FMGE 2024 — Biochemistry

Q: Zellweger syndrome is associated with which cellular organelle?

A. Peroxisomes. ***A. Peroxisomes*** - Zellweger syndrome is a severe disorder belonging to the group of **peroxisomal biogenesis disorders (PBDs)**, resulting from a failure to form functional peroxisomes due to mutations in *PEX* genes. - Functional peroxisomes are essential for the metabolism of substances like **very long-chain fatty acids (VLCFAs)** and plasmalogens; their absence leads to the accumulation of these toxic molecules. *B. Mitochondria* - **Mitochondria** are the powerhouse organelles responsible for cellular energy production via oxidative phosphorylation. - Mitochondrial disorders present with distinct features like lactic acidosis, myopathy, and neurodegeneration, which differ from the characteristic peroxisomal dysfunction seen in Zellweger syndrome. *C. Lysosomes* - **Lysosomes** are crucial for degrading cellular waste products and macromolecules; defects in these organelles cause **lysosomal storage diseases** (e.g., mucopolysaccharidoses). - Although some symptoms overlap, Zellweger syndrome is specifically defined by **peroxisomal dysfunction**, not primary lysosomal enzyme deficiency. *D. Ribosomes* - **Ribosomes** are responsible for synthesizing proteins via translation of mRNA. - Defects in ribosomes typically impair **global protein synthesis** or specific tissue development (ribosomopathies), which is distinct from the primary metabolic defects seen in Zellweger syndrome. [Practice more Biochemistry PYQs on OnCourse]

Q: A 3-month-old baby presents with severe hepatomegaly, cataracts in both eyes, lethargy, and hypotonia. Based on these symptoms, which enzyme deficiency is most likely involved?

Galactose-1-Phosphate Uridyl Transferase. ***Galactose-1-Phosphate Uridyl Transferase*** - **GALT** deficiency causes **Classic Galactosemia**, which is characterized by the accumulation of **galactose-1-phosphate**, leading to widespread organ damage. - The classic presentation in neonates includes **hepatomegaly**, **jaundice**, failure to thrive, hypoglycemia, **E. coli sepsis**, developmental delay, and pathognomonic **cataracts**. - This is the **most severe form** of galactosemia and matches all the clinical features in this case. *Galactokinase* - Deficiency of **Galactokinase (GALK)** leads to a milder form of galactosemia where **galactitol** accumulates, primarily causing **cataracts** in infancy. - Systemic symptoms like **hepatomegaly**, liver failure, and severe lethargy, as seen in this 3-month-old, are typically **absent** in GALK deficiency. *UDP-Galactose-4-Epimerase* - **GALE** deficiency is the rarest form of galactosemia with variable clinical severity. - While severe forms can present with symptoms similar to classic galactosemia, they are extremely rare and account for less than 5% of galactosemia cases. - The combination of severe hepatomegaly and cataracts at 3 months is most characteristic of **GALT deficiency**, not GALE deficiency. *Hepatic Glucose-6-Phosphatase* - Deficiency of this enzyme causes **Von Gierke disease** (GSD Type I), characterized by significant **hepatomegaly** and severe **fasting hypoglycemia**. - While it causes hepatomegaly and lethargy (from hypoglycemia), it is **not** associated with the development of **cataracts**, which differentiates it from galactosemia. [Practice more Biochemistry PYQs on OnCourse]

Q: Which substance is involved in the conjugation process in the liver?

Glucuronic Acid. ***Glucuronic Acid***- The conjugation process is a Phase II detoxification reaction in the liver that increases the compound's polarity and water solubility for excretion.- **Glucuronidation**, catalyzed by **UDP-glucuronosyltransferases (UGT)**, is the most common and critical conjugation pathway, where substrates (like **bilirubin** and drugs) are linked to **glucuronic acid** (provided by UDP-glucuronic acid).*Hyaluronic Acid*- This acid is a large, non-sulfated **glycosaminoglycan** and a primary component of the **extracellular matrix** and **synovial fluid**.- It functions mainly in tissue structure, hydration, and lubrication, not as a conjugating molecule in liver metabolism.*Gluconic Acid*- This is an oxidation product of **glucose**, often used in food and pharmaceutical industries (e.g., as a salt like ferrous gluconate).- While structurally related to glucose metabolites, it is **glucuronic acid**, not gluconic acid, that is utilized for Phase II **conjugation**.*Glycolic Acid*- This substance is the smallest **alpha-hydroxy acid (AHA)** and is widely known for its use as a chemical exfoliator in dermatology.- Although it is an endogenous metabolite, it is not involved in the major Phase II conjugation reactions in the liver; these reactions primarily utilize **glucuronic acid**, sulfate, or glutathione. [Practice more Biochemistry PYQs on OnCourse]

Q: A 6-month-old boy presents with recurrent bacterial and fungal infections, chronic diarrhea, and failure to thrive. He is diagnosed with severe combined immunodeficiency due to an autosomal recessive inheritance pattern. Which enzyme deficiency is responsible?

