Genetic Disorders and Biochemical Pathology — MCQs

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

Question 471: Glycogen storage diseases include all the following except?

A. Fabry's disease
B. Fragile syndrome
C. Krabbe's disease
D. All of these (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (All of these)** because none of the conditions listed are classified as Glycogen Storage Diseases (GSDs). GSDs are a group of inherited metabolic disorders caused by deficiencies in enzymes involved in glycogen synthesis or breakdown (e.g., Von Gierke, Pompe, Cori, and McArdle diseases). **Breakdown of Options:** 1. **Fabry’s Disease:** This is a **Lysosomal Storage Disorder (Sphingolipidosis)** caused by a deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of globotriaosylceramide (Gb3). It is X-linked recessive and presents with angiokeratomas, peripheral neuropathy, and renal failure. 2. **Krabbe’s Disease:** This is also a **Sphingolipidosis** caused by a deficiency of **galactocerebrosidase**. It leads to the destruction of myelin (demyelination) and is characterized by the presence of multinucleated **globoid cells** in the brain. 3. **Fragile X Syndrome:** This is a **Genetic Repeat Expansion Disorder** (not a metabolic storage disease). It is caused by a CGG trinucleotide repeat expansion in the *FMR1* gene on the X chromosome, leading to intellectual disability and macroorchidism. **High-Yield Clinical Pearls for NEET-PG:** * **GSD Type II (Pompe Disease)** is unique because it is both a GSD and a Lysosomal Storage Disorder (deficiency of Acid Maltase). * **Fabry’s Disease** and **Hunter Syndrome** are the two common X-linked recessive Lysosomal Storage Disorders; most others are Autosomal Recessive. * **Gaucher’s Disease** is the most common Lysosomal Storage Disorder (deficiency of Glucocerebrosidase; look for "crumpled tissue paper" cytoplasm).

Question 472: Acute intermittent porphyria is caused by the deficiency of which enzyme?

A. Uroporphyrinogen I synthase (Correct Answer)
B. Ferrochelatase
C. Uroporphyrinogen III synthase
D. Proto-oxidase

Explanation: **Explanation:** **Acute Intermittent Porphyria (AIP)** is an autosomal dominant metabolic disorder caused by a deficiency in **Uroporphyrinogen I synthase**, also known as **Porphobilinogen (PBG) deaminase** or Hydroxymethylbilane synthase. This enzyme is the third step in the heme biosynthesis pathway. Its deficiency leads to the accumulation of the precursors **delta-aminolevulinic acid (ALA)** and **porphobilinogen (PBG)** in the blood and urine, which are neurotoxic and responsible for the clinical symptoms. **Analysis of Options:** * **Option A (Correct):** Uroporphyrinogen I synthase (PBG deaminase) deficiency leads to AIP. * **Option B (Incorrect):** **Ferrochelatase** deficiency causes **Erythropoietic Protoporphyria**, characterized by cutaneous photosensitivity. * **Option C (Incorrect):** **Uroporphyrinogen III synthase** deficiency causes **Congenital Erythropoietic Porphyria (Gunther’s disease)**, leading to severe photosensitivity and erythrodontia. * **Option D (Incorrect):** **Protoporphyrinogen oxidase** (Proto-oxidase) deficiency causes **Variegate Porphyria**, which presents with both neurological symptoms and skin lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad of AIP:** 1. Abdominal pain (most common), 2. Neuropsychiatric symptoms (hallucinations/seizures), 3. Peripheral neuropathy. * **Key Diagnostic Feature:** Urine turns **dark/port-wine colored** upon standing (due to oxidation of PBG to porphobilin). * **Precipitating Factors:** Barbiturates, alcohol, and starvation (these induce CYP450 enzymes, depleting heme and triggering the pathway). * **Crucial Fact:** AIP is the only major porphyria that **does not** present with cutaneous photosensitivity.

Question 473: An albino individual marries a non-albino individual. What are the chances of their offspring being affected with albinism and what are the chances of their offspring being carriers?

