Genetic Disorders and Biochemical Pathology Practice Questions

Q: Alpha-1-antitrypsin deficiency is associated with a gene located on which chromosome?

Chromosome 14. **Explanation:** **Alpha-1-Antitrypsin (AAT) Deficiency** is an autosomal codominant disorder caused by mutations in the **SERPINA1 gene**. This gene is located on the **long arm of Chromosome 14 (14q32.1)**. AAT is a serine protease inhibitor (serpin) synthesized primarily in the liver; its main function is to inhibit neutrophil elastase in the lungs, thereby preventing alveolar destruction. * **Why Chromosome 14 is correct:** The SERPINA1 gene locus is part of a cluster of serpin genes located on 14q. Mutations here (most commonly the **PiZ variant**) lead to protein misfolding, causing liver cirrhosis (due to accumulation of polymers in hepatocytes) and panacinar emphysema (due to unchecked elastase activity). **Analysis of Incorrect Options:** * **Chromosome 10:** Associated with conditions like PTEN hamartoma tumor syndrome and RET proto-oncogene mutations (MEN 2). * **Chromosome 11:** Home to the Beta-globin gene cluster (Sickle cell anemia, Beta-thalassemia) and the WT1 gene (Wilms tumor). * **Chromosome 17:** Associated with NF1 (Neurofibromatosis type 1), TP53 (Li-Fraumeni syndrome), and BRCA1. **High-Yield Clinical Pearls for NEET-PG:** * **Histology:** Liver biopsy shows **PAS-positive, diastase-resistant globules** in periportal hepatocytes. * **Genetics:** The normal allele is **M**, and the most severe deficiency allele is **Z** (PiZZ phenotype). * **Clinical Presentation:** Suspect AAT deficiency in a young, non-smoker presenting with **panacinar emphysema** (classically involving lower lobes) and unexplained liver disease. * **Electrophoresis:** On serum protein electrophoresis, there is a characteristic **absence of the Alpha-1 globulin peak**.

Q: The following pedigree represents which of the following genetic disorders?

Osteogenesis imperfecta. ***Osteogenesis imperfecta*** - Shows **autosomal dominant inheritance** pattern with **vertical transmission** - affected individuals in every generation passing the condition to approximately **50% of their offspring**. - **Type I osteogenesis imperfecta** is the most common form with this inheritance pattern, caused by mutations in **COL1A1** or **COL1A2** genes affecting collagen production. *Wilson's disease* - Follows **autosomal recessive inheritance** pattern, which would show **horizontal transmission** with unaffected parents having affected children. - Caused by mutations in the **ATP7B gene**, leading to copper accumulation, and would not demonstrate the vertical inheritance pattern seen in this pedigree. *Alkaptonuria* - Exhibits **autosomal recessive inheritance**, characterized by **consanguineous marriages** and affected siblings from unaffected parents. - Results from **homogentisic acid oxidase deficiency**, causing dark urine and would not show the dominant inheritance pattern observed. *Cystic fibrosis* - Demonstrates **autosomal recessive inheritance** with **horizontal transmission** pattern, typically affecting siblings rather than successive generations. - Caused by **CFTR gene mutations** and would require both parents to be carriers, not the dominant pattern shown in this pedigree.

Q: Which of the following diseases is NOT a mitochondrial disorder?

Huntington's disease. **Explanation:** The correct answer is **Huntington’s disease** because it is an **autosomal dominant** neurodegenerative disorder caused by a trinucleotide repeat expansion (CAG) on Chromosome 4. It is a nuclear DNA defect, not a mitochondrial DNA (mtDNA) defect. **Why the other options are Mitochondrial Disorders:** Mitochondrial diseases typically involve defects in oxidative phosphorylation and exhibit **maternal inheritance** (mitochondrial inheritance). * **Leigh Syndrome:** A severe neurological disorder (subacute necrotizing encephalomyelopathy) often caused by mutations in mtDNA-encoded subunits of the electron transport chain (though nuclear mutations can also occur). * **Leber’s Hereditary Optic Neuropathy (LHON):** A classic example of mitochondrial inheritance characterized by bilateral painless loss of central vision due to mutations in NADH dehydrogenase subunits. * **Myoclonic Epilepsy with Ragged Red Fibers (MERRF):** Caused by a mutation in the mitochondrial tRNA-Lysine gene. It is characterized by myoclonus, ataxia, and the presence of "ragged red fibers" (clumps of diseased mitochondria) on Gomori trichrome stain. **NEET-PG High-Yield Pearls:** 1. **Maternal Inheritance:** Mitochondrial disorders are passed only from mothers to all children. Affected fathers do not pass the trait. 2. **Heteroplasmy:** The coexistence of mutated and wild-type mtDNA within a single cell, explaining the variable clinical severity. 3. **Tissues Affected:** Organs with high energy demands (Brain, Heart, Skeletal Muscle) are most commonly involved. 4. **Huntington’s Disease Key Fact:** It exhibits **anticipation** (earlier onset in successive generations), especially during paternal transmission.

