Genetic Disorders and Biochemical Pathology — MCQs

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

Question 161: Increased VMA is found in which of the following conditions?

A. Pheochromocytoma (Correct Answer)
B. Tyrosinemia
C. Parkinsonism
D. Phenylketonuria

Explanation: **Explanation:** **1. Why Pheochromocytoma is correct:** Pheochromocytoma is a catecholamine-secreting tumor arising from the chromaffin cells of the adrenal medulla. These cells produce excessive amounts of **epinephrine and norepinephrine**. In the body, these catecholamines are metabolized by enzymes **COMT** (Catechol-O-methyltransferase) and **MAO** (Monoamine oxidase). The final end-product of this metabolic pathway is **Vanillylmandelic Acid (VMA)**. Therefore, a 24-hour urinary VMA test is a classic diagnostic marker for Pheochromocytoma. **2. Why the other options are incorrect:** * **Tyrosinemia:** This is a defect in tyrosine catabolism (e.g., deficiency of fumarylacetoacetate hydrolase in Type I). It leads to the accumulation of succinylacetone and tyrosine, not VMA. * **Parkinsonism:** This condition is characterized by a **deficiency of Dopamine** in the nigrostriatal pathway. Since dopamine levels are low, its metabolites (like Homovanillic acid/HVA) are typically decreased, not increased. * **Phenylketonuria (PKU):** This is caused by a deficiency of **Phenylalanine hydroxylase**, leading to high levels of Phenylalanine and its metabolites (phenylpyruvate, phenyllactate). It does not involve the catecholamine degradation pathway. **Clinical Pearls for NEET-PG:** * **VMA vs. Metanephrines:** While VMA is a traditional marker, **urinary or plasma metanephrines** are now considered more sensitive and are the preferred screening test for Pheochromocytoma. * **HVA (Homovanillic Acid):** This is the end-metabolite of **Dopamine**. It is often elevated in **Neuroblastoma** (a common childhood tumor). * **Rule of 10s:** Pheochromocytoma is famously known as the 10% tumor (10% bilateral, 10% malignant, 10% extra-adrenal, 10% familial).

Question 162: Deficiency of lysosomal alpha-1,4 and alpha-1,6 glucosidase is seen in which condition?

A. Von Gierke disease
B. Cori disease
C. Pompe disease (Correct Answer)
D. Tarui disease

Explanation: ### Explanation The correct answer is **Pompe disease (GSD Type II)**. **1. Why Pompe Disease is Correct:** Pompe disease is unique among Glycogen Storage Diseases (GSDs) because it is a **lysosomal storage disorder**. The deficient enzyme is **Lysosomal acid alpha-1,4-glucosidase** (also known as **Acid Maltase**). This enzyme is responsible for breaking down glycogen within lysosomes. While most glycogenolysis occurs in the cytosol, about 1-3% of glycogen is degraded in lysosomes. A deficiency leads to massive accumulation of glycogen in the lysosomes of all organs, most significantly affecting the **heart and skeletal muscles**. **2. Analysis of Incorrect Options:** * **A. Von Gierke Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-Phosphatase**. It primarily affects the liver and kidneys, presenting with severe fasting hypoglycemia and hepatomegaly. * **B. Cori Disease (GSD Type III):** Caused by a deficiency of the **Debranching enzyme** (α-1,6-glucosidase in the cytosol). It results in the accumulation of "limit dextrin" (abnormally short outer chains). * **C. Tarui Disease (GSD Type VII):** Caused by a deficiency of **Phosphofructokinase (PFK)** in muscles and RBCs, leading to exercise intolerance and hemolytic anemia. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart)." * **Clinical Triad:** Cardiomegaly (massive), Hypotonia ("Floppy baby"), and early death due to heart failure. * **Key Feature:** It is the only GSD that is also a Lysosomal Storage Disease. * **Biochemical Marker:** Normal blood glucose levels (unlike Type I) because cytosolic glycogenolysis remains intact. * **Diagnosis:** PAS-positive material in lysosomes on muscle biopsy.

