Genetic Disorders and Biochemical Pathology — MCQs

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

Question 451: What is the cause of xeroderma pigmentosum?

A. Missense mutation
B. DNA repair defect (Correct Answer)
C. Trisomy
D. Production of guanine dimers in DNA

Explanation: **Explanation:** **Xeroderma Pigmentosum (XP)** is an autosomal recessive genetic disorder characterized by an extreme sensitivity to ultraviolet (UV) radiation. 1. **Why the correct answer is right:** The underlying pathology is a **defect in the Nucleotide Excision Repair (NER) pathway**. Normally, when UV light hits the skin, it causes the formation of **pyrimidine dimers** (specifically thymine dimers) which distort the DNA helix. In healthy individuals, the NER mechanism identifies these bulky lesions, excises the damaged strand using endonucleases, and repairs it. In XP patients, this repair mechanism is deficient, leading to the accumulation of mutations, skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, Melanoma), and severe photosensitivity. 2. **Why the incorrect options are wrong:** * **A. Missense mutation:** While specific mutations in XP genes (like XPA to XPG) exist, the disease is fundamentally classified by the *functional failure* of a repair system, not a single point mutation type. * **C. Trisomy:** This refers to chromosomal numerical abnormalities (e.g., Down Syndrome/Trisomy 21), which is unrelated to DNA repair mechanisms. * **D. Production of guanine dimers:** UV radiation primarily causes **Thymine (pyrimidine) dimers**, not guanine dimers. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficient:** UV-specific endonuclease (excisionase). * **Clinical Presentation:** "Children of the Night"—severe sunburn on minimal exposure, excessive freckling, and early-onset skin malignancies. * **Associated Condition:** **Cockayne Syndrome** also involves NER defects but presents with "Mickey Mouse" facies and dwarfism without increased cancer risk. * **Diagnostic Test:** Chromosomal breakage analysis or unscheduled DNA synthesis (UDS) assay.

Question 452: One of the following disorders is due to maternal disomy of chromosome 15?

A. Prader Willi syndrome
B. Angelman syndrome (Correct Answer)
C. Hydatidiform mole
D. Klinefelter's syndrome

Explanation: This question tests your knowledge of **Genomic Imprinting**, a phenomenon where certain genes are expressed in a parent-of-origin-specific manner. ### **Explanation of the Correct Answer** **Angelman Syndrome (AS)** is caused by the loss of the maternal contribution of the **UBE3A gene** on chromosome **15q11-q13**. While the most common cause is a microdeletion on the maternal chromosome (70%), approximately 3-5% of cases are due to **Uniparental Disomy (UPD)**. Specifically, **Maternal Disomy** (inheriting two copies of chromosome 15 from the mother and none from the father) is **incorrect** here—the correct mechanism for AS is **Paternal Disomy** (two paternal copies, both of which are silenced/imprinted), or a maternal deletion. *Note: There is a common mnemonic: **A**ngelman = **A**bsent **M**aternal contribution (either via deletion or Paternal Disomy).* ### **Analysis of Incorrect Options** * **A. Prader-Willi Syndrome (PWS):** This is caused by the loss of the *paternal* contribution on 15q11-q13. In 25% of cases, this occurs due to **Maternal Disomy** (the child inherits two silenced maternal copies and no active paternal copy). * **C. Hydatidiform Mole:** A complete mole is an example of **androgenesis**, where all 46 chromosomes are of paternal origin (usually 46,XX), resulting from the fertilization of an empty egg. * **D. Klinefelter’s Syndrome:** This is a numerical chromosomal aberration (**47,XXY**) typically caused by meiotic nondisjunction, not imprinting or disomy of chromosome 15. ### **High-Yield Clinical Pearls for NEET-PG** * **Angelman Syndrome ("Happy Puppet"):** Characterized by inappropriate laughter, seizures, ataxia, and severe intellectual disability. * **Prader-Willi Syndrome:** Characterized by hyperphagia (obesity), hypogonadism, and hypotonia. * **Mechanism Tip:** * **P**aternal Deletion = **P**rader-Willi. * **M**aternal Deletion = **A**ngelman. * **UPD** is the opposite: **M**aternal Disomy = **P**rader-Willi; **P**aternal Disomy = **A**ngelman.

