Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology Indian Medical PG Practice Questions and MCQs

Question 671: 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?

A. TCF7L2 gene variant (Correct Answer)
B. SLC30A8 gene variant
C. PPAR-gamma gene variant
D. KCNJ11 gene variant

Explanation: ***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.

Question 672: How does a defect in the enzyme 21-hydroxylase affect the synthesis of adrenal hormones?

A. Decreases aldosterone and cortisol synthesis (Correct Answer)
B. Decreases androgen synthesis
C. Decreases cortisol production
D. Leads to decreased estrogen production

Explanation: ***Decreases aldosterone and cortisol synthesis*** - 21-hydroxylase is a crucial enzyme in the adrenal steroid synthesis pathway, necessary for converting **progesterone** to **11-deoxycorticosterone** (a precursor to aldosterone) and **17-hydroxyprogesterone** to **11-deoxycortisol** (a precursor to cortisol). - A defect in this enzyme directly impairs the production of both **cortisol** and **aldosterone**, leading to their deficiency. - This is the hallmark of **congenital adrenal hyperplasia (CAH)** due to 21-hydroxylase deficiency. *Decreases cortisol production* - While cortisol production is decreased, this option is incomplete as it fails to mention the concurrent decrease in **aldosterone** synthesis, which is also a significant consequence of 21-hydroxylase deficiency. - The deficiency impacts multiple steroid pathways, not just the cortisol pathway in isolation. *Leads to decreased estrogen production* - Estrogen synthesis primarily occurs in the **gonads** and **peripheral tissues** from androgens, not directly regulated by 21-hydroxylase in the adrenal cortex. - A 21-hydroxylase defect leads to an **accumulation of androgen precursors** (like DHEA), which can be converted to estrogens peripherally, sometimes even increasing estrogen levels in certain forms of congenital adrenal hyperplasia. *Decreases androgen synthesis* - This is **incorrect** - a 21-hydroxylase defect causes a shunting of precursors towards the **androgen pathway**, leading to an **increase in androgen synthesis**, not a decrease. - This excess androgen production is responsible for the **virilization** observed in individuals with congenital adrenal hyperplasia due to 21-hydroxylase deficiency, including ambiguous genitalia in females and precocious puberty in males.

Question 673: A patient is diagnosed with MELAS syndrome after presenting with stroke-like episodes and lactic acidosis. What is the underlying defect?

A. Nuclear DNA mutation
B. Mitochondrial DNA mutation (Correct Answer)
C. Lysosomal storage defect
D. Glycogen storage disease

Explanation: ***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.

Question 674: Which enzyme deficiency leads to the accumulation of ceramide trihexoside in Fabry disease?

A. β-glucosidase deficiency
B. Hexosaminidase A deficiency
C. α-galactosidase A deficiency (Correct Answer)
D. Sphingomyelinase deficiency

Explanation: ***α-galactosidase A deficiency*** - **Fabry disease** is an **X-linked recessive** lysosomal storage disorder caused by a deficiency of the enzyme **α-galactosidase A**. - This enzyme deficiency leads to the accumulation of its substrate, **globotriaosylceramide (Gb3)**, also known as **ceramide trihexoside**, in various tissues. *β-glucosidase deficiency* - This deficiency is characteristic of **Gaucher disease**, leading to the accumulation of **glucocerebroside**. - Patients typically present with **hepatosplenomegaly**, **bone pain**, and **pancytopenia**, which are not features of Fabry disease. *Hexosaminidase A deficiency* - This enzyme deficiency is associated with **Tay-Sachs disease**, which results in the accumulation of **GM2 gangliosides**. - Tay-Sachs disease primarily affects the **nervous system**, causing neurodegeneration in infancy. *Sphingomyelinase deficiency* - This deficiency causes **Niemann-Pick disease**, leading to the accumulation of **sphingomyelin** in lysosomes. - Clinical features include **hepatosplenomegaly**, **neurodegeneration** (especially in Type A), and characteristic **foam cells**.

Question 675: Which enzyme deficiency is associated with Lesch-Nyhan syndrome, which is characterized by self-mutilation and hyperuricemia?

