Genetic Disorders and Biochemical Pathology Practice Questions

Q: For which of the following diseases is enzyme replacement therapy available?

Gaucher's disease. **Explanation:** **Gaucher’s Disease (Correct Answer):** Gaucher’s disease is a lysosomal storage disorder caused by a deficiency of the enzyme **β-glucocerebrosidase**, leading to the accumulation of glucocerebroside in macrophages. It was the first lysosomal storage disorder for which **Enzyme Replacement Therapy (ERT)** was successfully developed. Recombinant enzymes like **Imiglucerase**, Velaglucerase alfa, and Taliglucerase are the standard of care, significantly reducing hepatosplenomegaly and improving hematological parameters. **Why the other options are incorrect:** * **Albinism:** This is a defect in melanin synthesis (typically due to **Tyrosinase** deficiency). It is not a lysosomal storage disorder and cannot be treated with ERT because the enzyme cannot be targeted effectively to melanocytes via the bloodstream. * **Niemann-Pick Disease:** While ERT (Olipudase alfa) has recently been approved for Niemann-Pick Type B (Acid Sphingomyelinase deficiency), it is not yet the "classic" or widely established answer in standard medical examinations compared to Gaucher’s. Type A involves severe neurodegeneration where ERT cannot cross the blood-brain barrier. * **Metachromatic Leukodystrophy (MLD):** Caused by **Arylsulfatase A** deficiency. While gene therapy (Atidarsagene autotemcel) has shown promise, ERT is not the standard established treatment for MLD in the context of NEET-PG questions. **NEET-PG High-Yield Pearls:** * **Gaucher Cells:** Characterized by a "wrinkled paper" or "crumpled silk" appearance of the cytoplasm. * **Other ERT-treatable diseases:** Fabry’s disease (α-galactosidase A), Pompe disease (α-glucosidase), and Hurler syndrome (Iduronidase). * **Targeting Mechanism:** ERT for lysosomal disorders relies on the **Mannose-6-Phosphate (M6P)** receptor-mediated uptake of the enzyme into the cells.

Q: Barth syndrome is due to a defect in which of the following?

Cardiolipin. **Explanation:** **Barth Syndrome** (also known as 3-Methylglutaconic aciduria type II) is an X-linked recessive disorder caused by a mutation in the **TAZ gene**, which encodes the protein **Tafazzin**. Tafazzin is an acyltransferase responsible for the remodeling and maturation of **Cardiolipin**, a unique phospholipid found exclusively in the inner mitochondrial membrane. 1. **Why Cardiolipin is Correct:** Cardiolipin is essential for stabilizing the mitochondrial respiratory chain complexes and maintaining the structural integrity of the cristae. In Barth syndrome, defective remodeling leads to a deficiency of mature cardiolipin, resulting in mitochondrial dysfunction. This manifests clinically as the triad of **dilated cardiomyopathy, skeletal myopathy, and neutropenia.** 2. **Why Other Options are Incorrect:** * **Thermogenin (UCP1):** This is an uncoupling protein found in brown adipose tissue that generates heat. It is not associated with Barth syndrome. * **Ubiquinone (Coenzyme Q10):** This is a mobile electron carrier in the electron transport chain. While CoQ10 deficiency causes mitochondrial diseases, it is not the primary defect in Barth syndrome. * **Cytochrome:** Cytochromes (like Cytochrome c) are heme-containing proteins in the ETC. Mutations in cytochrome oxidase can cause Leigh syndrome, but not Barth syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked Recessive (affects males). * **Biochemical Marker:** Elevated **3-methylglutaconic acid** in urine. * **Key Clinical Features:** "Cardio-Skeletal-Neutro" (Cardiomyopathy, Myopathy, Neutropenia, and Growth retardation). * **Mnemonic:** Think of **"Barth"** as **"Heart"** (Cardiomyopathy) and **"Cardio"**-lipin.

Q: Different mutations in the same genetic locus causing similar or identical phenotypes is termed as?