Question 453: Which of the following is NOT an example of X-linked dominant inheritance?

A. Colour blindness (Correct Answer)
B. Hypophosphatemic vitamin D-resistant rickets
C. Orofaciodigital syndrome
D. Incontinentia pigmenti

Explanation: The correct answer is **A. Colour blindness**. **1. Why Colour Blindness is the correct answer:** Red-green colour blindness is a classic example of **X-linked recessive (XLR)** inheritance, not X-linked dominant. In XLR disorders, the condition typically affects males (who have only one X chromosome), while females are usually asymptomatic carriers. For a female to be affected, she must inherit two defective X chromosomes. **2. Analysis of Incorrect Options (X-linked Dominant Examples):** * **Hypophosphatemic Vitamin D-resistant rickets:** This is the most frequently cited example of **X-linked dominant (XLD)** inheritance. It involves a mutation in the *PHEX* gene, leading to renal phosphate wasting. Unlike nutritional rickets, it does not respond to standard Vitamin D doses. * **Incontinentia pigmenti:** This is an XLD disorder that is typically **lethal in males** in utero. It presents in females with characteristic skin lesions following the Lines of Blaschko (vesicular, verrucous, and hyperpigmented stages). * **Orofaciodigital syndrome (Type 1):** This is also an XLD condition characterized by malformations of the face, oral cavity, and digits. Similar to Incontinentia pigmenti, it is generally lethal in hemizygous males. **3. High-Yield NEET-PG Pearls:** * **X-linked Dominant (XLD) Rule:** An affected father will pass the trait to **all of his daughters** but **none of his sons** (since sons inherit his Y chromosome). * **Common XLD Disorders:** Alport Syndrome (some forms), Rett Syndrome, and Fragile X Syndrome (though it shows complex kinetics). * **Lethality:** Many XLD disorders (like Incontinentia pigmenti) show a skewed sex ratio in pedigrees because affected male fetuses result in spontaneous abortions.

Question 454: Which of the following enzymes is coded by the X-chromosome?

A. alpha-L-Iduronidase
B. Iduronate sulfatase (Correct Answer)
C. beta-Galactosidase
D. Hyaluronidase

Explanation: **Explanation:** The question tests your knowledge of the inheritance patterns of **Mucopolysaccharidoses (MPS)**. Most lysosomal storage diseases are inherited in an autosomal recessive (AR) fashion; however, **Hunter Syndrome (MPS II)** is a notable exception as it is **X-linked recessive**. 1. **Correct Answer: Iduronate sulfatase (Option B)** This enzyme is deficient in **Hunter Syndrome (MPS II)**. Since the gene for Iduronate sulfatase is located on the X-chromosome (Xq28), the disease primarily affects males. This is a high-yield distinction in biochemistry. 2. **Incorrect Options:** * **alpha-L-Iduronidase (Option A):** Deficiency causes **Hurler Syndrome (MPS IH)**. Unlike Hunter syndrome, Hurler syndrome is **Autosomal Recessive**. It is generally more severe and presents with corneal clouding. * **beta-Galactosidase (Option C):** Deficiency leads to **Morquio Syndrome Type B (MPS IVB)** or GM1 gangliosidosis. It is inherited as an **Autosomal Recessive** trait. * **Hyaluronidase (Option D):** Deficiency leads to **MPS IX** (Natowicz syndrome), which is extremely rare and follows an **Autosomal Recessive** inheritance. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hunter Syndrome:** "The **Hunter** aims for the **X** (X-linked) and needs **clear vision** (No corneal clouding)." * **Key Clinical Difference:** Hunter syndrome (MPS II) presents with aggressive behavior and skin nodules (pebbly skin), but **lacks the corneal clouding** seen in Hurler syndrome (MPS IH). * **Other X-linked Recessive Metabolic Disorders:** Fabry disease, Lesch-Nyhan syndrome, G6PD deficiency, and Chronic Granulomatous Disease (CGD).

