Genetic Disorders and Biochemical Pathology — MCQs

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

Question 261: Different mutations in the same genetic locus causing similar or identical phenotypes is termed as?

A. Phenotypic heterogeneity
B. Allelic heterogeneity (Correct Answer)
C. Locus heterogeneity
D. Non-allelic heterogeneity

Explanation: ### 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).

Question 262: Genes responsible for beta-thalassemia are located on which chromosome?

A. Chromosome 7
B. Chromosome 11 (Correct Answer)
C. Chromosome 16
D. Chromosome 18

Explanation: ### Explanation **Correct Option: B. Chromosome 11** The synthesis of hemoglobin involves two distinct gene clusters. The **beta-globin gene cluster** is located on the short arm (p-arm) of **Chromosome 11**. Beta-thalassemia is caused by mutations (usually point mutations) in this gene, leading to reduced ($\beta^+$) or absent ($\beta^0$) synthesis of beta-globin chains. Since humans have two copies of Chromosome 11, beta-thalassemia follows an autosomal recessive inheritance pattern. **Analysis of Incorrect Options:** * **Option A (Chromosome 7):** This is the location of the **CFTR gene**, mutations of which cause Cystic Fibrosis. It is not involved in hemoglobin synthesis. * **Option C (Chromosome 16):** This is a high-yield distractor. Chromosome 16 houses the **alpha-globin gene cluster**. Mutations or deletions here result in **Alpha-thalassemia**. Remember: "Alpha is on 16, Beta is on 11." * **Option D (Chromosome 18):** This is associated with **Edwards Syndrome** (Trisomy 18). It does not contain major globin gene clusters. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mutation Type:** While Alpha-thalassemia is primarily due to **gene deletions**, Beta-thalassemia is most commonly due to **point mutations** (splicing, promoter, or nonsense mutations). 2. **Hb Composition:** In $\beta$-thalassemia major, there is a compensatory increase in **HbF ($\alpha_2\gamma_2$)** and **HbA2 ($\alpha_2\delta_2$)** because $\beta$-chain production is deficient. 3. **Microscopy:** Look for **Target cells** (codocytes) and hypochromic microcytic RBCs on a peripheral smear. 4. **Mnemonic:** **"B-E-L-E-V-E-N"** (Beta on Eleven).

Question 263: Calcification of intervertebral discs is characteristic of which condition?

A. Alkaptonuria (Correct Answer)
B. Phenylketonuria
C. Gout
D. Rickets

Explanation: ### Explanation **Correct Answer: A. Alkaptonuria** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, leading to the accumulation of **Homogentisic Acid (HGA)**. The underlying pathology involves the oxidation of HGA into a melanin-like polymer called **alkapton**. This pigment deposits in connective tissues, a process known as **Ochronosis**. In the musculoskeletal system, these deposits weaken the collagen matrix of the intervertebral discs, leading to premature degeneration and **dystrophic calcification**. On X-ray, this presents as a classic "wafer-like" calcification of multiple intervertebral discs, often resulting in "bamboo spine" appearance similar to ankylosing spondylitis. **Why Incorrect Options are Wrong:** * **B. Phenylketonuria:** Caused by Phenylalanine Hydroxylase deficiency. It presents with intellectual disability, "mousy" odor, and hypopigmentation, but does not involve disc calcification. * **C. Gout:** Caused by monosodium urate crystal deposition. While it affects joints (Podagra), it typically involves peripheral joints and causes "punched-out" erosions rather than disc calcification. * **D. Rickets:** A defect in bone mineralization (Vitamin D deficiency). It leads to softening of bones (osteomalacia) and widening of growth plates, not pathological calcification of discs. **Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (blue-black pigmentation of sclera and ear cartilage), 3. Ochronotic arthritis (large joints and spine). * **Diagnostic Test:** Ferric Chloride test (turns deep blue/green) and Silver Nitrate test. * **Management:** High doses of Vitamin C (antioxidant) and **Nitisinone** (inhibits HGA production).

Question 264: All of the following are examples of uniparental disomy except?

