Genetic Disorders and Biochemical Pathology Practice Questions

Q: A 5-year-old presents with intellectual disability, self-mutilation, and hyperuricemia. What enzyme defect is most likely?

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT). ***Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)*** - Deficiency of **HGPRT** an enzyme in the **purine salvage pathway** leads to **Lesch-Nyhan syndrome**, characterized by **hyperuricemia**, **self-mutilation**, **intellectual disability**, and **dystonia**. - The accumulation of **uric acid** due to defective salvage leads to the characteristic symptoms. *Adenosine deaminase deficiency* - This deficiency causes **severe combined immunodeficiency (SCID)** due to the accumulation of toxic metabolites in lymphocytes. - It does not typically present with **self-mutilation** or **hyperuricemia**. *Xanthine oxidase deficiency* - This deficiency leads to **xanthinuria**, characterized by **low uric acid levels** and an increased risk of **xanthine kidney stones**. - The clinical presentation does not include **intellectual disability** or **self-mutilation**. *Glucose-6-phosphatase deficiency* - This enzyme deficiency causes **Glycogen Storage Disease Type Ia (von Gierke disease)**, characterized by **hypoglycemia**, **lactic acidosis**, and **hepatomegaly**. - It is not associated with **self-mutilation** or the primary neurological/behavioral features seen in Lesch-Nyhan syndrome.

Q: Which of the following is a common genetic mutation leading to hypoxia-induced polycythemia in patients with chronic mountain sickness?

VHL gene mutation. ***VHL gene mutation*** - **Von Hippel-Lindau (VHL) gene mutations** are associated with **Chuvash polycythemia**, a form of **primary familial congenital polycythemia (PFCP)**, which mimics chronic mountain sickness at normoxia by causing overproduction of red blood cells due to impaired **HIF-1α degradation**. - The VHL protein is a component of the **ubiquitin ligase complex** that targets **HIF-1α** for degradation under normoxic conditions. Dysfunction of VHL leads to constitutive activation of HIF-1α, over-expression of **erythropoietin (EPO)**, and subsequent **polycythemia** even at normal oxygen levels. *EGFR gene mutation* - **EGFR mutations** are commonly found in non-small cell lung cancer and are associated with tumor growth and response to targeted therapies, but they are not directly involved in the pathogenesis of **polycythemia** or its response to hypoxia. - These mutations primarily affect **cell proliferation and survival** pathways in cancerous cells, rather than erythropoiesis. *JAK2 V617F mutation* - The **JAK2 V617F mutation** is a hallmark of **myeloproliferative neoplasms (MPNs)**, particularly **polycythemia vera (PV)**, leading to constitutive activation of the JAK-STAT pathway and uncontrolled erythropoiesis independent of erythropoietin. - While it causes **polycythemia**, this is typically considered a primary, acquired bone marrow disorder, not directly linked to the hypoxia-sensing pathway dysregulation seen in **chronic mountain sickness**. *HIF1α gene mutation* - While **HIF-1α** is central to the hypoxic response, direct activating mutations in the **HIF1α gene** itself are generally not the most common genetic cause of **hypoxia-induced polycythemia** in humans. - The dysregulation typically occurs upstream, often through mutations in the **VHL gene**, which controls HIF-1α stability.

Q: In individuals of South Indian descent with a family history of type 2 diabetes, which genetic variant is most commonly associated with an increased risk of developing this condition?

TCF7L2 gene variant. ***TCF7L2 gene variant*** - The **TCF7L2 gene variant** is consistently identified as the strongest genetic risk factor for **Type 2 Diabetes (T2DM)** across various populations, including South Indians, due to its role in **insulin secretion** and **glucose homeostasis**. - Its association with increased T2DM risk is particularly significant in individuals with a **family history**, highlighting its broad impact on genetic predisposition. *SLC30A8 gene variant* - The **SLC30A8 gene** encodes a **zinc transporter** in pancreatic beta cells and is also associated with T2DM risk. - While relevant, its effect size and prevalence as a primary risk factor are generally less pronounced compared to TCF7L2, particularly in the South Indian population where TCF7L2 shows stronger association. *PPAR-gamma gene variant* - **PPAR-gamma** is involved in **adipogenesis** and **insulin sensitivity**, and variants in this gene are associated with T2DM and metabolic syndrome. - However, the **TCF7L2 gene** has consistently shown a stronger and more prevalent association with T2DM risk in the general population, including South Indians, compared to PPAR-gamma variants. *KCNJ11 gene variant* - The **KCNJ11 gene** encodes a subunit of the **ATP-sensitive potassium channel** in pancreatic beta cells, crucial for insulin secretion. Variants in this gene are linked to monogenic forms of diabetes, such as **Neonatal Diabetes Mellitus** and **Maturity-Onset Diabetes of the Young (MODY)**. - While it plays a role in glucose metabolism, its contribution to the common polygenic form of **Type 2 Diabetes** is less significant than that of TCF7L2, and it is not typically cited as the most common genetic risk factor in a broad population context like South Indians with a family history of T2DM.

