor. * **D. Leucine transaminase:** This is the first step in leucine metabolism. Deficiency is rare and does not result in the accumulation of isovaleric acid. **3. Clinical Pearls for NEET-PG:** * **Odor Association Summary:** * **Sweaty Feet:** Isovaleric Acidemia (Isovaleryl-CoA DH deficiency) or Glutaric Aciduria Type II. * **Maple Syrup/Burnt Sugar:** MSUD (BCKDH deficiency). * **Mousy/Musty:** Phenylketonuria (PKU). * **Cabbage-like/Rancid Butter:** Tyrosinemia Type I. * **Rotten Fish:** Trimethylaminuria. * **High-Yield Fact:** Isovaleric acidemia often presents in neonates with vomiting, metabolic acidosis, and a "cherry-red" appearance of the tongue, alongside the characteristic odor.

Question 1

Tryptophan is glucogenic and ketogenic by producing which of the following?

Accepted Answer

Acetyl CoA and alanine

Answer

Acetoacetate and fumarate

Answer

Acetoacetate and arginine

Answer

Arginine and alanine

Question 2

Which amino acid is regarded as the collector of amino groups?

Accepted Answer

Glutamate

Answer

Aspartate

Answer

Arginine

Answer

Cysteine

Question 3

A 2-month-old infant presented with hepatic crisis, peripheral neuropathy, and Fanconi-like syndrome. Laboratory investigations showed elevated transaminases, low coagulation factors, and elevated succinylacetone in serum and urine. What is the most likely diagnosis?

Accepted Answer

Tyrosinemia

Answer

Phenylketonuria

Answer

Homocystinuria

Answer

Hawkinsinuria

Question 4

All of the following may be raised in carcinoid syndrome, except?

Accepted Answer

VMA

Answer

5HIAA

Answer

5HT

Answer

5HTP

Question 5

A 30-year-old chronically malnourished male presented with weakness, lassitude, anorexia, poor appetite, abdominal discomfort, and diarrhea. He also exhibits apathy, lethargy, and mild disorientation. On examination, glossitis and dermatitis in sun-exposed areas are noted. What is the chromosome on which the gene for the metabolic disorder that mimics these findings is present?

Accepted Answer

5p

Answer

21p

Answer

6p

Answer

8p

Question 6

A 15-year-old girl is seen by a dermatologist for removal of multiple squamous cell carcinomas of the skin. The patient has nearly white hair, pink irises, very pale skin, and a history of burning easily when exposed to the sun. This patient's condition is caused by a disorder involving which of the following substances?

Accepted Answer

Aromatic amino acids

Answer

Branched chain amino acids

Answer

Glycolipids

Answer

Glycoproteins

Question 7

With ageing, a slight decrease in cognitive impairment is seen due to an increase in the level of which of the following?

Accepted Answer

Homocysteine

Answer

Methionine

Answer

Cysteine

Answer

Taurine

Question 8

Branched-chain ketoaciduria is a defect of catabolism of all the following amino acids except?

Accepted Answer

Methionine

Answer

Leucine

Answer

Isoleucine

Answer

Valine

Question 9

Which of the following is NOT an essential amino acid?

Accepted Answer

Arginine

Answer

Methionine

Answer

Valine

Answer

Tryptophan

Question 10

Sweaty feet odor in body fluids occurs due to a deficiency of which of the following?

Accepted Answer

FAD linked dehydrogenase

Answer

Biotin

Answer

Thiamin linked decarboxylase

Answer

Leucine transaminase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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