Genetic Disorders and Biochemical Pathology — MCQs

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

Question 781: The uronic acid level in urine is elevated in what condition?

A. Tyrosinosis
B. Maple syrup urine disease
C. Mucopolysaccharidosis (Correct Answer)
D. Niemann-Pick disease

Explanation: ***Mucopolysaccharidosis*** - **Mucopolysaccharidoses (MPS)** are a group of genetic disorders caused by the deficiency of lysosomal enzymes responsible for breaking down **glycosaminoglycans (GAGs)**, which are also known as mucopolysaccharides. - The accumulation of these undegraded GAGs, which contain **uronic acids**, leads to their excretion in the urine, hence the elevated uronic acid levels. *Tyrosinosis* - **Tyrosinosis (Tyrosinemia)** is a metabolic disorder characterized by the inability to properly break down the amino acid **tyrosine**. - It leads to the accumulation of *tyrosine* and its derivatives, not uronic acids, in the body fluids. *Maple syrup urine disease* - **Maple syrup urine disease (MSUD)** is an autosomal recessive metabolic disorder affecting the metabolism of branched-chain amino acids (BCAAs): **leucine, isoleucine, and valine**. - The disease is characterized by a distinctive sweet smell in the urine, reminiscent of maple syrup, due to the accumulation of these amino acids and their ketoacid derivatives, not uronic acids. *Niemann-Pick disease* - **Niemann-Pick disease** is a group of rare, inherited metabolic disorders that affect the body's ability to metabolize fats (**lipids**) within cells. - It results from a deficiency of an enzyme called acid sphingomyelinase or a defect in intracellular cholesterol trafficking, leading to the accumulation of sphingomyelin and cholesterol in various organs.

Question 782: Defective fumarylacetoacetate hydrolase enzyme is associated with:

A. Type 1 Tyrosinemia (Correct Answer)
B. Type 2 Tyrosinemia
C. Type 3 Tyrosinemia
D. Type 4 Tyrosinemia

Explanation: ***Type 1 Tyrosinemia*** - Type 1 Tyrosinemia (also known as tyrosinemia type I or hepatorenal tyrosinemia) results from a deficiency of the enzyme **fumarylacetoacetate hydrolase (FAH)**, the last enzyme in the tyrosine degradation pathway. - This deficiency leads to the accumulation of toxic metabolites such as **fumarylacetoacetate** and **succinylacetone**, causing severe liver, kidney, and neurological dysfunction. - Clinical features include hepatomegaly, cirrhosis, renal tubular dysfunction, and increased risk of hepatocellular carcinoma. *Type 2 Tyrosinemia* - Type 2 Tyrosinemia (oculocutaneous tyrosinemia or Richner-Hanhart syndrome) is caused by a deficiency of the enzyme **tyrosine aminotransferase (TAT)**. - This defect primarily affects the eyes and skin, leading to **corneal lesions** and **palmoplantar hyperkeratosis**, without the severe liver and kidney damage seen in Type 1. *Type 3 Tyrosinemia* - Type 3 Tyrosinemia is a rare disorder resulting from a deficiency of **4-hydroxyphenylpyruvate dioxygenase (HPD)**. - This type is generally milder and may present with neurological symptoms like intellectual disability, ataxia, and seizures, but not the severe visceral damage characteristic of Type 1. *Type 4 Tyrosinemia* - There is **no recognized medical condition** described as "Type 4 Tyrosinemia." - The classification of tyrosinemia typically includes types I, II, and III, each associated with a specific enzymatic defect in the tyrosine degradation pathway.

Question 783: Which of the following statements about polymorphism is true?

A. Single phenotype linked to a single locus with multiple abnormal alleles.
B. Single locus with multiple normal alleles. (Correct Answer)
C. Single locus with multiple abnormal alleles, not linked to a specific phenotype.
D. Single phenotype linked to a single locus with both normal and abnormal alleles.

Explanation: ***Single locus with multiple normal alleles.*** - **Polymorphism** refers to the existence of multiple alleles at a **single genetic locus** within a population. - For a variant to be considered a polymorphism, the most common allele must have a frequency of **less than 99%**, meaning at least two alleles are common. *Single locus with multiple abnormal alleles, not linked to a specific phenotype.* - While polymorphism involves multiple alleles at a single locus, classifying them as "abnormal" is misleading, as polymorphism often refers to **variations that are common** in the population and not necessarily disease-causing or abnormal. - The definition emphasizes the presence of multiple alleles, not their clinical implications, and many polymorphisms have **no overt phenotypic effect**. *Single phenotype linked to a single locus with both normal and abnormal alleles.* - Polymorphism primarily describes **genetic variation (alleles)**, not direct links to a single phenotype. A single locus can influence **multiple phenotypes**, and a single phenotype can be influenced by multiple loci. - Grouping alleles as "normal" and "abnormal" oversimplifies the concept; **many polymorphisms are neutral** or beneficial, and some "normal" alleles can become "abnormal" in certain contexts. *Single phenotype linked to a single locus with multiple abnormal alleles.* - This option incorrectly narrows the definition by focusing on a **single phenotype** and exclusively "abnormal" alleles. Polymorphism encompasses any common variation, regardless of its phenotypic effect or whether the alleles are considered abnormal. - Many polymorphic variations are **silent mutations** or variations that do not result in overt phenotypic changes or disease.