A. No affected children, all children are carriers (Correct Answer)
B. All children are normal
C. 50% of children are carriers
D. 50% of children are affected and 50% are carriers

Explanation: **Explanation:** **1. Understanding the Correct Answer (Option A):** Albinism (specifically Oculocutaneous Albinism) is a classic **Autosomal Recessive (AR)** disorder, most commonly caused by a deficiency in the enzyme **tyrosinase**. * An albino individual has the genotype **aa** (homozygous recessive). * A non-albino individual (unless specified as a carrier) is assumed to be **AA** (homozygous dominant). * **The Cross:** aa × AA results in 100% **Aa** offspring. * **Result:** All children are phenotypically normal but are obligate **carriers** (heterozygous). Since no child receives two recessive alleles, none are affected. **2. Analysis of Incorrect Options:** * **Option B:** While the children are phenotypically "normal," this option is incomplete because it fails to account for their status as genetic carriers. * **Option C:** This would occur if one parent was a carrier (Aa) and the other was homozygous dominant (AA). In our case, the albino parent must pass the 'a' gene to 100% of the offspring. * **Option D:** This scenario (Pseudodominance) only occurs if the non-albino parent is a **carrier (Aa)**. In NEET-PG questions, if a person is described simply as "non-albino" or "normal," assume they are homozygous dominant (AA) unless the family history suggests otherwise. **3. Clinical Pearls for NEET-PG:** * **Biochemical Defect:** Tyrosinase deficiency leads to a failure in converting **Tyrosine to DOPA** and subsequently to Melanin. * **Inheritance Pattern:** Most enzyme deficiencies (like Albinism, PKU, and Galactosemia) follow an **Autosomal Recessive** pattern. * **High-Yield Fact:** If an albino (aa) marries a carrier (Aa), the risk of an affected child is 50%. This is a common "trick" variation in exams. * **Associated Conditions:** Look for **Chediak-Higashi Syndrome** in questions involving albinism plus recurrent infections (immunodeficiency).

Question 474: On exercise testing, what is a characteristic finding in a patient with McArdle disease?

A. Increased exercise endurance
B. Increased blood glucose in the blood drawn from the exercising forearm vein
C. Increased blood lactate in the blood drawn from the exercising forearm vein
D. Relatively normal blood lactate in the blood drawn from the exercising forearm vein (Correct Answer)

Explanation: **Explanation:** **McArdle Disease (GSD Type V)** is caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**. This enzyme is essential for glycogenolysis, the process of breaking down muscle glycogen into glucose-1-phosphate to provide energy during anaerobic exercise. **1. Why the Correct Answer is Right:** In healthy individuals, intense exercise leads to anaerobic glycolysis, which produces lactate. In McArdle disease, the metabolic block prevents the breakdown of glycogen into glucose units. Consequently, there is no substrate for glycolysis to proceed, and **no lactate is produced**. During an ischemic forearm exercise test, the characteristic finding is a **failure of blood lactate levels to rise** (remaining relatively normal or flat), despite a significant rise in ammonia levels. **2. Why the Incorrect Options are Wrong:** * **Option A:** Patients have **decreased** exercise endurance due to the inability to access glycogen stores, leading to early fatigue, muscle cramps, and potential myoglobinuria. * **Option B:** Blood glucose does not typically increase in the venous drainage of an exercising muscle; rather, the muscle relies on internal glycogen or systemic glucose. * **Option C:** This is the physiological response in a healthy person. An increase in lactate excludes a diagnosis of McArdle disease. **3. NEET-PG High-Yield Clinical Pearls:** * **"Second Wind" Phenomenon:** A classic clinical feature where patients experience a decrease in heart rate and improved exercise tolerance after 10–15 minutes of exercise, once the body switches to using free fatty acids and blood glucose. * **Diagnosis:** Definitive diagnosis is made via muscle biopsy (showing subsarcolemmal glycogen deposits) or genetic testing (*PYGM* gene). * **Complication:** Severe exercise can lead to **rhabdomyolysis**, resulting in burgundy-colored urine (myoglobinuria) and potential acute renal failure. * **Lab Finding:** Elevated baseline Creatine Kinase (CK) is common.