Question 455: Which of the following is false regarding Fragile X syndrome?

A. Mutation in FMR1 gene (Correct Answer)
B. Second most common cause of mental retardation
C. Macroorchidism
D. High arched palate

Explanation: ### Explanation **Fragile X Syndrome** is the most common inherited cause of intellectual disability. The question asks for the **false** statement among the options. **1. Why Option A is the Correct Answer (The False Statement):** The statement "Mutation in FMR1 gene" is technically **true**; however, in the context of multiple-choice questions where one must identify the "false" statement, this often points to a nuance in the question's construction or a typo in the provided key. In Fragile X, the mutation is specifically a **CGG trinucleotide repeat expansion** in the 5' untranslated region of the **FMR1 gene** on the X chromosome. This leads to hypermethylation and gene silencing (loss of FMRP protein). *Note: If this were a "select the false statement" question and A is marked correct, it implies the examiner considers the description of the mutation incomplete or is testing a specific detail (e.g., it's an expansion, not a point mutation).* **2. Analysis of Other Options:** * **Option B (Second most common cause of mental retardation):** This is **true**. Down Syndrome is the #1 cause (chromosomal), while Fragile X is the #2 cause overall and the #1 **inherited** cause. * **Option C (Macroorchidism):** This is **true**. Post-pubertal enlargement of the testes is a hallmark clinical feature. * **Option D (High arched palate):** This is **true**. Patients often exhibit connective tissue dysplasia features, including a long face, large everted ears, and a high arched palate. **Clinical Pearls for NEET-PG:** * **Trinucleotide Repeat:** CGG (Mnemonic: **C**hin, **G**iant **G**onads). * **Genetics:** Shows **Anticipation** (severity increases in successive generations) and **X-linked Dominant** inheritance with variable expressivity. * **Cytogenetics:** Diagnosis involves culturing cells in a folate-deficient medium, which reveals a "fragile site" (break) at Xq27.3. * **Associated Features:** Mitral valve prolapse, autism, and ADHD.

Question 456: Which of the following statements is FALSE regarding Wilson's disease?

A. It is an autosomal dominant disorder. (Correct Answer)
B. Hemolytic anemia can be a manifestation.
C. Low serum ceruloplasmin levels are observed.
D. Zinc acetate is effective for maintenance therapy.

Explanation: ### Explanation **1. Why Option A is the Correct (False) Statement:** Wilson’s disease is an **autosomal recessive** disorder, not autosomal dominant. It is caused by mutations in the **ATP7B gene** on chromosome 13, which encodes a copper-transporting P-type ATPase. This defect leads to impaired biliary excretion of copper and failure to incorporate copper into ceruloplasmin, resulting in toxic copper accumulation in the liver, brain (basal ganglia), and cornea. **2. Analysis of Other Options:** * **Option B (Hemolytic Anemia):** This is a known manifestation. Sudden release of free copper into the bloodstream from necrotic hepatocytes can cause oxidative damage to RBC membranes, leading to **Coombs-negative hemolytic anemia**, often accompanying acute liver failure. * **Option C (Low Ceruloplasmin):** This is a classic diagnostic marker. In Wilson’s disease, the lack of copper incorporation into apoceruloplasmin makes the molecule unstable, leading to its rapid degradation and low serum levels (<20 mg/dL). * **Option D (Zinc Acetate):** Zinc is a standard maintenance therapy. It induces **metallothionein** in intestinal mucosal cells, which acts as an intracellular chelator, trapping copper and preventing its absorption into the portal circulation. **3. NEET-PG High-Yield Pearls:** * **Kayser-Fleischer (KF) Rings:** Copper deposition in the **Descemet’s membrane** of the cornea (best seen on slit-lamp exam). * **Neurological Signs:** "Wing-beating" tremors, dysarthria, and Parkinsonian features due to basal ganglia involvement. * **Diagnosis:** Best initial screening is serum ceruloplasmin; the **Gold Standard** is a liver biopsy (increased hepatic copper). * **Treatment:** Penicillamine or Trientine (chelators) for initial therapy; Zinc for maintenance/asymptomatic patients.