Question 784: The type of mutation that leads to the replacement of valine for glutamate in sickle cell disease is?

A. Point mutation (Correct Answer)
B. Silent mutation
C. Nonsense mutation
D. None of the options

Explanation: ***Point mutation*** - A **point mutation** involves a change in a single nucleotide base in the DNA sequence. - In sickle cell disease, a single base change (**adenine to thymine** in the β-globin gene codon 6) results in the substitution of **valine** for **glutamate** at position 6 of the β-globin chain. - More specifically, this is a **missense mutation** (a type of point mutation that changes the amino acid sequence), resulting in the production of hemoglobin S (HbS) instead of normal hemoglobin A (HbA). - This substitution alters the physical properties of hemoglobin, causing RBCs to sickle under low oxygen conditions. *Silent mutation* - A **silent mutation** is a type of point mutation that changes a single nucleotide but does **not** change the amino acid sequence due to the degeneracy of the genetic code. - In sickle cell disease, the mutation causes an amino acid change (**glutamate → valine**), so it is not a silent mutation. *Nonsense mutation* - A **nonsense mutation** is a point mutation that results in a **premature stop codon**, leading to a truncated and often non-functional protein. - In sickle cell disease, the mutation leads to an **amino acid substitution**, not a premature stop codon, so this is incorrect. *None of the options* - This option is incorrect because the replacement of glutamate with valine in sickle cell disease is specifically caused by a **point mutation** (missense type).

Question 785: Pedigree analysis chart is a tool used for which of the following purposes?

A. Used for growth monitoring
B. To assess side effects during chemotherapy
C. To assess developmental delay in infants
D. Used to see genetic transmission (Correct Answer)

Explanation: ***Used to see genetic transmission*** - A **pedigree chart** is a visual representation of biological relationships and genetic traits across multiple generations of a family. - It helps determine the **inheritance pattern** of a particular trait or disease within a family, such as autosomal dominant, recessive, or X-linked. *Used for growth monitoring* - **Growth monitoring** typically involves plotting an individual's weight, height, and head circumference on growth charts over time. - While family data might be considered, a pedigree chart is not the primary tool for directly monitoring an individual's physical growth. *To assess side effects during chemotherapy* - Assessing **side effects during chemotherapy** involves clinical evaluations, laboratory tests (e.g., blood counts, organ function tests), and patient self-reporting. - A pedigree chart provides information about family history and genetic predisposition, not direct monitoring of treatment side effects. *To assess developmental delay in infants* - **Developmental delay assessment** involves observing a child's milestone achievement in areas like motor skills, language, and cognition, often using standardized screening tools. - While some developmental delays can have genetic causes that might be highlighted in a pedigree, the chart itself does not directly assess developmental progress.

Question 786: What is the metabolic defect in primary oxaluria type II?

A. Glycine cleavage system
B. Alanine glyoxalate amino transferase
C. Excess vitamin C
D. D Glycerate dehydrogenase (Correct Answer)

Explanation: ***D Glycerate dehydrogenase*** - Primary oxaluria type II (PH2) specifically results from a deficiency in the enzyme **D-glycerate dehydrogenase** (also known as glyoxylate reductase/hydroxypyruvate reductase). - This deficiency leads to the accumulation of **L-glyceric acid** and **oxalate**, causing kidney stones and kidney failure. *Glycine cleavage system* - Defects in the glycine cleavage system are associated with **nonketotic hyperglycinemia**, a condition involving high levels of glycine. - This defect does not primarily cause the accumulation of oxalate. *Alanine glyoxalate amino transferase* - A deficiency in **alanine-glyoxylate aminotransferase (AGT)** is the underlying defect in **primary oxaluria type I (PH1)**. - PH1 is the most common and severe form of primary hyperoxaluria, leading to increased oxalate production. *Excess vitamin C* - While **excessive vitamin C intake** can contribute to increased urinary oxalate excretion in some individuals, it is not a genetic metabolic defect. - It is an exogenous factor, not an intrinsic enzyme deficiency causing primary hyperoxaluria.