Allelic heterogeneity. ### Explanation **1. Why Allelic Heterogeneity is Correct:** Allelic heterogeneity refers to a situation where **different mutations at the same genetic locus** (within the same gene) result in the same or a very similar clinical phenotype. In medical genetics, a single gene can be damaged in various ways—such as missense, nonsense, or frameshift mutations—yet all lead to the loss of function of the same protein, causing the same disease. * **Classic Example:** **Cystic Fibrosis.** Over 2,000 different mutations in the *CFTR* gene have been identified; while the specific molecular defect varies, they all manifest as Cystic Fibrosis. **2. Analysis of Incorrect Options:** * **Locus Heterogeneity:** This occurs when mutations at **different genetic loci** (different genes) produce the same phenotype. For example, Osteogenesis Imperfecta can be caused by mutations in either the *COL1A1* or *COL1A2* genes. * **Phenotypic Heterogeneity:** This is the opposite concept, where **different mutations in the same gene** result in **strikingly different clinical phenotypes**. For example, mutations in the *RET* proto-oncogene can cause either Hirschsprung disease (loss of function) or Multiple Endocrine Neoplasia type 2 (gain of function). * **Non-allelic Heterogeneity:** This is essentially a synonym for Locus Heterogeneity. **3. NEET-PG High-Yield Pearls:** * **Beta-Thalassemia** is a prime example of allelic heterogeneity (hundreds of different mutations in the $\beta$-globin gene). * **Compound Heterozygote:** A patient with two different mutant alleles at the same locus (common in allelic heterogeneity) rather than being a true homozygote. * **Pleiotropy:** One single mutation causing multiple, seemingly unrelated phenotypic effects (e.g., Marfan Syndrome affecting the eyes, heart, and skeleton).

Q: All of the following are examples of uniparental disomy except?

Bloom syndrome. **Explanation:** The correct answer is **Bloom syndrome** because it is an **autosomal recessive** disorder characterized by chromosomal instability, not uniparental disomy (UPD). It is caused by a mutation in the *BLM* gene (RECQL3), which encodes a DNA helicase. This leads to excessive sister chromatid exchanges, resulting in short stature, photosensitive rashes, and a high predisposition to various cancers. **Uniparental Disomy (UPD)** occurs when an individual inherits two copies of a chromosome (or part of a chromosome) from one parent and no copy from the other. The incorrect options are classic examples of UPD: * **Prader-Willi Syndrome (PWS):** Approximately 25–30% of cases are caused by **maternal UPD** of chromosome 15 (inheriting two maternal copies, lacking the paternal 15q11-q13 region). * **Angelman Syndrome (AS):** Approximately 3–7% of cases are caused by **paternal UPD** of chromosome 15 (inheriting two paternal copies, lacking the maternal 15q11-q13 region). * **Russell-Silver Syndrome:** About 10% of cases are due to **maternal UPD of chromosome 7**. It presents with intrauterine growth restriction (IUGR), triangular facies, and limb asymmetry. **High-Yield Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** PWS and AS are the "poster children" for imprinting. Remember: **P**ader-Willi = **P**aternal deletion; **A**ngelman = **M**aternal deletion (or vice versa via UPD). * **Bloom Syndrome Hallmark:** Look for "quadriradial figures" on cytogenetic analysis and "sister chromatid exchange" (SCE). * **Beckwith-Wiedemann Syndrome:** Another high-yield UPD example (paternal UPD of chromosome 11).

Q: The provided diagram indicates what type of inheritance?

Mitochondrial. ***Mitochondrial*** - Shows **strict maternal transmission** where all children of affected mothers inherit the condition, regardless of sex - **No paternal transmission** occurs since mitochondria are inherited exclusively from the mother through the egg cytoplasm *Autosomal recessive* - Requires **two affected alleles** for expression, typically showing affected individuals with unaffected parents - Demonstrates **horizontal pattern** with affected siblings but unaffected parents, unlike the vertical maternal pattern seen here *Autosomal dominant* - Shows **vertical transmission** through multiple generations but affects **both maternal and paternal lineages** - Typically has **50% risk** for each child of an affected parent, not the 100% maternal transmission pattern observed *X-linked recessive* - Primarily affects **males** due to hemizygous state, with affected males connected through **carrier females** - Shows **criss-cross inheritance** pattern where affected grandfathers pass the trait through carrier daughters to affected grandsons

Q: Menke's disease is due to a defect in the metabolism of which element?