Q: Friedreich's ataxia is caused by which of the following genetic mechanisms?

Expanded trinucleotide repeat. **Explanation:** **Friedreich's Ataxia (FRDA)** is an autosomal recessive neurodegenerative disorder. The correct answer is **Expanded trinucleotide repeat** because the disease is caused by an unstable expansion of the **GAA** triplet repeat in the first intron of the **FXN gene** on chromosome 9. This expansion leads to transcriptional silencing (epigenetic knockdown) of the gene, resulting in a deficiency of the protein **Frataxin**. Frataxin is essential for mitochondrial iron metabolism; its deficiency leads to iron accumulation, oxidative stress, and impaired ATP production, primarily affecting the dorsal root ganglia, spinocerebellar tracts, and the heart. **Why other options are incorrect:** * **Point mutation & Missense mutation:** While rare point mutations can occur in the FXN gene, they account for less than 5% of cases. The hallmark and primary mechanism (95%+) is the triplet repeat expansion. * **Inversion:** This involves a segment of DNA being reversed end-to-end (e.g., Hemophilia A). It is not the mechanism underlying Friedreich's ataxia. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive (unique among most trinucleotide repeat disorders like Huntington’s or Fragile X, which are Dominant/X-linked). * **Repeat Sequence:** **GAA** (Mnemonic: Friedreich is **GAA**y/GAA). * **Clinical Triad:** Progressive ataxia, HOCM (Hypertrophic Obstructive Cardiomyopathy—the most common cause of death), and Diabetes Mellitus. * **Skeletal findings:** Kyphoscoliosis and Pes cavus (high-arched feet).

Q: All of the following conditions have autosomal dominant inheritance except?

Fabry disease. **Explanation:** The correct answer is **Fabry disease** because it follows an **X-linked recessive** inheritance pattern, whereas the other options are classic examples of autosomal dominant (AD) disorders. 1. **Fabry Disease (Choice A):** This is a lysosomal storage disorder caused by a deficiency of the enzyme **$\alpha$-galactosidase A**, leading to the accumulation of globotriaosylceramide. Unlike most other sphingolipidoses (which are autosomal recessive), Fabry disease and Hunter syndrome are **X-linked**. Clinical hallmarks include angiokeratomas, peripheral neuropathy (acroparesthesia), hypohidrosis, and progressive renal/cardiac failure. 2. **Marfan’s Syndrome (Choice B):** This is an **AD** connective tissue disorder caused by mutations in the **FBN1 gene** on chromosome 15, which encodes **Fibrillin-1**. It typically presents with tall stature, ectopia lentis, and aortic root dilation. 3. **Osteogenesis Imperfecta (Choice C):** Most common types (Type I-IV) are inherited in an **AD** fashion, resulting from mutations in **COL1A1 or COL1A2** genes. It is characterized by "brittle bones," blue sclera, and hearing loss. 4. **Ehlers-Danlos Syndrome (Choice D):** While EDS is a heterogeneous group, the most common types (like the Classical and Hypermobility types) follow an **AD** inheritance pattern. It involves defects in collagen synthesis or processing, leading to skin hyperextensibility and joint hypermobility. **High-Yield NEET-PG Pearls:** * **Mnemonic for X-linked Recessive:** "**H**unter's **F**abry **G**ames **A**re **C**ool" (**H**unter, **F**abry, **G**6PD, **A**LDP/Adrenoleukodystrophy, **C**GDR/Color blindness). * Most structural protein defects (collagen, fibrillin, spectrin) are **Autosomal Dominant**. * Most enzyme deficiencies (metabolic pathways) are **Autosomal Recessive**, with Fabry and Hunter being the notable X-linked exceptions.