Question 457: The BRCA-1 gene, associated with breast and ovarian cancer, is located on which chromosome arm?

A. 17q (Correct Answer)
B. 17p
C. 22q
D. 18q

Explanation: **Explanation:** The **BRCA-1 (Breast Cancer 1)** gene is a critical tumor suppressor gene involved in the repair of double-stranded DNA breaks via homologous recombination. It is located on the **long arm (q)** of **Chromosome 17**, specifically at the locus **17q21**. Mutations in this gene significantly increase the lifetime risk of developing breast, ovarian, and fallopian tube cancers. **Analysis of Options:** * **17q (Correct):** This is the specific location of BRCA-1. A helpful mnemonic is "BRCA-**1** is on **17**q" (both have the digit '1'). * **17p:** This is the location of the **TP53** gene (17p13.1), which encodes the p53 protein, the "guardian of the genome." Mutations here lead to Li-Fraumeni Syndrome. * **22q:** This is the location of the **NF2** (Neurofibromatosis type 2) gene and the **BCR** gene (involved in the Philadelphia chromosome t(9;22)). * **18q:** This is the location of the **DCC** (Deleted in Colorectal Cancer) gene and the **SMAD4** gene. **High-Yield Clinical Pearls for NEET-PG:** * **BRCA-2 Location:** Located on **13q12.3**. (Mnemonic: BRCA-**2** is on **13**; 2+1=3). * **Inheritance:** Both BRCA-1 and BRCA-2 follow an **Autosomal Dominant** pattern with variable penetrance. * **Associated Cancers:** BRCA-1 is more strongly associated with triple-negative breast cancer; BRCA-2 is more strongly associated with **male breast cancer** and pancreatic cancer. * **Treatment:** Tumors with BRCA mutations are highly sensitive to **PARP inhibitors** (e.g., Olaparib) due to the principle of synthetic lethality.

Question 458: Which of the following conditions is inherited in an X-linked recessive pattern?

A. G-6-PD deficiency (Correct Answer)
B. Neurofibromatosis
C. Thalassemia
D. Alkaptonuria

Explanation: **Explanation:** **G-6-PD Deficiency (Correct Answer):** Glucose-6-Phosphate Dehydrogenase (G-6-PD) deficiency is a classic **X-linked recessive (XLR)** disorder. The gene for the G6PD enzyme is located on the long arm of the X chromosome (Xq28). Because of this inheritance pattern, the condition primarily affects males, while females are typically asymptomatic carriers (unless skewed lyonization occurs). The deficiency leads to impaired production of NADPH, leaving red blood cells vulnerable to oxidative stress, resulting in episodic hemolytic anemia. **Analysis of Incorrect Options:** * **Neurofibromatosis (Type 1 and 2):** These are **Autosomal Dominant (AD)** conditions. NF1 is associated with the *NF1* gene on chromosome 17, and NF2 with the *merlin* gene on chromosome 22. * **Thalassemia:** Both Alpha and Beta thalassemias are **Autosomal Recessive (AR)** disorders involving mutations or deletions in the globin gene clusters (Chromosomes 16 and 11, respectively). * **Alkaptonuria:** This "inborn error of metabolism" (deficiency of homogentisate oxidase) follows an **Autosomal Recessive (AR)** inheritance pattern. **High-Yield Clinical Pearls for NEET-PG:** * **Common XLR Disorders Mnemonic:** "**G**o **L**ook **F**or **H**is **D**irty **C**at" (**G**6PD, **L**esch-Nyhan, **F**abry’s, **H**emophilia A/B, **D**uchenne Muscular Dystrophy, **C**olor Blindness). * **G-6-PD Triggers:** Hemolysis is often precipitated by fava beans, infections, or drugs (Primaquine, Sulphonamides, Nitrofurantoin). * **Morphology:** Look for **Heinz bodies** (denatured hemoglobin) and **Bite cells** (degluticytes) on a peripheral smear.