Question 163: If a chromosome divides along an axis perpendicular to its usual axis of division, what will it form?

A. Ring chromosome
B. Isochromosome (Correct Answer)
C. Acrocentric chromosome
D. Subtelocentric chromosome

Explanation: ### Explanation **Correct Answer: B. Isochromosome** **Mechanism:** Normally, during cell division (anaphase), a chromosome divides **longitudinally** (parallel to its long axis), separating the two sister chromatids. An **isochromosome** is formed when the centromere divides **transversely** (perpendicular to the usual axis). This abnormal division results in one daughter chromosome consisting of two short arms (p) and the other consisting of two long arms (q). Consequently, an isochromosome is a structural abnormality where one arm is lost and the other is duplicated, leading to partial monosomy and partial trisomy. **Analysis of Incorrect Options:** * **A. Ring chromosome:** Formed when a chromosome sustains breaks at both ends (telomeres), followed by the fusion of the remaining "sticky" ends into a circular shape. * **C. Acrocentric chromosome:** This refers to the normal morphology of chromosomes (like 13, 14, 15, 21, and 22) where the centromere is located very close to one end, resulting in one very short arm. * **D. Subtelocentric chromosome:** A descriptive term for a chromosome where the centromere is located between the midpoint and the tip (similar to submetacentric). **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Isochromosome:** The most frequent isochromosome involves the long arm of the X chromosome: **i(Xq)**. * **Clinical Correlation:** i(Xq) is seen in approximately 15-20% of cases of **Turner Syndrome** (45,X/46,X,i(Xq)). These patients present with classic features like short stature and streak ovaries. * **Oncology Link:** Isochromosome 12p—**i(12p)**—is a highly specific diagnostic marker for **Germ Cell Tumors** (e.g., seminomas).

Question 164: Which of the following is NOT an indicator of bone formation?

A. Osteocalcin
B. Alkaline phosphatase
C. Hydroxyproline (Correct Answer)
D. Bone scan

Explanation: ### Explanation Bone remodeling is a continuous process involving **bone formation** (by osteoblasts) and **bone resorption** (by osteoclasts). To distinguish between these processes, specific biochemical markers are used. **Why Hydroxyproline is the correct answer:** Hydroxyproline is an amino acid found in high concentrations in **collagen**. When bone matrix is broken down by osteoclasts during **resorption**, hydroxyproline is released into the blood and excreted in the urine. Therefore, it is a marker of **bone resorption/destruction**, not formation. Note: It is less specific than newer markers like NTx and CTx because it can also be derived from dietary intake and non-skeletal collagen. **Analysis of Incorrect Options:** * **Osteocalcin:** This is a non-collagenous protein synthesized specifically by **osteoblasts**. It is considered a highly specific marker for bone formation and reflects osteoblastic activity. * **Alkaline Phosphatase (ALP):** Specifically the **bone-specific isoenzyme (B-ALP)**, it is secreted by osteoblasts during the mineralization process. Elevated levels indicate increased bone formation (e.g., Paget’s disease, rickets). * **Bone Scan:** Using Technetium-99m MDP, a bone scan detects areas of increased **osteoblastic activity** (bone turnover). "Hot spots" on a scan indicate active bone formation, often as a reaction to injury, infection, or malignancy. ### High-Yield Clinical Pearls for NEET-PG: * **Markers of Bone Formation:** Osteocalcin, Bone-specific ALP, and P1NP (Procollagen type 1 N-terminal propeptide—the most sensitive marker). * **Markers of Bone Resorption:** Urinary Hydroxyproline, Urinary Pyridinoline, and Serum/Urinary **CTx and NTx** (Cross-linked telopeptides). * **Acid Phosphatase (TRAP):** Tartrate-resistant acid phosphatase is a specific marker for **osteoclast** activity (resorption).

Question 165: Which of the following conditions is due to a point mutation?

