Genetic Disorders and Biochemical Pathology — MCQs

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

Question 291: A patient presents with muscle cramps on exercise and positive myoglobinemia. What is the most likely disorder?

A. Pompe's disease
B. Myotonia congenita
C. Myotonic dystrophy
D. McArdle's disease (Correct Answer)

Explanation: **Explanation:** The clinical presentation of **muscle cramps during exercise** followed by **myoglobinuria** (indicated by myoglobinemia) is the classic triad of **McArdle’s Disease (GSD Type V)**. **1. Why McArdle’s Disease is Correct:** McArdle’s disease is caused by a deficiency of **muscle glycogen phosphorylase** (myophosphorylase). This enzyme is essential for glycogenolysis—breaking down muscle glycogen into glucose-1-phosphate to provide energy during anaerobic exercise. Without it, muscles face an acute energy crisis during exertion, leading to painful cramps, rhabdomyolysis (muscle breakdown), and the release of myoglobin into the blood and urine. **2. Why Other Options are Incorrect:** * **Pompe’s Disease (GSD Type II):** Caused by **acid maltase** deficiency. It primarily presents as progressive muscle weakness and **cardiomegaly** (in infants). It does not typically cause exercise-induced cramps or acute myoglobinuria. * **Myotonia Congenita:** A chloride channelopathy characterized by **delayed muscle relaxation** (stiffness) after voluntary contraction (e.g., difficulty releasing a handshake), not exercise-induced breakdown. * **Myotonic Dystrophy:** A multisystem genetic disorder presenting with muscle wasting, cataracts, frontal balding, and arrhythmias. It involves "anticipation" due to CTG repeats, not acute metabolic crises during exercise. **3. High-Yield Clinical Pearls for NEET-PG:** * **"Second Wind" Phenomenon:** A hallmark of McArdle’s where symptoms improve after a few minutes of exercise as the body switches to using fatty acids and blood glucose. * **Ischemic Forearm Exercise Test:** Shows a **failure of blood lactate to rise** (since glycogen cannot be converted to lactate), while ammonia levels rise significantly. * **Biochemical Defect:** Deficiency of Myophosphorylase (encoded by the *PYGM* gene).

Question 292: The nitroprusside test is used for the diagnosis of which of the following diseases?

A. Phenylketonuria
B. Tyrosinemia
C. Maple syrup urine disease
D. Homocystinuria (Correct Answer)

Explanation: **Explanation:** The **Cyanide-Nitroprusside test** is a screening tool used to detect the presence of sulfur-containing amino acids in the urine, specifically **Cystine** and **Homocysteine**. In **Homocystinuria**, there is a deficiency of the enzyme *Cystathionine beta-synthase (CBS)*, leading to an accumulation of homocysteine. When sodium cyanide is added to the urine, it reduces the disulfide bonds of homocystine into sulfhydryl groups (homocysteine). These groups then react with sodium nitroprusside to produce a characteristic **magenta/deep red color**, confirming a positive result. **Analysis of Incorrect Options:** * **A. Phenylketonuria (PKU):** Diagnosed using the **Ferric Chloride test**, which produces a blue-green color due to phenylpyruvic acid. * **B. Tyrosinemia:** Also detected by the Ferric Chloride test (transient green) or the **Millon’s test** (detects tyrosine metabolites). * **C. Maple Syrup Urine Disease (MSUD):** Diagnosed using the **DNPH (2,4-Dinitrophenylhydrazine) test**, which detects alpha-keto acids, resulting in a yellow-white precipitate. **High-Yield Clinical Pearls for NEET-PG:** * **Differential Diagnosis:** Both Homocystinuria and **Cystinuria** give a positive Nitroprusside test. To differentiate, remember that Homocystinuria presents with intellectual disability and downward lens subluxation (*Ectopia lentis*), whereas Cystinuria presents with renal stones. * **Treatment Tip:** About 50% of Homocystinuria cases are **Pyridoxine (Vitamin B6) responsive**. * **Vascular Risk:** Homocystinuria is a high-yield cause of premature atherosclerosis and thromboembolism in young patients.

Question 293: Which leukodystrophy is characterized by the inability to properly catabolize very-long-chain fatty acids (VLCFA) within peroxisomes, leading to elevated levels of VLCFA in serum?