D. Adenosine deaminase. ***Adenosine deaminase*** - The **autosomal recessive** form of **Severe Combined Immunodeficiency (SCID)** is most commonly caused by a deficiency in **Adenosine Deaminase (ADA)**, accounting for about 15% of all SCID cases. - ADA deficiency leads to the accumulation of toxic metabolites (*dATP*), which are highly toxic to rapidly dividing cells, especially **T and B lymphocytes**, resulting in profound lymphopenia and immunodeficiency. *Phosphomannose isomerase* - Deficiency in **Phosphomannose Isomerase (PMI)** causes Congenital Disorder of Glycosylation Type Ib (**CDG-Ib**), which presents with protein-losing enteropathy, hypoglycemia, and failure to thrive, but usually *not* recurrent bacterial and fungal infections severe enough to be classified as SCID. - CDG-Ib is a generalized metabolic disorder affecting **glycosylation**, primarily presenting with liver and gastrointestinal issues. *Ornithine transcarbamylase* - **Ornithine Transcarbamylase (OTC)** deficiency is the most common urea cycle disorder, typically presenting with acute **hyperammonemia** (lethargy, seizures, coma) after an initial period, especially following protein intake, not specifically severe SCID with recurrent infections. - OTC deficiency results in the impaired conversion of **carbamoyl phosphate** and **ornithine** to citrulline, leading to elevated ammonia levels. *Hypoxanthine-guanine phosphoribosyltransferase* - Deficiency in **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)** is responsible for **Lesch-Nyhan Syndrome**, an X-linked recessive disorder characterized by overproduction of **uric acid** (**hyperuricemia**), neurological dysfunction, and self-mutilation. - Although a purine metabolism disorder, it does not cause primary immunodeficiency like SCID; it mainly affects the **nervous system** and purine salvage pathway. [Practice more Biochemistry PYQs on OnCourse]

Q: Iron absorption is decreased in which of the following?

Tea. ***Tea*** - Contains **tannins and polyphenols** that bind to non-heme iron in the gastrointestinal tract - Forms **insoluble iron complexes** that cannot be absorbed - Can reduce iron absorption by **60-70%** when consumed with meals - Most significant dietary inhibitor of iron absorption *Incorrect: Amla* - Indian gooseberry is rich in **vitamin C (ascorbic acid)** - Vitamin C **enhances iron absorption** by reducing ferric (Fe³⁺) to ferrous (Fe²⁺) form - Also prevents formation of insoluble iron compounds *Incorrect: Lemon* - Rich in **vitamin C and citric acid** - Both compounds **enhance iron absorption** significantly - Citric acid chelates iron, keeping it soluble and bioavailable *Incorrect: Sprouting* - **Reduces phytate content** in grains and legumes - Phytates are iron absorption inhibitors - Sprouting therefore **enhances iron bioavailability** [Practice more Biochemistry PYQs on OnCourse]

6 Previous Year Questions with Answers & Explanations

Questions

Zellweger syndrome is associated with which cellular organelle?

A 3-month-old baby presents with severe hepatomegaly, cataracts in both eyes, lethargy, and hypotonia. Based on these symptoms, which enzyme deficiency is most likely involved?

Which substance is involved in the conjugation process in the liver?

A 6-month-old boy presents with recurrent bacterial and fungal infections, chronic diarrhea, and failure to thrive. He is diagnosed with severe combined immunodeficiency due to an autosomal recessive inheritance pattern. Which enzyme deficiency is responsible?

Iron absorption is decreased in which of the following?

Which type of ion channel is affected by mutations in the CFTR gene?

FMGE 2024 - Biochemistry FMGE Practice Questions and MCQs

Question 1: Zellweger syndrome is associated with which cellular organelle?