A. Russell-Silver syndrome
B. Prader-Willi syndrome
C. Angelman syndrome
D. Bloom syndrome (Correct Answer)

Explanation: **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).

Question 265: Which gene is associated with Wilson's disease?

A. ATP7A
B. ATP7B (Correct Answer)
C. ADP7A
D. ADP7B

Explanation: **Explanation:** **Wilson’s Disease** (Hepatolenticular Degeneration) is an autosomal recessive disorder of copper metabolism. The correct answer is **ATP7B** because this gene, located on **chromosome 13**, encodes a P-type ATPase copper-transporting protein. This protein is primarily expressed in the liver and is responsible for two critical functions: 1. Transporting copper into the Golgi apparatus for incorporation into **ceruloplasmin**. 2. Facilitating the excretion of excess copper into the **bile**. Mutations in *ATP7B* lead to copper accumulation in the liver, brain (basal ganglia), and cornea. **Analysis of Incorrect Options:** * **ATP7A:** This gene is associated with **Menkes Disease** ("Kinky Hair Syndrome"). While it also encodes a copper-transporting ATPase, it is responsible for copper absorption from the GI tract. A defect here leads to systemic copper deficiency, not overload. * **ADP7A & ADP7B:** These are distractor options. The transporters involved in these metal-transport disorders utilize ATP for active transport (ATPases), not ADP. **High-Yield Clinical Pearls for NEET-PG:** * **Kayser-Fleischer (KF) rings:** Copper deposition in the Descemet membrane of the cornea (best seen via slit-lamp exam). * **Biochemical Triad:** Low serum ceruloplasmin, increased urinary copper excretion, and increased hepatic copper content. * **Neurological Sign:** "Giant Panda Face" appearance on MRI of the midbrain. * **Treatment:** Penicillamine (chelator) or Zinc (inhibits intestinal absorption).

Question 266: Which of the following genetic disorders is caused by defective transport of tryptophan?

A. Hartnup disease (Correct Answer)
B. Maple syrup urine disease
C. Alkaptonuria
D. Phenylketonuria

Explanation: **Explanation:** **Hartnup disease** is the correct answer because it is caused by a mutation in the **SLC6A19 gene**, which encodes a sodium-dependent neutral amino acid transporter. This defect occurs in the proximal renal tubules and the intestinal mucosa, specifically impairing the absorption and reabsorption of **tryptophan**. Since tryptophan is a precursor for **Niacin (Vitamin B3)**, patients develop symptoms resembling Pellagra (Dermatitis, Diarrhea, Dementia). **Analysis of Incorrect Options:** * **Maple syrup urine disease (MSUD):** Caused by a deficiency in the **Branched-chain alpha-keto acid dehydrogenase** complex. It affects the metabolism of Leucine, Isoleucine, and Valine, not the transport of tryptophan. * **Alkaptonuria:** A defect in the enzyme **Homogentisate oxidase** in the tyrosine catabolic pathway, leading to the accumulation of homogentisic acid (causing dark urine and ochronosis). * **Phenylketonuria (PKU):** Caused by a deficiency of **Phenylalanine hydroxylase** (or its cofactor BH4), leading to toxic accumulation of phenylalanine. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Look for "Pellagra-like skin rash" in a child with "cerebellar ataxia" and "neutral aminoaciduria." * **Diagnosis:** Confirmed by detecting high levels of neutral amino acids in the urine (chromatography), while dibasic amino acids (COAL) remain normal. * **Treatment:** High-protein diet and **Nicotinic acid (Niacin) supplementation** to bypass the tryptophan-to-niacin pathway deficiency.

Question 267: What is a cause of hypophosphatemic rickets?