Question 475: Which of the following statements is NOT true about Refsum disease?

A. Autosomal recessive inheritance
B. Lysosomal disorder (Correct Answer)
C. Defective fatty acid oxidation
D. Neurocutaneous syndrome

Explanation: **Explanation:** Refsum disease is a rare metabolic disorder characterized by the accumulation of **phytanic acid** in tissues. **Why Option B is the correct (False) statement:** Refsum disease is a **Peroxisomal disorder**, not a lysosomal disorder. It is specifically caused by a deficiency in the peroxisomal enzyme **Phytanoyl-CoA hydroxylase**. This enzyme is essential for **Alpha-oxidation**, the process required to break down phytanic acid (a branched-chain fatty acid derived from chlorophyll in the diet). **Analysis of other options:** * **Option A (Autosomal recessive):** This is true. Like most inborn errors of metabolism, Refsum disease follows an autosomal recessive inheritance pattern. * **Option C (Defective fatty acid oxidation):** This is true. It specifically involves a defect in the **alpha-oxidation** of branched-chain fatty acids. Since the methyl group is present on the beta-carbon of phytanic acid, beta-oxidation is blocked, necessitating initial alpha-oxidation. * **Option D (Neurocutaneous syndrome):** This is true. Refsum disease presents with both neurological and dermatological features, including **Retinitis pigmentosa**, peripheral neuropathy, cerebellar ataxia, and **Ichthyosis** (scaly skin). **High-Yield Clinical Pearls for NEET-PG:** * **Dietary Management:** The primary treatment is the restriction of green leafy vegetables (chlorophyll), ruminant fats, and dairy products. * **Classic Tetrad:** 1. Retinitis pigmentosa (earliest sign), 2. Peripheral neuropathy, 3. Cerebellar ataxia, 4. High CSF protein (without pleocytosis). * **Zellweger Syndrome:** Contrast Refsum with Zellweger; both are peroxisomal, but Zellweger involves a total loss of peroxisome biogenesis (PEX gene mutations) and is much more severe.

Question 476: Menke's disease is a disorder of which of the following transport mechanisms?

A. Impaired zinc transport
B. Impaired copper transport (Correct Answer)
C. Impaired magnesium transport
D. Impaired molybdenum transport

Explanation: **Explanation:** **Menkes Disease (Kinky Hair Syndrome)** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This gene encodes a P-type ATPase responsible for the efflux of copper from enterocytes into the portal circulation. In Menkes disease, copper is absorbed by intestinal cells but cannot be transported further, leading to **systemic copper deficiency**. 1. **Why Copper Transport is Correct:** Copper is a vital cofactor for several enzymes. Its deficiency leads to: * **Lysyl oxidase failure:** Results in defective collagen cross-linking (causing arterial tortuosity and bone abnormalities). * **Tyrosinase failure:** Leads to hypopigmentation. * **Cytochrome c oxidase failure:** Causes neurodegeneration and hypotonia. * **Dopamine β-hydroxylase failure:** Leads to catecholamine derangements. 2. **Why Other Options are Incorrect:** * **Zinc Transport:** Impaired zinc absorption (due to SLC39A4 mutation) causes **Acrodermatitis Enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Magnesium Transport:** Disorders like Gitelman syndrome or Familial Hypomagnesemia with Hypercalciuria involve magnesium, but do not present with the "kinky hair" phenotype. * **Molybdenum Transport:** Molybdenum cofactor deficiency is a rare metabolic disorder presenting with early-onset seizures and lens ectopia, unrelated to copper metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Steely/Kinky hair (Pili torti), growth retardation, and progressive neurological deterioration. * **Diagnosis:** Low serum copper and low serum ceruloplasmin levels. * **Contrast with Wilson’s Disease:** Wilson’s is a defect in **ATP7B**, leading to copper *overload* (toxic accumulation in liver/brain), whereas Menkes is a defect in **ATP7A**, leading to systemic *deficiency*.

Question 477: Mucopolysaccharidoses are caused by?