A. Color blindness
B. Sickle cell anemia (Correct Answer)
C. Diabetes
D. Porphyria

Explanation: **Explanation:** **Sickle Cell Anemia (Correct Answer):** Sickle cell anemia is the classic example of a **missense point mutation**. It occurs due to a single nucleotide substitution (A to T) in the 6th codon of the **β-globin gene** on chromosome 11. This results in the replacement of **Glutamic acid** (polar/negative charge) with **Valine** (non-polar/hydrophobic). This single amino acid change causes hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of RBCs, hemolysis, and vaso-occlusive crises. **Analysis of Incorrect Options:** * **Color Blindness:** This is typically an **X-linked recessive** disorder caused by **unequal crossing over** or deletions/rearrangements in the opsin genes, rather than a single point mutation. * **Diabetes Mellitus:** This is a **multifactorial/polygenic** disorder involving a complex interplay of multiple genetic loci and environmental factors (Type 2) or autoimmune destruction (Type 1). * **Porphyria:** While some forms can involve point mutations, the group as a whole is heterogeneous. Most porphyrias (like Acute Intermittent Porphyria) are **Autosomal Dominant** and involve various types of mutations (insertions, deletions, or splice-site mutations) leading to enzyme deficiencies in the heme synthesis pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Transition vs. Transversion:** The mutation in Sickle Cell (GAG → GTG) is a **transversion** (purine to pyrimidine). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because Valine is less negative than Glutamic acid. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria.

Question 166: Glycogen storage diseases include all of the following except?

A. Krabbe's disease
B. Fabry's disease
C. Glycogen storage disease
D. None of the above (Correct Answer)

Explanation: ### Explanation **Concept Overview** Glycogen Storage Diseases (GSDs) are a group of inherited metabolic disorders caused by deficiencies in enzymes involved in glycogen synthesis or breakdown. In contrast, Lysosomal Storage Disorders (LSDs) involve the accumulation of undigested macromolecules (like sphingolipids) due to lysosomal enzyme deficiencies. **Why "None of the above" is correct:** The question asks which of the listed options are **not** Glycogen Storage Diseases. However, the options provided (Krabbe’s and Fabry’s) are both classified as **Sphingolipidoses** (a subtype of Lysosomal Storage Diseases), not GSDs. Since the question structure implies selecting an exception, and the options provided are indeed not GSDs, the "None of the above" choice acts as a distractor or indicates a technical error in the question's phrasing. In a standard NEET-PG format, if the question asks "which of the following is NOT a GSD," and both A and B are not GSDs, the answer reflects that the premise of the options is distinct from GSD pathology. **Analysis of Options:** * **Krabbe’s disease:** This is a sphingolipidosis caused by a deficiency of **Galactocerebrosidase**. It leads to the accumulation of galactocerebroside, primarily affecting the myelin sheath (CNS/PNS). * **Fabry’s disease:** This is an X-linked recessive sphingolipidosis caused by **$\alpha$-galactosidase A** deficiency, leading to the accumulation of Ceramide trihexoside. * **Glycogen storage disease:** This is a tautological option and obviously belongs to the GSD category. **NEET-PG High-Yield Pearls:** * **GSD Type I (von Gierke):** Deficiency of Glucose-6-Phosphatase; presents with severe hypoglycemia and hepatomegaly. * **GSD Type II (Pompe):** Deficiency of Lysosomal $\alpha$-1,4-glucosidase; unique because it is both a **GSD and a Lysosomal Storage Disease**. * **Fabry’s Disease Triad:** Episodic peripheral neuropathy, angiokeratomas, and hypohidrosis. * **Krabbe’s Hallmark:** Presence of **Globoid cells** on histology.

Question 167: Which of the following occurs in the lipidosis known as Tay-Sachs disease?