A. Vitamin D deficiency
B. Chronic renal failure (CRF) (Correct Answer)
C. X-linked hypophosphatemic rickets
D. Fanconi syndrome

Explanation: **Explanation:** **Hypophosphatemic rickets** is characterized by low serum phosphate levels leading to defective mineralization of the bone matrix (osteoid). While several conditions cause phosphate wasting, **Chronic Renal Failure (CRF)** is a classic cause of secondary hypophosphatemic rickets (often termed "Renal Osteodystrophy"). **Why Chronic Renal Failure (CRF) is correct:** In CRF, the kidneys fail to excrete phosphate, leading to **hyperphosphatemia**. This triggers a rise in **FGF-23** (Fibroblast Growth Factor 23), which inhibits the 1-alpha-hydroxylase enzyme. Consequently, there is a deficiency of active Vitamin D ($1,25(OH)_2D$), leading to poor calcium absorption and secondary hyperparathyroidism. The combination of high PTH and FGF-23 promotes significant urinary phosphate wasting and bone resorption, resulting in rickets/osteomalacia. **Analysis of Incorrect Options:** * **Vitamin D deficiency:** Causes "Nutritional Rickets." While it leads to low phosphate (due to secondary hyperparathyroidism), it is primarily classified as calcipenic rickets, not primarily hypophosphatemic. * **X-linked hypophosphatemic rickets (XLH):** This is a genetic cause due to *PHEX* gene mutations. While it *is* a cause of hypophosphatemic rickets, in the context of many standard medical examinations, CRF is often highlighted as the systemic pathological cause involving complex endocrine feedback (FGF-23/PTH axis). * **Fanconi Syndrome:** Causes Type 2 Proximal Renal Tubular Acidosis. While it leads to phosphaturia, it is a generalized proximal tubule dysfunction (losing glucose, amino acids, etc.) rather than an isolated phosphate-driven pathology. **High-Yield NEET-PG Pearls:** * **FGF-23:** The "phosphatonin" that is elevated in both XLH and CRF; it decreases phosphate reabsorption in the proximal tubule. * **Hallmark of Hypophosphatemic Rickets:** Normal serum Calcium, Low serum Phosphate, and High Alkaline Phosphatase (ALP). * **Treatment:** Unlike nutritional rickets, this does not respond to Vitamin D alone; it requires oral phosphate supplementation and calcitriol.

Question 268: All of the following are true about von Gierke's disease except?

A. Glucose-6-phosphatase deficiency
B. Hypoglycemia unresponsive to epinephrine but not glucagon
C. Glycogen accumulates in kidney and liver
D. Hyperglycemia (Correct Answer)

Explanation: **Explanation:** **Von Gierke’s Disease (Glycogen Storage Disease Type I)** is a metabolic disorder characterized by the inability to perform the final step of both glycogenolysis and gluconeogenesis. **1. Why "Hyperglycemia" is the correct (False) statement:** The hallmark of von Gierke’s disease is **severe fasting hypoglycemia**, not hyperglycemia. Because the enzyme Glucose-6-phosphatase is deficient, the liver cannot convert Glucose-6-phosphate into free glucose. Consequently, glucose cannot be released into the bloodstream during fasting, leading to profound low blood sugar. **2. Analysis of other options:** * **Option A (G6Pase deficiency):** This is the primary biochemical defect. Type Ia is a deficiency of the enzyme itself, while Type Ib is a deficiency of the translocase. * **Option B (Unresponsive to epinephrine/glucagon):** In a healthy individual, these hormones trigger glycogen breakdown to raise blood sugar. In von Gierke’s, the pathway is blocked at the final step; thus, administering epinephrine or glucagon fails to increase blood glucose levels (instead, it further increases lactate). * **Option C (Accumulation in kidney/liver):** Glucose-6-phosphatase is normally expressed in the liver, kidney, and intestinal mucosa. Its absence leads to massive glycogen deposition in these organs, resulting in **hepatomegaly** and **nephromegaly**. **Clinical Pearls for NEET-PG:** * **Metabolic "4 Hypers":** Hyperuricemia (Gout), Hyperlipidemia, Hyperlactatemia, and Ketosis (though hypoglycemia is the primary driver). * **Appearance:** "Doll-like" facies due to fat deposition and stunted growth. * **Diagnosis:** Confirmed by gene analysis or liver biopsy (showing increased glycogen of normal structure). * **Treatment:** Frequent oral cornstarch (slow-release glucose) and avoidance of fructose/galactose.