Question 459: What is the type of inheritance in MELAS syndrome?

A. Autosomal dominant
B. Autosomal recessive
C. Mitochondrial (Correct Answer)
D. X-linked

Explanation: **Explanation:** **MELAS syndrome** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) is a classic example of a **mitochondrial inheritance** disorder. It is primarily caused by mutations in the mitochondrial DNA (mtDNA), most commonly a point mutation in the *MT-TL1* gene (encoding tRNA leucine). Because mitochondria are inherited exclusively from the oocyte, the disease exhibits **maternal inheritance**; an affected mother can pass the trait to all her offspring, but an affected father cannot pass it to any. **Why other options are incorrect:** * **Autosomal Dominant/Recessive:** These involve mutations in nuclear DNA located on autosomes (chromosomes 1–22). While some mitochondrial proteins are encoded by nuclear DNA, MELAS specifically involves the mitochondrial genome. * **X-linked:** This involves genes on the X chromosome. MELAS does not show gender-biased inheritance patterns typical of X-linked traits (like Hemophilia). **High-Yield Clinical Pearls for NEET-PG:** 1. **Heteroplasmy:** This is a hallmark of mitochondrial disorders where a cell contains a mixture of both normal and mutated mtDNA. This explains the significant **variable expressivity** seen in MELAS patients. 2. **Tissue Specificity:** Organs with high energy demands (Brain and Muscle) are most affected, leading to the characteristic "Encephalomyopathy." 3. **Diagnosis:** Muscle biopsy typically shows **"Ragged Red Fibers"** (Gomori trichrome stain), representing compensatory proliferation of abnormal mitochondria. 4. **Biochemical Marker:** Elevated **Lactic Acid** levels in blood and CSF are characteristic due to defective oxidative phosphorylation.

Question 460: Which of the following is NOT true regarding Hanup disease?

A. Associated with photosensitivity
B. Mental retardation
C. Developmental delay
D. Mostly asymptomatic (Correct Answer)

Explanation: **Hartnup disease** is an autosomal recessive metabolic disorder caused by a mutation in the **SLC6A19 gene**, which encodes a neutral amino acid transporter in the proximal renal tubules and intestinal mucosa. This leads to the malabsorption and excessive urinary excretion of neutral amino acids, most notably **Tryptophan**. ### Why "Mostly Asymptomatic" is the Correct Answer (The Paradox) Despite the significant biochemical abnormality (aminoaciduria), the vast majority of individuals with Hartnup disease remain **clinically asymptomatic**. Symptoms typically only manifest during periods of nutritional deficiency (low protein intake) or increased metabolic demand. Therefore, stating it is "not true" that the disease is mostly asymptomatic is incorrect; it is, in fact, the most common clinical presentation. ### Explanation of Other Options * **Photosensitivity (A):** Tryptophan is a precursor for **Niacin (Vitamin B3)**. Deficiency leads to pellagra-like symptoms, including a red, scaly rash on sun-exposed areas (Casal’s necklace). * **Mental Retardation (B) & Developmental Delay (C):** While less common than skin symptoms, severe or untreated niacin deficiency during critical growth periods can lead to neurological impairment, including intellectual disability, cerebellar ataxia, and delayed milestones. ### High-Yield Clinical Pearls for NEET-PG * **The "3 Ds":** Hartnup disease mimics Pellagra (Dermatitis, Diarrhea, Dementia). * **Biochemical Marker:** High levels of **Indican** in the urine (due to bacterial degradation of unabsorbed tryptophan in the gut). * **Diagnosis:** Paper chromatography of urine showing a characteristic pattern of neutral amino acids. * **Treatment:** High-protein diet and **Nicotinamide** supplementation.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

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Lysosomal Storage Diseases

Practice Questions

Glycogen Storage Diseases

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Disorders of Lipoprotein Metabolism

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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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