Q: A patient is diagnosed with MELAS syndrome after presenting with stroke-like episodes and lactic acidosis. What is the underlying defect?

Mitochondrial DNA mutation. ***Mitochondrial DNA mutation*** - **MELAS syndrome** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) is specifically caused by mutations in **mitochondrial DNA (mtDNA)**. - These mutations impair the function of the **mitochondrial respiratory chain**, leading to energy production deficits in high-energy demand tissues like the brain and muscles. *Nuclear DNA mutation* - While many genetic disorders are caused by nuclear DNA mutations, **MELAS** is distinctly characterized by its mode of inheritance and origin in the **mitochondrial genome**. - **Nuclear DNA mutations** cause problems in a vast array of cellular functions, but not the specific triad of MELAS symptoms with mtDNA inheritance. *Lysosomal storage defect* - **Lysosomal storage diseases** involve enzyme deficiencies that lead to the accumulation of specific substrates within lysosomes. - These typically present with organomegaly, skeletal abnormalities, and neurodegeneration, but not the stroke-like episodes and lactic acidosis seen in MELAS. *Glycogen storage disease* - **Glycogen storage diseases** are characterized by enzyme defects in glycogen synthesis or breakdown, leading to abnormal glycogen accumulation or depletion. - Clinical features usually include hypoglycemia, hepatomegaly, and myopathy, which differ from the primary neurological and metabolic symptoms of MELAS.

Q: Which enzyme is most commonly deficient in patients with Alzheimer's disease?

Choline acetyltransferase. ***Choline acetyltransferase*** - **Choline acetyltransferase (ChAT)** is the enzyme responsible for synthesizing **acetylcholine**, a neurotransmitter crucial for memory and learning. - In Alzheimer's disease, there is a characteristic **degeneration of cholinergic neurons** and a significant reduction in ChAT activity, leading to **acetylcholine deficiency**. *Acetylcholinesterase* - **Acetylcholinesterase (AChE)** is the enzyme that breaks down acetylcholine in the synaptic cleft. - While it's involved in cholinergic signaling, its deficiency is not the primary pathology in Alzheimer's; rather, **inhibitors of AChE** are used therapeutically to increase acetylcholine levels. *Monoamine oxidase* - **Monoamine oxidase (MAO)** is involved in the metabolism of monoamine neurotransmitters like dopamine, norepinephrine, and serotonin. - Its dysfunction is more commonly associated with conditions like **Parkinson's disease** and depression, not the primary deficit in Alzheimer's. *Dopamine β-hydroxylase* - **Dopamine β-hydroxylase (DBH)** catalyzes the conversion of dopamine to norepinephrine. - Dysregulation of norepinephrine pathways can occur in Alzheimer's, but a primary deficiency of DBH is **not a hallmark** of the disease.

Q: In the context of lysosomal storage diseases, what is the consequence of glucocerebrosidase deficiency?

