Genetic Disorders and Biochemical Pathology — MCQs

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

Question 211: The genetic defect associated with decreased response to interferon therapy for Hepatitis C is:

A. IL28B (Correct Answer)
B. IL15
C. c-KIT
D. NAT2

Explanation: **Explanation:** The correct answer is **IL28B** (Interleukin 28B). **1. Why IL28B is correct:** IL28B is a gene that encodes **Interferon-lambda 3 (IFN-λ3)**. Extensive genome-wide association studies (GWAS) have identified that single nucleotide polymorphisms (SNPs) near the IL28B gene are the strongest predictors of a patient's response to **Pegylated Interferon-alpha and Ribavirin** therapy in Chronic Hepatitis C (Genotype 1). Specifically, the **CC genotype** is associated with a high rate of sustained virologic response (SVR) and spontaneous viral clearance, whereas the **CT or TT genotypes** are associated with a significantly decreased response to therapy. **2. Why the other options are incorrect:** * **IL15:** This is a cytokine involved in the proliferation of T-cells and Natural Killer (NK) cells. While important in the immune response, it is not a genetic predictor for Hepatitis C treatment outcomes. * **c-KIT:** This is a proto-oncogene encoding a receptor tyrosine kinase. Mutations in c-KIT are associated with **Gastrointestinal Stromal Tumors (GIST)** and mastocytosis, not interferon response. * **NAT2 (N-acetyltransferase 2):** This gene is responsible for the "Acetylation" pathway of drug metabolism. Polymorphisms in NAT2 categorize individuals into **Fast or Slow Acetylators**, which is high-yield for drugs like **Isoniazid (INH)**, Hydralazine, and Procainamide, but irrelevant to Hepatitis C. **High-Yield Clinical Pearls for NEET-PG:** * **IL28B polymorphism** is the most important host genetic factor for predicting SVR in HCV Genotype 1. * **HCV Genotype 1** is generally more resistant to interferon than Genotypes 2 or 3. * With the advent of **Direct-Acting Antivirals (DAAs)** like Sofosbuvir, the clinical reliance on IL28B testing has decreased, but it remains a classic, high-yield biochemistry/genetics question.

Question 212: A family, after consuming wild mushrooms during a picnic, became ill. The youngest child exhibited the most severe symptoms. The family's illness is attributed to a primary cellular inability to accomplish which of the following?

A. Synthesize proteins (Correct Answer)
B. Synthesize lipids
C. Synthesize DNA
D. Synthesize carbohydrates

Explanation: **Explanation:** The clinical scenario describes **Amanita phalloides (Death Cap mushroom) poisoning**. This is a classic high-yield topic in biochemistry and toxicology. **1. Why "Synthesize proteins" is correct:** The primary toxin in these mushrooms is **$\alpha$-amanitin**. This potent toxin acts by specifically binding to and inhibiting **RNA Polymerase II** in eukaryotic cells. RNA Polymerase II is responsible for the transcription of DNA into **messenger RNA (mRNA)**. When mRNA synthesis is blocked, the cell loses the template required for translation at the ribosomes, leading to a complete cessation of **protein synthesis**. This results in massive cell death, particularly in the liver (centrilobular necrosis) and kidneys, leading to fulminant hepatic failure. **2. Why the incorrect options are wrong:** * **Synthesize lipids (B):** While lipid metabolism occurs in the liver, it is not the primary target of $\alpha$-amanitin. * **Synthesize DNA (C):** DNA synthesis (replication) involves DNA polymerases. $\alpha$-amanitin specifically targets RNA Polymerase II, not DNA polymerase. * **Synthesize carbohydrates (D):** While hypoglycemia is a common clinical finding in mushroom poisoning due to liver failure, it is a secondary effect of hepatocyte destruction, not the primary molecular mechanism of the toxin. **3. Clinical Pearls for NEET-PG:** * **RNA Polymerase I:** Synthesizes rRNA (located in the nucleolus). * **RNA Polymerase II:** Synthesizes mRNA and snRNA (Inhibited by $\alpha$-amanitin). * **RNA Polymerase III:** Synthesizes tRNA and 5S rRNA. * **Clinical Presentation:** Typically involves a latent period (6–24 hours), followed by severe GI distress, and eventually "Phase 3" characterized by hepatic and renal failure. * **Antidote:** Silibinin (milk thistle extract) or N-acetylcysteine are often used in management.