Q: What is true about Crigler-Najjar type II syndrome?

Diglucuronide deficiency. **Explanation:** Crigler-Najjar Syndrome (CNS) is caused by a deficiency of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**, which is responsible for conjugating bilirubin with glucuronic acid in the liver. **1. Why Option A is Correct:** In Crigler-Najjar Type II (Arias Syndrome), there is a partial deficiency of UGT1A1 (enzyme activity is usually <10% of normal). Because the enzyme is present but severely limited, it can only perform the first step of conjugation effectively. This results in the formation of **bilirubin monoglucuronide**, but there is a significant **deficiency of bilirubin diglucuronide** in the bile. **2. Why the other options are incorrect:** * **Option B (Recessive trait):** Unlike Type I, which is strictly autosomal recessive, Type II is typically inherited as an **autosomal dominant** trait with variable penetrance (though some cases show recessive patterns). * **Option C (Kernicterus):** Kernicterus is a hallmark of **Type I** (complete enzyme absence). In Type II, serum bilirubin levels are lower (usually 6–20 mg/dL), and **kernicterus is rare**, typically occurring only during periods of severe stress or illness. * **Option D (Phenobarbitone):** Phenobarbitone is a potent inducer of the UGT1A1 enzyme. It is **highly effective** in Type II, significantly reducing serum bilirubin levels. It has no effect in Type I because the enzyme is completely absent. **High-Yield Clinical Pearls for NEET-PG:** * **Type I vs. Type II:** The most definitive clinical test to differentiate the two is the **Phenobarbitone response test** (Positive in Type II; Negative in Type I). * **Bile Color:** Bile is colorless in Type I (no conjugated bilirubin) but pigmented in Type II. * **Gilbert Syndrome:** The mildest form of UGT1A1 deficiency (enzyme activity ~30% of normal), often asymptomatic until triggered by fasting or stress.

Q: Which of the following is not included under Garrod's tetrad?

Phenylketonuria. **Explanation** Sir Archibald Garrod, known as the "Father of Inborn Errors of Metabolism," first proposed the concept that certain diseases result from a genetic deficiency of a specific enzyme. In 1902, he described a group of four conditions collectively known as **Garrod’s Tetrad**. **Why Phenylketonuria (PKU) is the correct answer:** Phenylketonuria is **not** part of the original tetrad. Although PKU is a classic inborn error of metabolism (deficiency of Phenylalanine Hydroxylase), it was discovered much later, in 1934, by Ivar Asbjørn Følling. **Analysis of Garrod’s Tetrad (Incorrect Options):** The four conditions originally identified by Garrod are: 1. **Alkaptonuria (Option A):** The first one described; characterized by a deficiency of Homogentisate Oxidase leading to black urine. 2. **Albinism (Option A):** Specifically Oculocutaneous Albinism, involving a defect in Tyrosinase. 3. **Pentosuria (Option B):** A benign condition involving the excretion of L-xylulose due to a deficiency of L-xylulose reductase. 4. **Cystinuria (Option D):** A transport defect of COAL (Cystine, Ornithine, Arginine, Lysine) in the renal tubules, leading to hexagonal cystine stones. **High-Yield Clinical Pearls for NEET-PG:** * **Alkaptonuria:** Look for "Ochronosis" (pigmentation of connective tissue) and arthritis in clinical vignettes. * **Pentosuria:** Often confused with Diabetes Mellitus because it gives a positive Benedict’s test (reducing sugar), but it shows a negative Glucose Oxidase test. It is common in Ashkenazi Jews. * **Cystinuria:** Diagnosed via the **Cyanide-Nitroprusside test**. * **Garrod’s Hypothesis:** He coined the term "Inborn Errors of Metabolism" and was the first to suggest the "one gene, one enzyme" relationship.

Q: Nitisinone is an example of which of the following therapeutic approaches?