A. C. Lysosomes
B. D. Ribosomes
C. B. Mitochondria
D. A. Peroxisomes (Correct Answer)

Explanation: ***A. Peroxisomes*** - Zellweger syndrome is a severe disorder belonging to the group of **peroxisomal biogenesis disorders (PBDs)**, resulting from a failure to form functional peroxisomes due to mutations in *PEX* genes. - Functional peroxisomes are essential for the metabolism of substances like **very long-chain fatty acids (VLCFAs)** and plasmalogens; their absence leads to the accumulation of these toxic molecules. *B. Mitochondria* - **Mitochondria** are the powerhouse organelles responsible for cellular energy production via oxidative phosphorylation. - Mitochondrial disorders present with distinct features like lactic acidosis, myopathy, and neurodegeneration, which differ from the characteristic peroxisomal dysfunction seen in Zellweger syndrome. *C. Lysosomes* - **Lysosomes** are crucial for degrading cellular waste products and macromolecules; defects in these organelles cause **lysosomal storage diseases** (e.g., mucopolysaccharidoses). - Although some symptoms overlap, Zellweger syndrome is specifically defined by **peroxisomal dysfunction**, not primary lysosomal enzyme deficiency. *D. Ribosomes* - **Ribosomes** are responsible for synthesizing proteins via translation of mRNA. - Defects in ribosomes typically impair **global protein synthesis** or specific tissue development (ribosomopathies), which is distinct from the primary metabolic defects seen in Zellweger syndrome.

Question 2: A 3-month-old baby presents with severe hepatomegaly, cataracts in both eyes, lethargy, and hypotonia. Based on these symptoms, which enzyme deficiency is most likely involved?

A. Hepatic Glucose-6-Phosphatase
B. Galactose-1-Phosphate Uridyl Transferase (Correct Answer)
C. UDP-Galactose-4-Epimerase
D. Galactokinase

Explanation: ***Galactose-1-Phosphate Uridyl Transferase*** - **GALT** deficiency causes **Classic Galactosemia**, which is characterized by the accumulation of **galactose-1-phosphate**, leading to widespread organ damage. - The classic presentation in neonates includes **hepatomegaly**, **jaundice**, failure to thrive, hypoglycemia, **E. coli sepsis**, developmental delay, and pathognomonic **cataracts**. - This is the **most severe form** of galactosemia and matches all the clinical features in this case. *Galactokinase* - Deficiency of **Galactokinase (GALK)** leads to a milder form of galactosemia where **galactitol** accumulates, primarily causing **cataracts** in infancy. - Systemic symptoms like **hepatomegaly**, liver failure, and severe lethargy, as seen in this 3-month-old, are typically **absent** in GALK deficiency. *UDP-Galactose-4-Epimerase* - **GALE** deficiency is the rarest form of galactosemia with variable clinical severity. - While severe forms can present with symptoms similar to classic galactosemia, they are extremely rare and account for less than 5% of galactosemia cases. - The combination of severe hepatomegaly and cataracts at 3 months is most characteristic of **GALT deficiency**, not GALE deficiency. *Hepatic Glucose-6-Phosphatase* - Deficiency of this enzyme causes **Von Gierke disease** (GSD Type I), characterized by significant **hepatomegaly** and severe **fasting hypoglycemia**. - While it causes hepatomegaly and lethargy (from hypoglycemia), it is **not** associated with the development of **cataracts**, which differentiates it from galactosemia.

Question 3: Which substance is involved in the conjugation process in the liver?

A. Gluconic Acid
B. Hyaluronic Acid
C. Glucuronic Acid (Correct Answer)
D. Glycolic Acid

Explanation: ***Glucuronic Acid***- The conjugation process is a Phase II detoxification reaction in the liver that increases the compound's polarity and water solubility for excretion.- **Glucuronidation**, catalyzed by **UDP-glucuronosyltransferases (UGT)**, is the most common and critical conjugation pathway, where substrates (like **bilirubin** and drugs) are linked to **glucuronic acid** (provided by UDP-glucuronic acid).*Hyaluronic Acid*- This acid is a large, non-sulfated **glycosaminoglycan** and a primary component of the **extracellular matrix** and **synovial fluid**.- It functions mainly in tissue structure, hydration, and lubrication, not as a conjugating molecule in liver metabolism.*Gluconic Acid*- This is an oxidation product of **glucose**, often used in food and pharmaceutical industries (e.g., as a salt like ferrous gluconate).- While structurally related to glucose metabolites, it is **glucuronic acid**, not gluconic acid, that is utilized for Phase II **conjugation**.*Glycolic Acid*- This substance is the smallest **alpha-hydroxy acid (AHA)** and is widely known for its use as a chemical exfoliator in dermatology.- Although it is an endogenous metabolite, it is not involved in the major Phase II conjugation reactions in the liver; these reactions primarily utilize **glucuronic acid**, sulfate, or glutathione.

Question 4: A 6-month-old boy presents with recurrent bacterial and fungal infections, chronic diarrhea, and failure to thrive. He is diagnosed with severe combined immunodeficiency due to an autosomal recessive inheritance pattern. Which enzyme deficiency is responsible?