Question 213: Which of the following is an exception to mitochondrial inheritance?

A. Leigh disease
B. Mitochondrial encephalopathy
C. MELAS
D. Colour blindness (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Colour blindness**. **1. Why Colour Blindness is the Exception:** Mitochondrial inheritance (maternal inheritance) follows a non-Mendelian pattern where traits are passed exclusively from the mother to all her offspring because mitochondria in the zygote are derived solely from the oocyte. **Colour blindness**, however, is an **X-linked recessive disorder**. It follows Mendelian genetics, specifically criss-cross inheritance, where the gene is located on the X chromosome. It is not related to mitochondrial DNA (mtDNA). **2. Analysis of Incorrect Options:** * **Leigh Disease (Subacute Necrotizing Encephalomyelopathy):** While it can be nuclear-encoded, the classic form is often caused by mutations in mtDNA (e.g., ATP6 gene), making it a hallmark example of mitochondrial pathology. * **Mitochondrial Encephalopathy:** This is a broad category of disorders specifically caused by mutations in the mitochondrial genome affecting high-energy demanding tissues like the brain and muscles. * **MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes):** This is one of the most common mitochondrial disorders, typically caused by a mutation in the *MT-TL1* gene (encoding tRNA leucine). **3. High-Yield Clinical Pearls for NEET-PG:** * **Heteroplasmy:** A key feature of mitochondrial diseases where a cell contains a mixture of both normal and mutated mtDNA. This explains the **variable expressivity** seen in clinical practice. * **Tissues Affected:** Mitochondrial diseases primarily affect "high-energy" organs: the CNS (encephalopathy), Heart (cardiomyopathy), and Skeletal Muscle (myopathy/ragged red fibers). * **Mnemonic for Mitochondrial Diseases:** **M**ELAS, **M**ERRF (Myoclonic Epilepsy with Ragged Red Fibers), **L**HON (Leber’s Hereditary Optic Neuropathy), and **K**earns-Sayre Syndrome. * **Bottleneck Effect:** This occurs during oogenesis, leading to significant variability in the proportion of mutant mtDNA passed to offspring.

Question 214: Which of the following is NOT a single gene disorder?

A. Haemochromatosis
B. Cystic fibrosis
C. William's syndrome (Correct Answer)
D. Huntington's disease

Explanation: **Explanation:** The distinction between single-gene (Mendelian) disorders and chromosomal microdeletion syndromes is a high-yield topic in NEET-PG Biochemistry and Genetics. **Why William’s Syndrome is the correct answer:** Unlike the other options, **William’s Syndrome** is not caused by a mutation in a single gene. It is a **microdeletion syndrome** involving the contiguous loss of approximately 26–28 genes on the long arm of **chromosome 7 (7q11.23)**. The most notable gene lost in this region is the *ELN* (elastin) gene, which accounts for the cardiovascular features, but the overall phenotype results from the loss of multiple neighboring genes. **Analysis of Incorrect Options (Single Gene Disorders):** * **Haemochromatosis:** An **Autosomal Recessive** disorder typically caused by a single mutation in the *HFE* gene (most commonly C282Y), leading to iron overload. * **Cystic Fibrosis:** An **Autosomal Recessive** disorder caused by mutations in the *CFTR* gene on chromosome 7. The most common mutation is the deletion of phenylalanine at position 508 (ΔF508). * **Huntington’s Disease:** An **Autosomal Dominant** trinucleotide repeat disorder (CAG repeats) in the *HTT* gene on chromosome 4. **Clinical Pearls for NEET-PG:** * **William’s Syndrome Triad:** "Elfin" facies, Intellectual disability with "cocktail party personality" (extreme friendliness), and **Supravalvular Aortic Stenosis (SVAS)**. * **Diagnosis:** The gold standard for microdeletion syndromes like William's is **FISH** (Fluorescence In Situ Hybridization) or Microarray, not standard karyotyping. * **Hypercalcemia:** Patients often present with idiopathic infantile hypercalcemia due to increased sensitivity to Vitamin D.