A. Adenosine deaminase
B. Hypoxanthine-guanine phosphoribosyltransferase (Correct Answer)
C. Xanthine oxidase
D. Phosphoribosyl pyrophosphate synthetase

Explanation: ***Hypoxanthine-guanine phosphoribosyltransferase*** - Deficiency of **hypoxanthine-guanine phosphoribosyltransferase (HGPRT)** leads to an inability to salvage purines, resulting in their degradation to **uric acid**. - This overproduction of uric acid causes **hyperuricemia** and characteristic neurological symptoms, including **self-mutilation**, choreoathetosis, and cognitive dysfunction, defining Lesch-Nyhan syndrome. *Adenosine deaminase* - Deficiency in **adenosine deaminase (ADA)** leads to the accumulation of **adenosine** and **deoxyadenosine**, which are toxic to lymphocytes. - This causes severe combined immunodeficiency **(SCID)**, not Lesch-Nyhan syndrome. *Xanthine oxidase* - **Xanthine oxidase** is involved in the catabolism of purines, converting hypoxanthine to xanthine and then to uric acid. - Its deficiency leads to **xanthinuria**, characterized by low uric acid levels and **xanthine kidney stones**, which is distinct from Lesch-Nyhan syndrome. *Phosphoribosyl pyrophosphate synthetase* - **Phosphoribosyl pyrophosphate (PRPP) synthetase** is involved in the *de novo* purine synthesis pathway. - Overactivity (not deficiency) of this enzyme can lead to increased purine production and **hyperuricemia**, but a deficiency typically impairs purine synthesis and does not present with Lesch-Nyhan features.

Question 676: Which enzyme deficiency leads to the accumulation of homogentisic acid, resulting in dark urine and ochronosis?

A. Phenylalanine hydroxylase
B. Tyrosine hydroxylase
C. Homogentisate oxidase (Correct Answer)
D. Fumarylacetoacetate hydrolase

Explanation: ***Homogentisate oxidase*** - Deficiency of **homogentisate oxidase** is the underlying cause of **alkaptonuria**, a rare genetic disorder. - This enzyme is crucial for the metabolism of **tyrosine**, and its absence leads to the accumulation of **homogentisic acid**, which is excreted in the urine and oxidizes upon exposure to air, turning it dark. *Phenylalanine hydroxylase* - Deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, leading to the accumulation of phenylalanine. - PKU primarily results in neurodevelopmental issues and intellectual disability if untreated, not dark urine or ochronosis. *Tyrosine hydroxylase* - **Tyrosine hydroxylase** is involved in the synthesis of catecholamines (dopamine, norepinephrine, epinephrine). - Its deficiency leads to a shortage of these neurotransmitters, causing neurological symptoms but not the characteristic features of alkaptonuria. *Fumarylacetoacetate hydrolase* - Deficiency of **fumarylacetoacetate hydrolase** causes **tyrosinemia type I**, a severe metabolic disorder. - This condition results in liver and kidney damage, neurological crises, and a characteristic "cabbage-like" odor, distinct from the symptoms of alkaptonuria.

Question 677: A newborn exhibits severe mental retardation and a musty body odor. What is the likely diagnosis based on these clinical features?

A. Alkaptonuria
B. Phenylketonuria (Correct Answer)
C. Maple syrup urine disease
D. Homocystinuria

Explanation: ***Phenylketonuria*** - **Phenylketonuria (PKU)** is an autosomal recessive disorder characterized by the inability to metabolize **phenylalanine**, leading to its accumulation. - Classic symptoms in untreated individuals include **severe intellectual disability**, **seizures**, and a distinct **musty body odor** due to the accumulation of phenylacetic acid. *Alkaptonuria* - This disorder is characterized by the accumulation of **homogentisic acid**, which leads to **dark urine** upon standing and bluish-black pigmentation of cartilage and connective tissues (**ochronosis**). - It does not typically present with severe mental retardation or a musty body odor in infancy. *Maple syrup urine disease* - This metabolic disorder is characterized by the inability to metabolize **branched-chain amino acids** (leucine, isoleucine, and valine), leading to their accumulation. - Key features include neurological damage, feeding difficulties, and a characteristic **sweet, maple syrup-like odor in urine** and earwax, not a musty odor. *Homocystinuria* - This is a disorder of methionine metabolism, leading to the accumulation of **homocysteine** in the blood and urine. - Clinical features include **ectopia lentis** (dislocation of the lens), skeletal abnormalities, thrombotic events, and **developmental delay** or intellectual disability, but not a musty body odor.

Question 678: 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?

A. Impaired mitochondrial iron handling due to frataxin deficiency. (Correct Answer)
B. Increased oxidative stress resulting from mitochondrial dysfunction.
C. Decreased ATP synthesis due to impaired iron metabolism.
D. Progressive neuronal degeneration in the dorsal root ganglia.

