Genetic Disorders and Biochemical Pathology Practice Questions

Q: What is the cause of xeroderma pigmentosum?

DNA repair defect. **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.

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

Colour blindness. 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.

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

Iduronate sulfatase. **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).

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

It is an autosomal dominant disorder.. ### 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.

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

17q. **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.

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

G-6-PD deficiency. **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.

Q: What is the type of inheritance in MELAS syndrome?

Mitochondrial. **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.

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

Mostly asymptomatic. **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.

Question 1

What is the cause of xeroderma pigmentosum?

Accepted Answer

DNA repair defect

Answer

Missense mutation

Answer

Trisomy

Answer

Production of guanine dimers in DNA

Question 2

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

Accepted Answer

Angelman syndrome

Answer

Prader Willi syndrome

Answer

Hydatidiform mole

Answer

Klinefelter's syndrome

Question 3

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

Accepted Answer

Colour blindness

Answer

Hypophosphatemic vitamin D-resistant rickets

Answer

Orofaciodigital syndrome

Answer

Incontinentia pigmenti

Question 4

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

Accepted Answer

Iduronate sulfatase

Answer

alpha-L-Iduronidase

Answer

beta-Galactosidase

Answer

Hyaluronidase

Question 5

Which of the following is false regarding Fragile X syndrome?

Accepted Answer

Mutation in FMR1 gene

Answer

Second most common cause of mental retardation

Answer

Macroorchidism

Answer

High arched palate

Question 6

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

Accepted Answer

It is an autosomal dominant disorder.

Answer

Hemolytic anemia can be a manifestation.

Answer

Low serum ceruloplasmin levels are observed.

Answer

Zinc acetate is effective for maintenance therapy.

Question 7

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

Accepted Answer

17q

Answer

17p

Answer

22q

Answer

18q

Question 8

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

Accepted Answer

G-6-PD deficiency

Answer

Neurofibromatosis

Answer

Thalassemia

Answer

Alkaptonuria

Question 9

What is the type of inheritance in MELAS syndrome?

Accepted Answer

Mitochondrial

Answer

Autosomal dominant

Answer

Autosomal recessive

Answer

X-linked

Question 10

Which of the following is NOT true regarding Hanup disease?

Accepted Answer

Mostly asymptomatic

Answer

Associated with photosensitivity

Answer

Mental retardation

Answer

Developmental delay

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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