A. An increased rate of synthesis of proteoglycans
B. Defects in the degradation of proteoglycans (Correct Answer)
C. The synthesis of polysaccharides with an altered structure
D. An insufficient amount of proteolytic enzymes

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Mucopolysaccharidoses (MPS) are a group of hereditary metabolic disorders characterized by the accumulation of **Glycosaminoglycans (GAGs)**, formerly known as mucopolysaccharides. These GAGs are the carbohydrate component of **proteoglycans**. Under normal physiological conditions, GAGs are degraded within **lysosomes** by a series of specific enzymes. MPS occurs due to a deficiency or defect in these lysosomal hydrolases, leading to the incomplete degradation and subsequent accumulation of GAGs in various tissues. Therefore, the core pathology is a **defect in the degradation** of proteoglycans. **2. Why the Incorrect Options are Wrong:** * **Option A:** The rate of synthesis of proteoglycans is normal in MPS; the problem lies entirely in their clearance. * **Option C:** The structure of the polysaccharides synthesized is initially normal. The "altered" structures found in tissues are actually partially degraded intermediates that cannot be broken down further. * **Option D:** While enzymes are insufficient, they are **glycosidases/sulfatases** (which break down carbohydrates), not **proteolytic enzymes** (which break down proteins). **3. High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** All MPS are **Autosomal Recessive**, EXCEPT **Hunter Syndrome (MPS II)**, which is **X-linked Recessive**. * **Corneal Clouding:** Present in most MPS (e.g., Hurler Syndrome) but notably **ABSENT in Hunter Syndrome**. * **Key Enzyme Deficiencies:** * **Hurler (MPS IH):** $\alpha$-L-iduronidase. * **Hunter (MPS II):** Iduronate sulfatase. * **Sanfilippo (MPS III):** Most common MPS; involves degradation of Heparan sulfate. * **Diagnosis:** Initial screening via urinary GAG levels; definitive diagnosis via enzyme assay or genetic testing.

Question 478: Paternal 15 chromosome deletion is seen in which of the following conditions?

A. Angelman syndrome
B. Prader Willi syndrome (Correct Answer)
C. Down syndrome
D. Turner syndrome

Explanation: **Explanation:** The correct answer is **Prader-Willi Syndrome (PWS)**. This condition is a classic example of **Genomic Imprinting**, where the expression of a gene depends on whether it is inherited from the mother or the father. 1. **Why Prader-Willi Syndrome is correct:** In normal individuals, the PWS gene region on **Chromosome 15 (q11-q13)** is active on the paternal chromosome and silenced (imprinted) on the maternal chromosome. PWS occurs when the **paternal** contribution is lost, most commonly due to a **microdeletion (70%)** or Maternal Uniparental Disomy (25%). Clinical features include neonatal hypotonia, hyperphagia leading to obesity, hypogonadism, and small hands/feet. 2. **Why other options are incorrect:** * **Angelman Syndrome:** This involves the same locus (15q11-q13) but results from the loss of the **maternal** allele (specifically the *UBE3A* gene). It is characterized by inappropriate laughter ("Happy Puppet"), seizures, and ataxia. * **Down Syndrome:** Caused by **Trisomy 21** (an extra copy of chromosome 21), not a deletion. * **Turner Syndrome:** Caused by **Monosomy X (45, XO)**, involving the loss of an entire sex chromosome in females. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **P**aternal deletion = **P**rader-Willi; **M**aternal deletion = **A**ngelman (**M**ama **A**ngel). * **Uniparental Disomy (UPD):** PWS can occur if a child inherits two copies of chromosome 15 from the mother and none from the father. * **Diagnostic Gold Standard:** Chromosomal Microarray or DNA Methylation analysis (to detect abnormal imprinting patterns).

Question 479: Tay-Sachs disease is an inborn error of metabolism due to deficiency of which enzyme?