Question 215: All of the following are true about Wilson disease, except?

A. Serum copper level is usually lower than normal.
B. Low blood levels of ceruloplasmin.
C. Liver damage begins at the age of 18. (Correct Answer)
D. Free copper level is increased in the body.

Explanation: **Explanation** Wilson disease (Hepatolenticular degeneration) is an autosomal recessive disorder caused by a mutation in the **ATP7B gene** on chromosome 13. This defect impairs biliary copper excretion and the incorporation of copper into ceruloplasmin. **1. Why Option C is the correct answer (The False Statement):** Liver damage in Wilson disease does not wait until age 18; it typically begins in **early childhood** (as early as 6 years of age). Copper starts accumulating in the hepatocytes from birth. While neuropsychiatric symptoms often appear in the late teens or early 20s, hepatic involvement is the most common presentation in children and adolescents. **2. Analysis of other options:** * **Option A (True):** Total serum copper is usually **low** because 90% of serum copper is normally bound to ceruloplasmin, which is deficient in this disease. * **Option B (True):** Low ceruloplasmin (<20 mg/dL) is a hallmark diagnostic feature due to the failure of apo-ceruloplasmin to bind copper and its subsequent rapid degradation. * **Option D (True):** While total copper is low, **non-ceruloplasmin bound copper ("free copper")** is increased. This free copper deposits in tissues (liver, brain, cornea), causing oxidative damage. **High-Yield NEET-PG Pearls:** * **Kayser-Fleischer (KF) rings:** Copper deposition in the **Descemet’s membrane** of the cornea (best seen on slit-lamp exam). * **Diagnosis:** Best initial screening is serum ceruloplasmin; **Gold standard** is liver biopsy (increased hepatic copper). * **Treatment:** Drug of choice is **Penicillamine** (copper chelator). Alternatively, Trientine or Zinc (inhibits intestinal absorption) can be used. * **Neurology:** Characterized by "Wing-beating" tremors and "Panda sign" on MRI brain.

Question 216: Which enzyme is used to "flush" the sticky ends of DNA?

A. Klenow fragment (Correct Answer)
B. Polynucleotide kinase
C. Alkaline phosphatase
D. Primase

Explanation: **Explanation:** In recombinant DNA technology, "flushing" or "blunting" refers to the process of converting cohesive (sticky) ends of DNA into blunt ends. **Why Klenow Fragment is correct:** The **Klenow fragment** is a large protein fragment produced when DNA Polymerase I from *E. coli* is enzymatically cleaved by subtilisin. It retains two critical activities: 1. **5' → 3' polymerase activity:** It fills in 5' overhangs (recessed 3' ends) by adding dNTPs. 2. **3' → 5' exonuclease activity:** It removes 3' overhangs. By performing these actions, it creates uniform, double-stranded "blunt" ends, making it the enzyme of choice for "flushing" DNA ends before ligation. **Why other options are incorrect:** * **Polynucleotide kinase:** This enzyme adds a phosphate group to the 5'-hydroxyl terminus of DNA or RNA. It is used for radio-labeling or preparing DNA for ligation, not for altering the physical structure of sticky ends. * **Alkaline phosphatase:** This enzyme removes 5' phosphate groups from DNA. It is used to prevent self-ligation of vector DNA, not for blunting ends. * **Primase:** This is a type of RNA polymerase that synthesizes short RNA primers during DNA replication *in vivo*. It has no role in the laboratory manipulation of DNA ends. **High-Yield Clinical Pearls for NEET-PG:** * **Klenow vs. DNA Pol I:** The Klenow fragment lacks the **5' → 3' exonuclease activity** (which is present in the intact DNA Pol I). This lack of 5' → 3' exonuclease activity is crucial because it prevents the degradation of the primer/template being synthesized. * **S1 Nuclease:** Another enzyme used for blunting DNA, but it works by degrading single-stranded protrusions. * **Taq Polymerase:** Unlike Klenow, Taq adds a single "A" overhang at the 3' end (TA cloning), which is the opposite of flushing.