Question 787: Inheritance of Huntington's disease is

A. Autosomal Dominant (Correct Answer)
B. X-Linked Recessive
C. Autosomal Recessive
D. X-Linked Dominant

Explanation: ***AD*** - Huntington's chorea is inherited in an **autosomal dominant** manner, meaning only one copy of the mutated gene is sufficient to cause the disorder [1]. - The disease typically manifests in mid-adulthood, with progressively worsening movement disorders and cognitive decline [1]. *XR* - X-linked recessive disorders typically affect **males** and can be transmitted by carrier females; this is not the case for Huntington's chorea. - The inheritance pattern does not align with the typical clinical presentation of Huntington's, which does not show a gender bias. *XD* - X-linked dominant disorders often affect both sexes, but their inheritance pattern does not describe Huntington's chorea. - Symptoms and gene affected are clearly linked with **autosomal dominant** inheritance, not X-linked dominant. *AR* - Autosomal recessive conditions typically require two copies of the mutated gene, which is not applicable to Huntington's chorea. - This pattern generally leads to earlier onset conditions and significantly different clinical presentations than those observed in Huntington's. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 149-150.

Question 788: Which of the following disorders is most commonly associated with multifactorial inheritance?

A. Achondroplasia
B. Lysosomal storage disease
C. Cleft lip (Correct Answer)
D. Huntington disease

Explanation: ***Cleft lip*** - **Cleft lip** is a classic example of a **multifactorial disorder**, resulting from the interaction of multiple genes and environmental factors. - Its recurrence risk is typically observed to be around 2-5% in affected families, consistent with **multifactorial inheritance**. *Achondroplasia* - Achondroplasia is an **autosomal dominant disorder**, caused by a single gene mutation in the **FGFR3 gene**. - It does not primarily involve the complex interplay of multiple genes and environmental factors characteristic of multifactorial inheritance. *Lysosomal storage disease* - Lysosomal storage diseases are a group of **autosomal recessive disorders**, each caused by a defect in a specific lysosomal enzyme. - They follow classic **Mendelian inheritance patterns** rather than multifactorial models. *Huntington disease* - Huntington disease is an **autosomal dominant neurodegenerative disorder**, caused by a trinucleotide repeat expansion in the **HTT gene**. - It exhibits a clear dominant inheritance pattern and does not involve multiple genes or significant environmental contributions in its etiology.

Question 789: What enzyme deficiency is responsible for Type I phenylketonuria?

A. Tyrosine transaminase
B. Tyrosine hydroxylase
C. Phenylalanine transaminase
D. Phenylalanine hydroxylase (Correct Answer)

Explanation: ***Correct: Phenylalanine hydroxylase*** - **Phenylalanine hydroxylase (PAH)** is the enzyme responsible for converting **phenylalanine** to **tyrosine**. - A deficiency in PAH leads to the accumulation of phenylalanine, which is the hallmark of **Type I phenylketonuria (PKU)**. - Type I PKU is the classic form, accounting for approximately 98% of all PKU cases. *Incorrect: Phenylalanine transaminase* - This enzyme is involved in the transfer of an amino group from **phenylalanine** to an alpha-keto acid, forming **phenylpyruvate**. - While this alternative pathway becomes active in PKU (producing phenylpyruvate and other metabolites), its deficiency is not the primary cause of Type I PKU. *Incorrect: Tyrosine transaminase* - **Tyrosine transaminase** is involved in the metabolism of **tyrosine**, which is downstream of the phenylalanine hydroxylase pathway. - A deficiency in this enzyme would lead to problems with tyrosine degradation (tyrosinemia), not the accumulation of phenylalanine. *Incorrect: Tyrosine hydroxylase* - **Tyrosine hydroxylase** is an enzyme that catalyzes the conversion of **tyrosine** to **L-DOPA**, a precursor for catecholamines (dopamine, norepinephrine, epinephrine). - Its deficiency is associated with conditions affecting neurotransmitter synthesis (such as Segawa syndrome), not phenylketonuria.

Question 790: Which of the following statements regarding the inheritance of red-green color blindness is most accurate?

A. Protanopia is the most common type of color blindness.
B. Individuals with trichromacy can perceive all colors.
C. Color blindness is primarily inherited in an X-linked recessive pattern. (Correct Answer)
D. Color blindness results from a defect in one or more of the primary colors.

Explanation: ***Correct: Color blindness is primarily inherited in an X-linked recessive pattern.*** - The genes for red and green photopigments are located on the **X chromosome** - Males have only one X chromosome, so a single recessive allele causes color blindness - This explains why males are **much more frequently affected** than females - Females need two copies of the recessive allele (one on each X) to be affected *Incorrect: Protanopia is the most common type of color blindness.* - **Deuteranomaly** (green perception defect) is actually the most common form - Protanopia is a rarer dichromacy where red cones are missing - Results in difficulty distinguishing red, orange, and yellow from green *Incorrect: Individuals with trichromacy can perceive all colors.* - **Normal trichromacy** means three functional cone types with full color perception - However, **anomalous trichromacy** also has three cone types but with altered spectral sensitivity - Protanomaly and deuteranomaly are examples where color perception is impaired despite having three cone types *Incorrect: Color blindness results from a defect in one or more of the primary colors.* - Color blindness results from defects in **cone photoreceptors**, not in "primary colors" themselves - The defects involve genes coding for **red or green photopigments** - This leads to impaired detection of specific wavelengths of light

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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