A. Hexosaminidase-A (Correct Answer)
B. Aryl sulfatase-A
C. alpha-galactosidase
D. Galactocerebrosidase

Explanation: **Explanation:** Tay-Sachs disease is an autosomal recessive lysosomal storage disorder classified as a **GM2 Gangliosidosis**. It is caused by a deficiency of the enzyme **Hexosaminidase-A**, which leads to the toxic accumulation of GM2 gangliosides, primarily in the neurons of the brain and spinal cord. **Analysis of Options:** * **A. Hexosaminidase-A (Correct):** Deficiency leads to Tay-Sachs. The "A" in Hexosaminidase-A stands for "Acidic," and its absence results in the characteristic neurodegeneration. * **B. Aryl sulfatase-A:** Deficiency causes **Metachromatic Leukodystrophy**, characterized by the accumulation of cerebroside sulfate and central/peripheral demyelination. * **C. alpha-galactosidase:** Deficiency causes **Fabry disease**, an X-linked disorder leading to the accumulation of ceramide trihexoside. Clinical features include angiokeratomas and renal failure. * **D. Galactocerebrosidase:** Deficiency causes **Krabbe disease**, leading to the accumulation of galactocerebroside and the presence of characteristic "globoid cells" in the brain. **High-Yield Clinical Pearls for NEET-PG:** * **Cherry-red spot on macula:** A classic finding in Tay-Sachs (also seen in Niemann-Pick, but Tay-Sachs lacks hepatosplenomegaly). * **Onion-skin lysosomes:** Electron microscopy shows whorled configurations of membranes. * **No Hepatosplenomegaly:** This is the key clinical differentiator from Niemann-Pick disease (Sphingomyelinase deficiency). * **Genetics:** Common in the Ashkenazi Jewish population; caused by a mutation on Chromosome 15.

Question 480: Hepatomegaly is a feature of all of the following, EXCEPT:

A. Hurler's Disease
B. Hepatic porphyrias (Correct Answer)
C. Von Gierke's Disease
D. Nieman Pick Disease

Explanation: ### Explanation The correct answer is **B. Hepatic porphyrias**. **1. Why Hepatic Porphyrias is the correct answer:** Hepatic porphyrias (such as Acute Intermittent Porphyria) are disorders of heme biosynthesis characterized by the accumulation of porphyrin precursors (ALA and PBG). While the liver is the primary site of the metabolic defect, these conditions typically present with **neurovisceral symptoms** (abdominal pain, neuropsychiatric issues) rather than structural changes like organomegaly. Unlike storage disorders, there is no massive accumulation of bulky macromolecules within the hepatocytes to cause significant hepatomegaly. **2. Why the other options are incorrect:** * **Hurler’s Disease (MPS I):** This is a lysosomal storage disorder where **Dermatan and Heparan sulfate** accumulate in the reticuloendothelial system, leading to significant hepatosplenomegaly and coarse facial features. * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-Phosphatase**. Glycogen cannot be broken down into glucose and instead accumulates excessively in the liver and kidneys, leading to massive hepatomegaly and renomegaly. * **Niemann-Pick Disease:** A sphingolipidosis (deficiency of **Sphingomyelinase**) where sphingomyelin accumulates in macrophages. This results in prominent hepatosplenomegaly and a "cherry-red spot" on the macula. **Clinical Pearls for NEET-PG:** * **Massive Hepatomegaly:** Think Von Gierke’s (GSD I) or Gaucher’s Disease. * **Hepatomegaly + Hypoglycemia:** Think Glycogen Storage Diseases (Types I, III, VI). * **Hepatomegaly + Coarse Facies:** Think Mucopolysaccharidoses (Hurler/Hunter). * **Porphyria Tip:** The most common porphyria is *Porphyria Cutanea Tarda*, which may show mild liver damage/siderosis, but classic "Hepatic Porphyrias" are tested for their acute neurological presentations, not hepatomegaly.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

Practice Questions

Lysosomal Storage Diseases

Practice Questions

Glycogen Storage Diseases

Practice Questions

Disorders of Lipoprotein Metabolism

Practice Questions

Disorders of Purine and Pyrimidine Metabolism

Practice Questions

Hemoglobinopathies

Practice Questions

Porphyrias

Practice Questions

Biochemical Markers for Disease Diagnosis

Practice Questions

Newborn Screening for Genetic Disorders

Practice Questions

Enzyme Replacement Therapy

Practice Questions

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