Question 217: Mothers with phenylketonuria (PKU) are at increased risk of having children with which of the following conditions?

A. Hydrocephaly
B. Spina bifida
C. Skeletal dysplasia
D. Mental retardation (Correct Answer)

Explanation: **Explanation:** The correct answer is **Mental retardation**. This question addresses **Maternal PKU Syndrome**, a critical concept in biochemical pathology. **1. Why Mental Retardation is Correct:** In a mother with Phenylketonuria (PKU) who does not maintain a strict low-phenylalanine diet during pregnancy, high levels of phenylalanine (Phe) act as a **teratogen**. Although the fetus may be heterozygote (not having the disease itself), Phe readily crosses the placenta via active transport. High fetal Phe levels interfere with amino acid transport and protein synthesis in the developing brain, leading to **microcephaly** (not hydrocephaly) and profound **mental retardation**. Other features include congenital heart defects (especially Fallot's tetralogy) and intrauterine growth restriction (IUGR). **2. Why the Other Options are Incorrect:** * **A. Hydrocephaly:** Maternal PKU typically causes **Microcephaly** (small head size) due to impaired brain development, which is the opposite of hydrocephaly. * **B. Spina bifida:** This is a neural tube defect primarily associated with **Folic acid deficiency**, not phenylalanine toxicity. * **C. Skeletal dysplasia:** While IUGR occurs, specific skeletal dysplasias (like achondroplasia) are genetic bone disorders and not a classic feature of Maternal PKU. **Clinical Pearls for NEET-PG:** * **The "Diet for Life" Rule:** To prevent Maternal PKU Syndrome, women with PKU must maintain blood Phe levels between **2–6 mg/dL** starting *before* conception and throughout pregnancy. * **Enzyme Deficiency:** Classic PKU is due to a deficiency of **Phenylalanine Hydroxylase (PAH)**; the cofactor required is **Tetrahydrobiopterin (BH4)**. * **Screening:** PKU is screened using the **Guthrie Test** (bacterial inhibition assay). * **Key Triad of Maternal PKU:** Microcephaly, Mental Retardation, and Congenital Heart Disease.

Question 218: Deficiency of which enzyme is seen in Tay-Sach disease?

A. Galactosidase
B. Hexosaminidase (Correct Answer)
C. Acid lipase
D. Glucosidase

Explanation: **Explanation:** **Tay-Sachs Disease** is an autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **Hexosaminidase A**. This deficiency leads to the toxic accumulation of **GM2 gangliosides**, particularly in the neurons of the central nervous system. 1. **Why Hexosaminidase is correct:** The enzyme Hexosaminidase A is responsible for breaking down GM2 gangliosides. In its absence, these lipids accumulate in lysosomes, causing progressive neurodegeneration, developmental delay, and the characteristic **"cherry-red spot"** on the macula. 2. **Why other options are incorrect:** * **Galactosidase:** Deficiency of $\beta$-Galactosidase causes **Krabbe disease** (accumulation of galactocerebroside) or **GM1 gangliosidosis**. Deficiency of $\alpha$-Galactosidase A causes **Fabry disease**. * **Acid lipase:** Deficiency of lysosomal acid lipase leads to **Wolman disease** or Cholesteryl Ester Storage Disease (CESD). * **Glucosidase:** Deficiency of $\beta$-Glucosidase (glucocerebrosidase) causes **Gaucher disease**, the most common lysosomal storage disorder. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Tay-Sa**X** lacks He**X**osaminidase." * **Key Clinical Triad:** Progressive neurodegeneration + Cherry-red spot + **No hepatosplenomegaly** (this distinguishes it from Niemann-Pick disease). * **Histology:** Presence of **"onion-skin"** lysosomes (whorled configurations of membranes). * **Genetics:** Common in Ashkenazi Jewish populations; caused by a mutation on chromosome 15.

Question 219: A 12-year-old girl presents with tremors and emotional lability. Slit lamp examination of the eye shows golden brown discoloration in Descemet's membrane. What is the most appropriate diagnosis for this patient?