Substrate reduction therapy. **Explanation:** **Nitisinone** (NTBC) is the primary treatment for **Hereditary Tyrosinemia Type I (HT-1)**. HT-1 is caused by a deficiency of the enzyme *Fumarylacetoacetate Hydrolase (FAH)*, the final step in tyrosine catabolism. This deficiency leads to the accumulation of toxic metabolites like **succinylacetone**, which causes severe liver failure, renal tubular dysfunction, and hepatocellular carcinoma. **1. Why Substrate Reduction Therapy (SRT) is correct:** Nitisinone acts as a potent inhibitor of **4-hydroxyphenylpyruvate dioxygenase (4-HPPD)**, an enzyme upstream in the tyrosine degradation pathway. By blocking this enzyme, Nitisinone prevents the formation of maleylacetoacetate and fumarylacetoacetate, thereby reducing the production of the toxic substrate succinylacetone. This "metabolic bypass" shifts the pathology from a lethal condition to a manageable one (Tyrosinemia Type III-like state). **2. Why other options are incorrect:** * **Enzyme Activation:** This involves using small molecules to increase the residual activity of a mutant enzyme (e.g., Vitamin B6 for Homocystinuria). Nitisinone *inhibits* rather than activates. * **Enzyme Replacement Therapy (ERT):** This involves administering the functional recombinant enzyme intravenously (e.g., Alglucerase for Gaucher disease). * **Substrate Addition:** This involves providing the end-product that the body cannot produce (e.g., Citrulline in certain Urea Cycle Disorders). **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Marker:** Succinylacetone in urine is pathognomonic for Tyrosinemia Type I. * **Dietary Management:** Patients on Nitisinone must follow a **low-tyrosine and low-phenylalanine diet** to prevent corneal crystals and neurological issues caused by secondary hypertyrosinemia. * **Other SRT Examples:** **Miglustat** (used in Gaucher disease) inhibits glucosylceramide synthase, another classic example of SRT.

Question 1

Which enzyme deficiency is most commonly responsible for the presence of a long clitoris and fused vagina?

Accepted Answer

21 hydroxylase

Answer

11 hydroxylase

Answer

3 beta hydroxy steroid dehydrogenase

Answer

None of the above

Question 2

All of the following are associated with non-ketotic hypoglycemia, EXCEPT?

Accepted Answer

Von Gierke's disease

Answer

Insulinoma

Answer

Carnitine deficiency

Answer

MCAD deficiency

Question 3

Alpha-1-antitrypsin deficiency is associated with a gene located on which chromosome?

Accepted Answer

Chromosome 14

Answer

Chromosome 10

Answer

Chromosome 17

Answer

Chromosome 11

Question 4

The following pedigree represents which of the following genetic disorders?

Accepted Answer

Osteogenesis imperfecta

Answer

Wilson’s disease

Answer

Alkaptonuria

Answer

Cystic fibrosis

Question 5

Which of the following diseases is NOT a mitochondrial disorder?

Accepted Answer

Huntington's disease

Answer

Leigh syndrome

Answer

Leber's hereditary optic neuropathy

Answer

Myoclonic epilepsy

Question 6

Friedreich's ataxia is caused by which of the following genetic mechanisms?

Accepted Answer

Expanded trinucleotide repeat

Answer

Point mutation

Answer

Missense mutation

Answer

Inversion

Question 7

All of the following conditions have autosomal dominant inheritance except?

Accepted Answer

Fabry disease

Answer

Marfan's syndrome

Answer

Osteogenesis imperfecta

Answer

Ehlers Danlos syndrome

Question 8

What is true about Crigler-Najjar type II syndrome?

Accepted Answer

Diglucuronide deficiency

Answer

Recessive trait

Answer

Kernicterus is seen

Answer

Phenobarbitone is not useful

Question 9

Which of the following is not included under Garrod's tetrad?

Accepted Answer

Phenylketonuria

Answer

Albinism

Answer

Pentosuria

Answer

Cystinuria

Question 10

Nitisinone is an example of which of the following therapeutic approaches?

Accepted Answer

Substrate reduction therapy

Answer

Enzyme activation therapy

Answer

Enzyme replacement therapy

Answer

Substrate addition therapy

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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