Copper. **Explanation:** **Menkes Disease (Kinky Hair Syndrome)** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This gene encodes a copper-transporting ATPase responsible for the absorption of copper from the gastrointestinal tract and its distribution to various tissues. In Menkes disease, copper is trapped within intestinal mucosal cells, leading to severe systemic **copper deficiency**. Copper is a vital cofactor for several enzymes. Its deficiency leads to the characteristic clinical features: * **Lysyl Oxidase deficiency:** Causes defective collagen cross-linking, leading to arterial tortuosity and skeletal abnormalities. * **Tyrosinase deficiency:** Results in hypopigmentation. * **Cytochrome c oxidase deficiency:** Leads to neurodegeneration and hypothermia. * **Clinical Hallmark:** "Steely" or "Kinky" brittle hair due to defective keratin disulfide bond formation. **Why other options are incorrect:** * **Zinc:** Deficiency causes *Acrodermatitis enteropathica*, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Selenium:** Deficiency is associated with *Keshan disease* (dilated cardiomyopathy). * **Iron:** Metabolism defects lead to *Iron Deficiency Anemia* or *Hemochromatosis* (iron overload), not Menkes disease. **High-Yield NEET-PG Pearls:** 1. **ATP7A vs. ATP7B:** Remember **A** for **A**bsorption (Menkes/ATP7A - deficiency) and **B** for **B**iliary excretion (Wilson’s/ATP7B - overload). 2. **Diagnosis:** Low serum copper and low serum ceruloplasmin levels. 3. **Inheritance:** Menkes is X-linked Recessive (mostly affects males), whereas Wilson’s is Autosomal Recessive.

Q: Unconjugated hyperbilirubinemia is seen in which of the following conditions?

Gilbe syndrome. **Explanation:** Hyperbilirubinemia is classified into unconjugated (pre-microsomal) and conjugated (post-microsomal) based on where the metabolic defect occurs in the liver. **Why Gilbert Syndrome is Correct:** Gilbert syndrome is a common, benign autosomal recessive condition characterized by reduced activity of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**. This enzyme is responsible for conjugating bilirubin with glucuronic acid in the hepatocytes. A deficiency leads to impaired conjugation, resulting in isolated **unconjugated hyperbilirubinemia**, typically triggered by stress, fasting, or illness. **Analysis of Incorrect Options:** * **Rotor Syndrome:** An autosomal recessive condition caused by a defect in hepatic storage and re-uptake of bilirubin. It presents with **conjugated hyperbilirubinemia**. * **Dubin-Johnson Syndrome:** Caused by a mutation in the **MRP2 gene**, leading to defective excretion of conjugated bilirubin into the bile canaliculi. It presents with **conjugated hyperbilirubinemia** and a characteristic "black liver" due to melanin-like pigment deposition. * **Bile Duct Obstruction:** This is a post-hepatic (obstructive) cause of jaundice. Since the bilirubin has already been processed by the liver, it results in **conjugated hyperbilirubinemia** and features like pale stools and dark urine. **High-Yield Clinical Pearls for NEET-PG:** * **Crigler-Najjar Syndrome (Type I & II):** Also causes unconjugated hyperbilirubinemia due to UGT1A1 deficiency (Type I is total absence; Type II is severe deficiency). * **Distinguishing Dubin-Johnson vs. Rotor:** Dubin-Johnson shows a black liver and abnormal urinary coproporphyrin I levels (>80%), whereas Rotor syndrome has a normal-appearing liver. * **Gilbert Syndrome** is often a "spot diagnosis" in exams when a patient presents with mild jaundice after a period of fasting or infection with otherwise normal Liver Function Tests (LFTs).

Q: Which of the following is an X-linked dominant disorder?

Vitamin D resistant rickets. **Explanation:** **Vitamin D Resistant Rickets (Hypophosphatemic Rickets)** is the classic prototype of an **X-linked dominant (XLD)** disorder. It is caused by mutations in the *PHEX* gene on the X chromosome, leading to impaired phosphate reabsorption in the kidneys. In XLD inheritance, a single copy of the mutated gene on the X chromosome is sufficient to cause the disease in both males and females. A key pedigree feature is that an affected father will pass the trait to **all** of his daughters but **none** of his sons. **Analysis of Incorrect Options:** * **Fragile X Syndrome:** While often associated with X-linked inheritance, it is technically classified as an **X-linked dominant disorder with variable expressivity/incomplete penetrance**. However, in standard medical examinations, Vitamin D resistant rickets is the more "pure" and frequently tested example of XLD. * **Duchenne Muscular Dystrophy (DMD):** This is a classic **X-linked recessive** disorder. It primarily affects males, while females are typically asymptomatic carriers. * **Myotonic Dystrophy:** This is an **Autosomal Dominant** disorder characterized by trinucleotide repeat expansion (CTG) and the phenomenon of anticipation. **NEET-PG High-Yield Pearls:** * **X-linked Dominant Mnemonics:** Remember "**F**ragile **A**lport's **V**itamin **I**ncontinentia" (**F**ragile X, **A**lport Syndrome, **V**itamin D resistant rickets, **I**ncontinentia Pigmenti). * **Clinical Marker:** In Vitamin D resistant rickets, look for low serum phosphate, normal serum calcium, and high urinary phosphate despite normal Vitamin D intake. * **Pedigree Rule:** If a father is affected and has a healthy daughter, the disorder **cannot** be X-linked dominant.