A. Acute Rheumatic fever
B. Fabry's disease
C. Wilson's disease (Correct Answer)
D. Glycogen storage disease

Explanation: ### Explanation **Correct Answer: C. Wilson’s Disease** **1. Why it is correct:** The clinical triad of **neuropsychiatric symptoms** (tremors, emotional lability), **hepatic involvement**, and the pathognomonic **Kayser-Fleischer (KF) rings** is diagnostic of Wilson’s disease. KF rings are golden-brown deposits of copper in the **Descemet’s membrane** of the cornea, best visualized via slit-lamp examination. Biochemically, Wilson’s disease is an autosomal recessive disorder caused by mutations in the **ATP7B gene** (Chromosome 13). This leads to defective biliary copper excretion and failure to incorporate copper into ceruloplasmin, resulting in toxic copper accumulation in the liver, brain (basal ganglia), and eyes. **2. Why other options are incorrect:** * **A. Acute Rheumatic Fever:** While it can present with Sydenham’s chorea (involuntary movements), it is associated with streptococcal infections and carditis, not corneal copper deposits. * **B. Fabry’s Disease:** An X-linked lysosomal storage disorder. While it involves the eye, it presents with **cornea verticillata** (vortex keratopathy), not KF rings, and typically involves angiokeratomas and renal failure. * **D. Glycogen Storage Disease:** These primarily present with hypoglycemia, hepatomegaly, or muscular weakness, depending on the type (e.g., Von Gierke’s), but do not cause Descemet’s membrane discoloration. **3. NEET-PG High-Yield Pearls:** * **Best Initial Test:** Serum Ceruloplasmin (decreased; <20 mg/dL). * **Gold Standard Diagnosis:** Liver biopsy (increased copper content >250 µg/g dry weight). * **Most Sensitive Screening:** 24-hour urinary copper excretion (increased). * **Treatment of Choice:** Chelating agents like **D-Penicillamine** or Trientine; Zinc is used for maintenance (inhibits GI absorption). * **MRI Finding:** "Face of the Giant Panda" sign in the midbrain.

Question 220: Congenital bilateral absence of vas deferens is genetically related to which of the following conditions?

A. Noonan syndrome
B. Down syndrome
C. Kartagener syndrome
D. Cystic fibrosis (Correct Answer)

Explanation: **Explanation:** **Cystic Fibrosis (CF)** is the correct answer because **Congenital Bilateral Absence of the Vas Deferens (CBAVD)** is a highly specific clinical feature of this disorder. CF is an autosomal recessive condition caused by mutations in the **CFTR gene** (Chromosome 7), which encodes a chloride channel. In males, defective CFTR leads to thickened secretions that obstruct the developing vas deferens *in utero*, causing them to involute or fail to develop. Notably, up to 95% of males with CF are infertile due to CBAVD, and some individuals may present with CBAVD as their *only* clinical manifestation of a CFTR mutation. **Analysis of Incorrect Options:** * **Noonan Syndrome:** Characterized by short stature, webbed neck, and pulmonary stenosis. While cryptorchidism (undescended testes) is common, CBAVD is not a feature. * **Down Syndrome (Trisomy 21):** Associated with intellectual disability, cardiac defects (AVSD), and early-onset Alzheimer’s. While fertility may be reduced, the anatomy of the vas deferens is typically intact. * **Kartagener Syndrome:** A subset of Primary Ciliary Dyskinesia (PCD). It causes male infertility due to **immotile spermatozoa** (dynein arm defect), but the vas deferens is anatomically present. **NEET-PG High-Yield Pearls:** * **Genetics:** Most common mutation in CF is **ΔF508** (phenylalanine deletion), leading to protein misfolding and degradation in the ER. * **Diagnosis:** Sweat chloride test (>60 mEq/L) is the gold standard. * **CBAVD vs. Kartagener:** In exams, if the patient has obstructive azoospermia (absent vas), think **CF**; if they have normal sperm count but zero motility, think **Kartagener**. * **Associated finding:** CBAVD is often associated with seminal vesicle hypoplasia and low semen volume (<1 mL) with acidic pH.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

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Inborn Errors of Metabolism

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Lysosomal Storage Diseases

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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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