A. Ortho leukodystrophy
B. Metachromatic leukodystrophy
C. Adrenoleukodystrophy (Correct Answer)
D. Krabbe disease

Explanation: **Explanation:** **Adrenoleukodystrophy (ALD)** is an X-linked recessive disorder caused by a mutation in the **ABCD1 gene**, which encodes a peroxisomal membrane transporter protein. This defect prevents the entry of **Very-Long-Chain Fatty Acids (VLCFA)** into the peroxisome for **beta-oxidation**. Consequently, VLCFAs accumulate in the serum and tissues, particularly the adrenal cortex and the white matter of the CNS, leading to demyelination and adrenal insufficiency (Addison’s disease). **Analysis of Incorrect Options:** * **Metachromatic Leukodystrophy:** Caused by a deficiency of **Arylsulfatase A**, leading to the accumulation of **sulfatides**. It is characterized by "metachromasia" of nerves when stained with toluidine blue. * **Krabbe Disease (Globoid Cell Leukodystrophy):** Caused by a deficiency of **Galactocerebrosidase**, leading to the accumulation of galactocerebroside and psychosine. Histology shows characteristic multinucleated **globoid cells**. * **Ortho Leukodystrophy:** A general term for leukodystrophies with normal myelin breakdown products (like Pelizaeus-Merzbacher disease), but it is not specifically associated with VLCFA metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Peroxisomal Disorders:** ALD is the most common peroxisomal disorder. Another high-yield condition is **Zellweger Syndrome** (generalized peroxisome biogenesis defect), which also shows elevated VLCFAs but presents with more severe craniofacial dysmorphism and hepatomegaly. * **Clinical Triad of ALD:** Behavioral changes/cognitive decline, vision/hearing loss, and adrenal failure. * **Biochemical Marker:** Elevated **C24:0 and C26:0** fatty acids in plasma.

Question 294: In which condition is vanillylmandelic acid excreted in urine?

A. Gaucher's disease
B. Phenylketonuria
C. Pheochromocytoma (Correct Answer)
D. Hartnup's disease

Explanation: **Explanation:** **Pheochromocytoma** is a catecholamine-secreting tumor, typically arising from the chromaffin cells of the adrenal medulla. These tumors produce excessive amounts of epinephrine and norepinephrine. In the body, these catecholamines are metabolized by enzymes **Monoamine Oxidase (MAO)** and **Catechol-O-methyltransferase (COMT)**. The final end-product of this metabolic pathway is **Vanillylmandelic Acid (VMA)**. Consequently, elevated 24-hour urinary VMA levels serve as a classic diagnostic marker for Pheochromocytoma. **Analysis of Incorrect Options:** * **Gaucher’s disease:** A lysosomal storage disorder caused by a deficiency of **glucocerebrosidase**, leading to the accumulation of glucocerebroside. It is characterized by hepatosplenomegaly and "wrinkled tissue paper" appearance of macrophages. * **Phenylketonuria (PKU):** Caused by a deficiency of **phenylalanine hydroxylase**. This leads to the excretion of phenylpyruvate, phenyllactate, and phenylacetate (giving a "mousy" odor to urine), not VMA. * **Hartnup’s disease:** A defect in the transport of **neutral amino acids** (like Tryptophan) in the gut and kidneys. It results in pellagra-like symptoms due to niacin deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of 10s:** Pheochromocytoma is 10% bilateral, 10% extra-adrenal, 10% malignant, and 10% familial. * **Metanephrines:** While VMA is a common marker, **urinary and plasma metanephrines** are now considered more sensitive screening tests. * **Associated Syndromes:** Always screen for MEN 2A and 2B, von Hippel-Lindau (VHL) disease, and Neurofibromatosis type 1 in these patients.

Question 295: 5-hydroxyindole acetic acid (5-HIAA) in urine is seen in which of the following conditions?