Q: A clinical study of adults with a body mass index of at least 30 is undertaken. About 8% of these individuals do not have hyperphagia but are found to have normal levels of leptin and ghrelin, along with a diminished basal metabolic rate. A mutation in which of the following genes is most likely present in these individuals?

MC4R. **Explanation:** The correct answer is **MC4R (Melanocortin 4 Receptor)**. **1. Why MC4R is correct:** The **Melanocortin-4 Receptor (MC4R)** is a critical component of the leptin-melanocortin signaling pathway in the hypothalamus, which regulates energy homeostasis and food intake. Mutations in the *MC4R* gene are the **most common monogenic cause of human obesity**, accounting for up to 5-8% of cases of severe adult obesity. Unlike leptin or POMC deficiencies, which typically present with early-onset extreme hyperphagia, MC4R mutations can present with a **diminished basal metabolic rate (BMR)** and a more variable appetite profile in adulthood. Since the defect is at the receptor level (downstream), levels of upstream hormones like **Leptin** and **Ghrelin** remain normal. **2. Why other options are incorrect:** * **OB (Leptin) & OB-R (Leptin Receptor):** Mutations in the *OB* gene (causing leptin deficiency) or *OB-R* gene (causing leptin resistance) result in **extreme hyperphagia** and early-onset morbid obesity. In *OB* mutations, leptin levels would be undetectable, not normal. * **POMC (Pro-opiomelanocortin):** POMC is a precursor for $\alpha$-MSH, which activates MC4R. POMC deficiency causes severe obesity and hyperphagia, but it is classically associated with **hypoadrenalism** (due to lack of ACTH) and **red hair/pale skin** (due to lack of MSH). **Clinical Pearls for NEET-PG:** * **MC4R mutation:** Most common monogenic obesity; inherited in an autosomal dominant fashion. * **Leptin:** Produced by adipocytes (the "satiety hormone"); acts on the arcuate nucleus to inhibit NPY/AgRP (orexigenic) and stimulate POMC/CART (anorexigenic) neurons. * **Ghrelin:** Produced by P/D1 cells of the stomach fundus; the only peripheral orexigenic (appetite-stimulating) hormone. * **Prader-Willi Syndrome:** The most common *syndromic* cause of obesity, characterized by high ghrelin levels.

Question 1

For which of the following diseases is enzyme replacement therapy available?

Accepted Answer

Gaucher's disease

Answer

Albinism

Answer

Niemann-Pick disease

Answer

Metachromatic leukodystrophy

Question 2

Barth syndrome is due to a defect in which of the following?

Accepted Answer

Cardiolipin

Answer

Thermogenic

Answer

Ubiquinone

Answer

Cytochrome

Question 3

Different mutations in the same genetic locus causing similar or identical phenotypes is termed as?

Accepted Answer

Allelic heterogeneity

Answer

Phenotypic heterogeneity

Answer

Locus heterogeneity

Answer

Non-allelic heterogeneity

Question 4

All of the following are examples of uniparental disomy except?

Accepted Answer

Bloom syndrome

Answer

Russell-Silver syndrome

Answer

Prader-Willi syndrome

Answer

Angelman syndrome

Question 5

The provided diagram indicates what type of inheritance?

Accepted Answer

Mitochondrial

Answer

Autosomal recessive

Answer

Autosomal dominant

Answer

X-linked recessive

Question 6

Menke's disease is due to a defect in the metabolism of which element?

Accepted Answer

Copper

Answer

Zinc

Answer

Selenium

Answer

Iron

Question 7

Which of the following is not characteristic of vitamin-D resistant rickets?

Accepted Answer

Reduced calcium absorption from the gut

Answer

X-linked inheritance

Answer

Renal stones

Answer

Hypophosphatemia

Question 8

Unconjugated hyperbilirubinemia is seen in which of the following conditions?

Accepted Answer

Gilbe syndrome

Answer

Rotor syndrome

Answer

Dubin-Johnson syndrome

Answer

Bile duct obstruction

Question 9

Which of the following is an X-linked dominant disorder?

Accepted Answer

Vitamin D resistant rickets

Answer

Fragile X syndrome

Answer

Duchenne muscular dystrophy

Answer

Myotonic dystrophy

Question 10

A clinical study of adults with a body mass index of at least 30 is undertaken. About 8% of these individuals do not have hyperphagia but are found to have normal levels of leptin and ghrelin, along with a diminished basal metabolic rate. A mutation in which of the following genes is most likely present in these individuals?

Accepted Answer

MC4R

Answer

OB-R

Answer

OB

Answer

POMC

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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