Storage of lipid substrates in macrophages (Gaucher cells). ***Storage of lipid substrates in macrophages (Gaucher cells)*** - **Glucocerebrosidase** is an enzyme responsible for breaking down **glucocerebroside**. Its deficiency leads to the accumulation of this lipid. - This accumulation primarily occurs in **macrophages**, forming characteristic **Gaucher cells** with a "wrinkled tissue paper" appearance, leading to **Gaucher disease**. *Impaired glycogen metabolism (seen in Pompe disease)* - Impaired glycogen metabolism is characteristic of **glycogen storage diseases**, such as **Pompe disease**, which involves a deficiency of **acid alpha-glucosidase**. - This deficiency results in the accumulation of **glycogen** in lysosomes, primarily affecting muscle and liver. *Altered lipid metabolism in other pathways* - While glucocerebrosidase deficiency certainly alters lipid metabolism, this option is too general. The specific consequence involves the **storage of glucocerebroside**. - Many lysosomal storage diseases involve altered lipid metabolism, but each is distinct in the specific lipid and enzyme affected. *Storage of sphingomyelin in lysosomes (seen in Niemann-Pick disease)* - The storage of **sphingomyelin** in lysosomes is characteristic of **Niemann-Pick disease**, caused by a deficiency of the enzyme **sphingomyelinase**. - This leads to the accumulation of sphingomyelin in various organs, including the spleen, liver, lungs, and brain.

Q: A newborn presents with hypotonia, poor feeding, and a cherry-red spot on the retina. Which lysosomal enzyme deficiency is most likely?

Hexosaminidase A. ***Hexosaminidase A*** - Deficiency of **Hexosaminidase A** leads to **Tay-Sachs disease**, a lysosomal storage disorder characterized by the accumulation of **GM2 ganglioside**. - Classic symptoms include **hypotonia**, **poor feeding**, developmental regression, and a pathognomonic **cherry-red spot on the retina** (present in nearly all cases). - This is the most common cause of cherry-red spot in infancy. *Arylsulfatase A* - Deficiency of **arylsulfatase A** causes **metachromatic leukodystrophy**, which manifests with progressive **demyelination** and neurological decline. - It typically does not present with a cherry-red spot on the retina or the specific constellation of symptoms described. *β-Glucocerebrosidase* - Deficiency of **β-glucocerebrosidase** leads to **Gaucher disease**, characterized by the accumulation of **glucocerebroside** in macrophages. - Symptoms include **hepatosplenomegaly**, bone pain, and cytopenias, but not hypotonia or a cherry-red spot in infancy. *Sphingomyelinase* - Deficiency of **sphingomyelinase** causes **Niemann-Pick disease (Type A and B)**, involving the accumulation of **sphingomyelin**. - Type A can present with hepatosplenomegaly, neurodegeneration, and cherry-red spots (in ~50% of cases), but typically presents with **prominent hepatosplenomegaly** as an early feature, which is not mentioned in this clinical scenario. - The combination of profound hypotonia and poor feeding as presenting features, along with cherry-red spot and **absence of organomegaly**, favors Tay-Sachs over Niemann-Pick.

Q: A 30-year-old presents with recurrent kidney stones. Which enzyme deficiency should be investigated for primary hyperoxaluria?

Alanine-glyoxylate aminotransferase. ***Alanine-glyoxylate aminotransferase*** - This enzyme (AGT) is deficient in **primary hyperoxaluria type 1 (PH1)**, the most common and severe form of primary hyperoxaluria. - A deficiency in AGT leads to the accumulation of **glyoxylate**, which is then converted into **oxalate**, leading to increased urinary oxalate excretion and recurrent **calcium oxalate kidney stones**. *Glucose-6-phosphatase* - Deficiency of this enzyme is characteristic of **Glycogen Storage Disease Type I (von Gierke disease)**. - It leads to **hypoglycemia**, lactic acidosis, hyperuricemia, and hyperlipidemia, but not primary hyperoxaluria. *Phenylalanine hydroxylase* - A deficiency in this enzyme causes **phenylketonuria (PKU)**, an inborn error of amino acid metabolism. - PKU results in the accumulation of **phenylalanine** and its metabolites, leading to neurological damage if untreated, but is not associated with primary hyperoxaluria. *Lactase* - Lactase deficiency causes **lactose intolerance**, where the body cannot properly digest lactose, a sugar found in milk and dairy products. - Symptoms include bloating, diarrhea, and abdominal cramps after consuming lactose, and it is not related to primary hyperoxaluria or kidney stone formation.

Q: What enzyme deficiency is associated with the overproduction of uric acid and the development of gout?