A. Synthesis of a specific ganglioside is excessive
B. Xanthomas due to cholesterol deposition are observed
C. Phosphoglycerides accumulate in the brain
D. Ganglioside GM2 is not catabolized by lysosomal enzymes (Correct Answer)

Explanation: **Explanation:** **Tay-Sachs Disease** is a lysosomal storage disorder (specifically a GM2 gangliosidosis) caused by a deficiency of the enzyme **Hexosaminidase A**. 1. **Why Option D is Correct:** Under normal physiological conditions, Hexosaminidase A degrades Ganglioside GM2 into GM3. In Tay-Sachs, the absence of this enzyme prevents the catabolism (breakdown) of **Ganglioside GM2**. Consequently, GM2 accumulates to toxic levels within the lysosomes of neurons, leading to progressive neurodegeneration. 2. **Analysis of Incorrect Options:** * **Option A:** The pathology is due to a failure in **degradation**, not an excess in synthesis. The rate of ganglioside production remains normal. * **Option B:** Xanthomas are characteristic of Hyperlipidemias (e.g., Familial Hypercholesterolemia), where there is an accumulation of cholesterol/LDLs, not gangliosides. * **Option C:** Phosphoglycerides are major components of cell membranes but are not the primary storage material in Tay-Sachs. The specific lipid involved is a sphingolipid (Ganglioside GM2). **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficient:** Hexosaminidase A (Mnemonic: Tay-Sa**X** lacks He**X**osaminidase). * **Accumulated Substance:** GM2 Ganglioside. * **Clinical Triad:** Progressive neurodegeneration, developmental delay, and a **Cherry-red spot** on the macula. * **Distinguishing Feature:** Unlike Niemann-Pick disease, there is **NO hepatosplenomegaly** in Tay-Sachs. * **Histology:** "Onion-skin" appearance of lysosomes under electron microscopy. * **Genetics:** Autosomal Recessive; high prevalence in Ashkenazi Jews.

Question 168: A defect in the MRP2 gene results in which hereditary disorder of bilirubin metabolism?

A. Gilbert syndrome
B. Crigler-Najjar syndrome type I
C. Crigler-Najjar syndrome type II
D. Dubin-Johnson syndrome (Correct Answer)

Explanation: **Explanation:** The correct answer is **Dubin-Johnson syndrome**. This condition is an autosomal recessive disorder caused by a mutation in the **ABCC2 gene**, which encodes the **Multidrug Resistance-associated Protein 2 (MRP2)**. MRP2 is an ATP-dependent canalicular transporter responsible for the efflux of conjugated bilirubin from hepatocytes into the bile canaliculi. A defect in this protein leads to the accumulation of conjugated bilirubin within the liver cells, which then leaks back into the blood, causing **conjugated (direct) hyperbilirubinemia**. **Why the other options are incorrect:** * **Gilbert syndrome:** Caused by a mutation in the promoter region of the **UGT1A1 gene**, leading to reduced activity (approx. 30%) of the enzyme UDP-glucuronosyltransferase. It results in mild *unconjugated* hyperbilirubinemia. * **Crigler-Najjar syndrome type I:** Characterized by a **complete absence** of UGT1A1 enzyme activity. It causes severe, life-threatening *unconjugated* hyperbilirubinemia. * **Crigler-Najjar syndrome type II (Arias syndrome):** Characterized by a **marked reduction** (less than 10%) in UGT1A1 activity. It results in moderate *unconjugated* hyperbilirubinemia and usually responds to Phenobarbital. **High-Yield Clinical Pearls for NEET-PG:** * **Black Liver:** A hallmark of Dubin-Johnson syndrome is a grossly **black/darkly pigmented liver** due to the accumulation of epinephrine metabolites (melanin-like pigment) in lysosomes. * **Urinary Coproporphyrins:** In Dubin-Johnson, the *total* urinary coproporphyrin levels are normal, but **>80% is Coproporphyrin I** (normally, Coproporphyrin III predominates). * **Rotor Syndrome:** Often confused with Dubin-Johnson, but it lacks liver pigmentation and is caused by defects in OATP1B1 and OATP1B3 transporters.