Question 269: Which enzyme is deficient in Tyrosinemia type I?

A. Tyrosine Transaminase
B. Carbamoyl Phosphate Synthetase I
C. Fumarylacetoacetate hydrolase (Correct Answer)
D. Argininosuccinate synthetase

Explanation: **Explanation:** **Tyrosinemia Type I** (also known as hepatorenal tyrosinemia) is an autosomal recessive disorder caused by a deficiency of the enzyme **Fumarylacetoacetate hydrolase (FAH)**. This enzyme is responsible for the final step in the tyrosine degradation pathway, converting fumarylacetoacetate into fumarate and acetoacetate. When FAH is deficient, fumarylacetoacetate accumulates and is diverted into the formation of **succinylacetone**. Succinylacetone is a potent toxin that causes severe liver damage (cirrhosis, hepatocellular carcinoma) and renal tubular dysfunction (Fanconi syndrome). **Analysis of Incorrect Options:** * **Option A: Tyrosine Transaminase** deficiency causes **Tyrosinemia Type II** (Richner-Hanhart syndrome), characterized by painful corneal erosions and hyperkeratotic plaques on the palms and soles. * **Option B: Carbamoyl Phosphate Synthetase I** is the rate-limiting enzyme of the Urea Cycle; its deficiency leads to severe hyperammonemia but is unrelated to tyrosine metabolism. * **Option D: Argininosuccinate synthetase** deficiency causes **Citrullinemia Type I**, another urea cycle disorder presenting with lethargy and seizures due to ammonia toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Marker:** Elevated levels of **succinylacetone** in blood or urine are diagnostic for Tyrosinemia Type I. * **Clinical Presentation:** "Cabbage-like" odor, liver failure, and rickets (due to renal phosphate loss). * **Management:** **Nitisinone (NTBC)** is the drug of choice. It inhibits 4-hydroxyphenylpyruvate dioxygenase, preventing the formation of toxic metabolites. * **Diet:** Restriction of Phenylalanine and Tyrosine is essential.

Question 270: Abnormalities of copper metabolism are implicated in the pathogenesis of all the following conditions except?

A. Wilson's disease
B. Menkes' Kinky-hair syndrome
C. Indian childhood cirrhosis
D. Keshan disease (Correct Answer)

Explanation: **Explanation:** The correct answer is **Keshan disease** because it is a condition caused by a deficiency of **Selenium**, not copper. It is a cardiomyopathy primarily seen in children and young women in regions of China where the soil is selenium-deficient. Selenium is a vital component of the enzyme **glutathione peroxidase**, which protects the myocardium from oxidative damage. **Analysis of other options:** * **Wilson’s Disease (Hepatolenticular Degeneration):** An autosomal recessive disorder caused by mutations in the **ATP7B gene**. It leads to impaired biliary copper excretion and failure to incorporate copper into ceruloplasmin, resulting in copper toxicosis in the liver, brain (basal ganglia), and eyes. * **Menkes’ Kinky-hair Syndrome:** An X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This results in defective intestinal copper absorption and transport, leading to severe systemic copper deficiency. Clinical features include "steely" or "kinky" hair, seizures, and connective tissue defects. * **Indian Childhood Cirrhosis (ICC):** A progressive liver disorder associated with excessive **intake of copper** (traditionally from storing milk in brass or copper vessels) combined with a genetic susceptibility. It is characterized by massive copper deposition in hepatocytes. **High-Yield Clinical Pearls for NEET-PG:** * **ATP7A vs. ATP7B:** Remember **"A"** for **A**bsorption (Menkes/ATP7A) and **"B"** for **B**iliary excretion (Wilson/ATP7B). * **Kayser-Fleischer (KF) rings:** Copper deposition in the Descemet’s membrane of the cornea (Wilson's). * **Selenium Deficiency:** Associated with Keshan disease (cardiomyopathy) and Kashin-Beck disease (osteoarthritis). * **Ceruloplasmin:** Low levels are a screening marker for Wilson’s disease.

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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