Hypoxanthine-guanine phosphoribosyltransferase. ***Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)*** - A deficiency in **HGPRT** leads to an inability to salvage hypoxanthine and guanine, shunting these purine bases towards degradation. - This results in the **overproduction of uric acid** as the end product of purine metabolism, directly contributing to hyperuricemia and gout. *Adenosine deaminase* - Deficiency of **adenosine deaminase (ADA)** leads to the accumulation of deoxyadenosine and its derivatives, which are toxic to lymphocytes. - This causes **severe combined immunodeficiency (SCID)**, not gout or uric acid overproduction. *Xanthine oxidase* - **Xanthine oxidase** is the enzyme responsible for the final steps of purine degradation, converting hypoxanthine to xanthine and then xanthine to uric acid. - Inhibition of xanthine oxidase (e.g., by allopurinol) is a treatment for gout, meaning a **deficiency would *reduce* uric acid production**, not increase it. *Uricase* - **Uricase** is an enzyme that converts uric acid into allantoin, a more soluble compound. - Humans naturally lack this enzyme, which is why we excrete uric acid; its absence is normal and not a "deficiency" causing *overproduction* of uric acid, although some conditions leverage recombinant uricase for treatment.

Question 1

A 5-year-old presents with intellectual disability, self-mutilation, and hyperuricemia. What enzyme defect is most likely?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Answer

Adenosine deaminase deficiency

Answer

Xanthine oxidase deficiency

Answer

Glucose-6-phosphatase deficiency

Question 2

Which of the following is a common genetic mutation leading to hypoxia-induced polycythemia in patients with chronic mountain sickness?

Accepted Answer

VHL gene mutation

Answer

EGFR gene mutation

Answer

JAK2 V617F mutation

Answer

HIF1α gene mutation

Question 3

In individuals of South Indian descent with a family history of type 2 diabetes, which genetic variant is most commonly associated with an increased risk of developing this condition?

Accepted Answer

TCF7L2 gene variant

Answer

SLC30A8 gene variant

Answer

PPAR-gamma gene variant

Answer

KCNJ11 gene variant

Question 4

A patient is diagnosed with MELAS syndrome after presenting with stroke-like episodes and lactic acidosis. What is the underlying defect?

Accepted Answer

Mitochondrial DNA mutation

Answer

Nuclear DNA mutation

Answer

Lysosomal storage defect

Answer

Glycogen storage disease

Question 5

Which enzyme is most commonly deficient in patients with Alzheimer's disease?

Accepted Answer

Choline acetyltransferase

Answer

Acetylcholinesterase

Answer

Monoamine oxidase

Answer

Dopamine β-hydroxylase

Question 6

A 50-year-old woman with progressive ataxia is found to have an autosomal recessive mutation affecting the frataxin gene. Which of the following best describes a key aspect of the pathophysiology of her condition?

Accepted Answer

Impaired mitochondrial iron handling due to frataxin deficiency.

Answer

Increased oxidative stress resulting from mitochondrial dysfunction.

Answer

Decreased ATP synthesis due to impaired iron metabolism.

Answer

Progressive neuronal degeneration in the dorsal root ganglia.

Question 7

In the context of lysosomal storage diseases, what is the consequence of glucocerebrosidase deficiency?

Accepted Answer

Storage of lipid substrates in macrophages (Gaucher cells)

Answer

Altered lipid metabolism in other pathways

Answer

Impaired glycogen metabolism (seen in Pompe disease)

Answer

Storage of sphingomyelin in lysosomes (seen in Niemann-Pick disease)

Question 8

A newborn presents with hypotonia, poor feeding, and a cherry-red spot on the retina. Which lysosomal enzyme deficiency is most likely?

Accepted Answer

Hexosaminidase A

Answer

Arylsulfatase A

Answer

β-Glucocerebrosidase

Answer

Sphingomyelinase

Question 9

A 30-year-old presents with recurrent kidney stones. Which enzyme deficiency should be investigated for primary hyperoxaluria?

Accepted Answer

Alanine-glyoxylate aminotransferase

Answer

Glucose-6-phosphatase

Answer

Phenylalanine hydroxylase

Answer

Lactase

Question 10

What enzyme deficiency is associated with the overproduction of uric acid and the development of gout?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase

Answer

Adenosine deaminase

Answer

Xanthine oxidase

Answer

Uricase

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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