Question 169: Pellagra is caused by which of the following conditions?

A. Hartnup's disease (Correct Answer)
B. Cystinuria
C. Cystinosis
D. Type I Tyrosinemia

Explanation: **Explanation:** **Pellagra** is a clinical syndrome caused by a deficiency of **Niacin (Vitamin B3)**. While it is commonly associated with dietary deficiency, it can also occur due to **Hartnup’s disease** (Option A). 1. **Why Hartnup’s Disease is Correct:** Hartnup’s disease is an autosomal recessive disorder characterized by a defect in the **SLC6A19 transporter**, which is responsible for the absorption of neutral amino acids (especially **Tryptophan**) in the small intestine and renal tubules. Since Tryptophan is a metabolic precursor for Niacin (60 mg Tryptophan = 1 mg Niacin), its malabsorption leads to a secondary Niacin deficiency, manifesting as the "3 Ds" of Pellagra: **Dermatitis** (Casal’s necklace), **Diarrhea**, and **Dementia**. 2. **Why Other Options are Incorrect:** * **Cystinuria (B):** A defect in the transport of COAL (Cystine, Ornithine, Arginine, Lysine). It leads to renal stones (hexagonal crystals) but does not affect Niacin levels. * **Cystinosis (C):** A lysosomal storage disorder where cystine accumulates within lysosomes, leading to Fanconi syndrome and renal failure, not pellagra. * **Type I Tyrosinemia (D):** Caused by a deficiency of Fumarylacetoacetate hydrolase. It presents with liver failure, cabbage-like odor, and renal rickets. **High-Yield Clinical Pearls for NEET-PG:** * **The 4th D:** If untreated, Pellagra leads to **Death**. * **Hartnup Diagnosis:** Look for **Neutral Aminoaciduria** in the urine. * **Other causes of Pellagra:** Carcinoid syndrome (Tryptophan is diverted to Serotonin synthesis) and prolonged Isoniazid (INH) therapy (depletes Vitamin B6, a cofactor for Niacin synthesis).

Question 170: Which of the following is true about autosomal dominant inheritance?

A. Variable age of onset
B. Reduced penetrance
C. Variable expressibility
D. All of the above (Correct Answer)

Explanation: **Explanation:** Autosomal dominant (AD) disorders are characterized by the presence of a mutation in only one copy of a gene (heterozygous state). Unlike autosomal recessive conditions, AD traits exhibit several unique clinical phenomena that make their presentation complex. 1. **Variable Age of Onset:** Not all AD disorders are present at birth. Many manifest later in life (e.g., **Huntington’s disease**, which typically presents in the 30s or 40s). This is often due to the gradual accumulation of toxic metabolic products or delayed protein dysfunction. 2. **Reduced (Incomplete) Penetrance:** This refers to an "all-or-none" phenomenon where an individual carries the dominant mutation but does not manifest the clinical phenotype at all. For example, some individuals with the **BRCA1** mutation may never develop cancer. 3. **Variable Expressivity:** This describes the range of signs and symptoms that can occur in different people with the same genetic condition. In **Neurofibromatosis Type 1**, one patient may only have café-au-lait spots, while their sibling may have extensive neurofibromas and skeletal deformities. **Why "All of the above" is correct:** All three features—variable onset, reduced penetrance, and variable expressivity—are hallmark characteristics of autosomal dominant inheritance patterns, distinguishing them from the more uniform presentation of recessive disorders. **High-Yield Clinical Pearls for NEET-PG:** * **Vertical Transmission:** AD disorders typically appear in every generation. * **Pleiotropy:** A single gene mutation affecting multiple organ systems (e.g., Marfan Syndrome affecting eyes, heart, and skeleton). * **Anticipation:** Symptoms become more severe or appear at an earlier age in succeeding generations (common in triplet repeat disorders like Myotonic Dystrophy). * **De novo mutations:** Many AD cases (like Achondroplasia) arise from new mutations in the germline of a parent, often associated with advanced paternal age.

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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