Explanation: ***Impaired mitochondrial iron handling due to frataxin deficiency*** - Frataxin is a **mitochondrial protein** essential for **iron-sulfur cluster assembly**, which in turn is critical for many mitochondrial enzymes. - Deficiency leads to mitochondrial iron overload and a functional iron deficit in the **cytosol**, impairing heme synthesis and iron-sulfur cluster formation. *Increased oxidative stress resulting from mitochondrial dysfunction* - While increased **oxidative stress** is a consequence of frataxin deficiency, it is not the primary mechanism of pathophysiology. - The underlying cause of the mitochondrial dysfunction and subsequent oxidative stress is the **impaired iron handling**. *Decreased ATP synthesis due to impaired iron metabolism* - Decreased **ATP synthesis** does occur but is a downstream effect, not the initial key aspect of the pathophysiology. - The initial problem is the disrupted **iron metabolism**, specifically within the mitochondria, which subsequently impacts energy production. *Progressive neuronal degeneration in the dorsal root ganglia* - **Neuronal degeneration**, particularly in the **dorsal root ganglia**, is a significant clinical symptom and long-term consequence of the disease, but it's not the primary pathophysiological mechanism itself. - This degeneration results from the **cellular damage** caused by the underlying mitochondrial dysfunction and impaired iron handling.

Question 679: What is the biochemical effect of a deficiency in the enzyme hexosaminidase A on ganglioside metabolism?

A. Accumulation of GM2 ganglioside due to impaired degradation (Correct Answer)
B. No change in sphingomyelin synthesis
C. No significant change in ceramide production
D. Impaired glycolipid degradation without specific accumulation

Explanation: ***Accumulation of GM2 ganglioside due to impaired degradation*** - Hexosaminidase A is responsible for the catabolism of **GM2 ganglioside** by removing the terminal N-acetylgalactosamine. - Deficiency leads to the **lysosomal accumulation** of GM2 ganglioside, primarily in neural tissues, causing neurodegeneration. *No change in sphingomyelin synthesis* - Sphingomyelin synthesis and degradation are primarily managed by **sphingomyelinase**, an enzyme distinct from hexosaminidase A. - A deficiency in hexosaminidase A specifically affects **ganglioside metabolism**, not sphingomyelin. *No significant change in ceramide production* - Ceramide is a precursor to many sphingolipids, including gangliosides, and its production is upstream of ganglioside catabolism. - Hexosaminidase A deficiency impacts the **degradation of specific gangliosides**, not the synthesis of ceramide. *Impaired glycolipid degradation without specific accumulation* - Hexosaminidase A deficiency leads to a **very specific accumulation** (GM2 ganglioside), not a general, non-specific impairment of glycolipid degradation. - The disease associated with this deficiency (Tay-Sachs disease) is characterized by this **particular lipid storage**.

Question 680: What is the underlying biochemical defect in phenylketonuria (PKU) that leads to cognitive impairment if left untreated?

A. Deficiency in phenylalanine hydroxylase, leading to accumulation of phenylalanine (Correct Answer)
B. Deficiency in tyrosinase, leading to lack of melanin production
C. Deficiency in homogentisate oxidase, leading to ochronosis
D. Deficiency in glucose-6-phosphatase, leading to hypoglycemia

Explanation: ***Deficiency in phenylalanine hydroxylase, leading to accumulation of phenylalanine*** - **Phenylketonuria (PKU)** is primarily caused by a genetic deficiency of the enzyme **phenylalanine hydroxylase (PAH)**. - This deficiency prevents the conversion of **phenylalanine** to **tyrosine**, leading to toxic accumulation of phenylalanine and its byproducts in the blood and brain, causing severe **cognitive impairment**. *Deficiency in tyrosinase, leading to lack of melanin production* - A deficiency in **tyrosinase** is characteristic of **albinism**, which results in reduced or absent melanin production and pigmentation issues. - While *tyrosine* is related to *phenylalanine* metabolism, this defect does not cause the cognitive impairment seen in PKU. *Deficiency in homogentisate oxidase, leading to ochronosis* - This defect describes **alkaptonuria**, a rare metabolic disorder where the body cannot properly break down *tyrosine* and *phenylalanine* into simpler molecules. - It leads to the accumulation of **homogentisic acid**, causing **ochronosis** (darkening of connective tissues) and arthritis, but not the primary cognitive impairment of PKU. *Deficiency in glucose-6-phosphatase, leading to hypoglycemia* - This is the underlying biochemical defect in **Type I glycogen storage disease (von Gierke's disease)**. - It results in the inability to release glucose from stored glycogen, leading to severe **hypoglycemia** and hepatomegaly, which is unrelated to PKU.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

Practice Questions

Lysosomal Storage Diseases

Practice Questions

Glycogen Storage Diseases

Practice Questions

Disorders of Lipoprotein Metabolism

Practice Questions

Disorders of Purine and Pyrimidine Metabolism

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Hemoglobinopathies

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Porphyrias

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Biochemical Markers for Disease Diagnosis

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Newborn Screening for Genetic Disorders

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Enzyme Replacement Therapy

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