A. B. Ornithine transcarbamylase
B. C. Hypoxanthine-guanine phosphoribosyltransferase
C. A. Phosphomannose isomerase
D. D. Adenosine deaminase (Correct Answer)

Explanation: ***Adenosine deaminase*** - The **autosomal recessive** form of **Severe Combined Immunodeficiency (SCID)** is most commonly caused by a deficiency in **Adenosine Deaminase (ADA)**, accounting for about 15% of all SCID cases. - ADA deficiency leads to the accumulation of toxic metabolites (*dATP*), which are highly toxic to rapidly dividing cells, especially **T and B lymphocytes**, resulting in profound lymphopenia and immunodeficiency. *Phosphomannose isomerase* - Deficiency in **Phosphomannose Isomerase (PMI)** causes Congenital Disorder of Glycosylation Type Ib (**CDG-Ib**), which presents with protein-losing enteropathy, hypoglycemia, and failure to thrive, but usually *not* recurrent bacterial and fungal infections severe enough to be classified as SCID. - CDG-Ib is a generalized metabolic disorder affecting **glycosylation**, primarily presenting with liver and gastrointestinal issues. *Ornithine transcarbamylase* - **Ornithine Transcarbamylase (OTC)** deficiency is the most common urea cycle disorder, typically presenting with acute **hyperammonemia** (lethargy, seizures, coma) after an initial period, especially following protein intake, not specifically severe SCID with recurrent infections. - OTC deficiency results in the impaired conversion of **carbamoyl phosphate** and **ornithine** to citrulline, leading to elevated ammonia levels. *Hypoxanthine-guanine phosphoribosyltransferase* - Deficiency in **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)** is responsible for **Lesch-Nyhan Syndrome**, an X-linked recessive disorder characterized by overproduction of **uric acid** (**hyperuricemia**), neurological dysfunction, and self-mutilation. - Although a purine metabolism disorder, it does not cause primary immunodeficiency like SCID; it mainly affects the **nervous system** and purine salvage pathway.

Question 5: Iron absorption is decreased in which of the following?

A. Amla
B. Lemon
C. Tea (Correct Answer)
D. Sprouting

Explanation: ***Tea*** - Contains **tannins and polyphenols** that bind to non-heme iron in the gastrointestinal tract - Forms **insoluble iron complexes** that cannot be absorbed - Can reduce iron absorption by **60-70%** when consumed with meals - Most significant dietary inhibitor of iron absorption *Incorrect: Amla* - Indian gooseberry is rich in **vitamin C (ascorbic acid)** - Vitamin C **enhances iron absorption** by reducing ferric (Fe³⁺) to ferrous (Fe²⁺) form - Also prevents formation of insoluble iron compounds *Incorrect: Lemon* - Rich in **vitamin C and citric acid** - Both compounds **enhance iron absorption** significantly - Citric acid chelates iron, keeping it soluble and bioavailable *Incorrect: Sprouting* - **Reduces phytate content** in grains and legumes - Phytates are iron absorption inhibitors - Sprouting therefore **enhances iron bioavailability**

Question 6: Which type of ion channel is affected by mutations in the CFTR gene?

A. Calcium
B. Chloride (Correct Answer)
C. Potassium
D. Sodium

Explanation: ***Chloride***- The **CFTR** (Cystic Fibrosis Transmembrane Conductance Regulator) gene encodes an **ATP-gated chloride channel** that controls the movement of chloride ions across epithelial cell membranes.- Mutations in the CFTR gene cause defective chloride secretion and increased sodium and water reabsorption, leading to the dehydrated, thick mucus characteristic of **Cystic Fibrosis (CF)**.*Sodium*- While the lack of functional CFTR leads to excessive **sodium reabsorption** via the epithelial sodium channel (ENaC) in airways, CFTR itself is a chloride channel, not a sodium channel.- ENaC hyperactivity is a *secondary* effect resulting from the failure of CFTR to inhibit ENaC activity appropriately.*Potassium*- The primary role of the CFTR protein is not the regulation of potassium ions; potassium channels are distinct proteins involved in maintaining resting membrane potential and cell volume.- CFTR activity and the resultant disease phenotype are directly linked to chloride imbalance, not potassium transport defects.*Calcium*- CFTR does not function as a calcium channel; calcium channels are separate entities crucial for many cellular processes, including neurotransmitter release and muscle contraction.- Although intracellular calcium levels can sometimes modulate CFTR activity through signaling pathways, the channel protein itself transports chloride.