A. Carcinoid syndrome (Correct Answer)
B. Pheochromocytoma
C. Hirschsprung's disease
D. Wilm's tumor

Explanation: **Explanation:** **1. Why Carcinoid Syndrome is Correct:** Carcinoid syndrome arises from neuroendocrine tumors (usually in the ileum) that secrete excessive amounts of **Serotonin (5-hydroxytryptamine)**. In the body, Serotonin is metabolized by the enzyme **Monoamine Oxidase (MAO)** into its primary end-product, **5-hydroxyindole acetic acid (5-HIAA)**, which is then excreted in the urine. Therefore, a 24-hour urinary 5-HIAA test is the gold-standard biochemical marker for diagnosing and monitoring carcinoid tumors. **2. Why the Other Options are Incorrect:** * **Pheochromocytoma:** This tumor of the adrenal medulla secretes catecholamines (Epinephrine/Norepinephrine). The diagnostic urinary markers are **Vanillylmandellic acid (VMA)** and **Metanephrines**, not 5-HIAA. * **Hirschsprung's disease:** This is a congenital condition characterized by the absence of ganglion cells in the distal colon. It is a structural/motility disorder, not a secretory biochemical disorder. * **Wilm’s Tumor (Nephroblastoma):** This is a common pediatric renal tumor. While it may present with hypertension due to renin production, it is not associated with serotonin metabolism. **3. NEET-PG High-Yield Pearls:** * **Precursor:** Serotonin is synthesized from the amino acid **Tryptophan**. In carcinoid syndrome, up to 60% of dietary tryptophan is diverted to serotonin production, which can lead to **Pellagra** (Niacin deficiency) because tryptophan is also a precursor for Niacin (Vitamin B3). * **Clinical Triad:** Flushing, Diarrhea, and Right-sided heart failure (Tricuspid regurgitation/Pulmonary stenosis). * **Dietary Caution:** Patients must avoid serotonin-rich foods (bananas, walnuts, pineapples, avocados) before the 5-HIAA test to prevent false positives.

Question 296: Which of the following is NOT a feature of Hermansky-Pudlak syndrome?

A. Autosomal dominant inheritance (Correct Answer)
B. Oculocutaneous albinism
C. Bleeding disorder
D. Pulmonary fibrosis

Explanation: **Explanation:** Hermansky-Pudlak Syndrome (HPS) is a rare multi-system genetic disorder characterized by defects in **lysosome-related organelles (LROs)**, such as melanosomes and platelet dense granules. **1. Why Option A is correct:** Hermansky-Pudlak syndrome is inherited in an **Autosomal Recessive** pattern, not autosomal dominant. It is caused by mutations in several genes (HPS1 through HPS11) that encode proteins involved in the biogenesis of protein complexes required for organelle trafficking. **2. Why other options are incorrect (Features of HPS):** * **Oculocutaneous Albinism (Option B):** Due to defective melanosome formation and distribution, patients present with hypopigmentation of the skin, hair, and eyes, along with visual acuity issues and nystagmus. * **Bleeding Disorder (Option C):** This is a hallmark of HPS caused by a **storage pool deficiency of platelets**. Specifically, there is an absence or marked reduction of **dense granules** (delta granules), leading to impaired platelet aggregation and prolonged bleeding time. * **Pulmonary Fibrosis (Option D):** This is the most serious complication (especially in HPS-1 and HPS-4 subtypes). It results from the accumulation of a ceroid-lipofuscin-like material in alveolar macrophages, leading to restrictive lung disease, typically manifesting in the third or fourth decade of life. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of HPS:** Albinism + Bleeding diathesis + Ceroid lipofuscin lysosomal storage. * **Diagnostic Test:** Electron microscopy of platelets showing **absence of dense bodies**. * **Differential Diagnosis:** **Chediak-Higashi Syndrome** (also features albinism but is distinguished by giant lysosomal granules in neutrophils and frequent pyogenic infections).

Question 297: Zellweger syndrome is characterized by a defect in which of the following organelles?

A. Endoplasmic reticulum
B. Lysosome
C. Mitochondria
D. Peroxisome (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Peroxisome** Zellweger syndrome (also known as cerebrohepatorenal syndrome) is an autosomal recessive disorder caused by mutations in **PEX genes**. These genes encode **peroxins**, which are proteins essential for the assembly and biogenesis of peroxisomes [1]. In this condition, peroxisomes are either absent or non-functional, leading to a failure in the **alpha-oxidation** and **beta-oxidation of Very Long Chain Fatty Acids (VLCFAs)** [2]. This results in the toxic accumulation of VLCFAs and phytanic acid in tissues, particularly the brain and liver [2]. **Why Incorrect Options are Wrong:** * **A. Endoplasmic Reticulum:** The ER is involved in protein folding and lipid synthesis. Defects here lead to conditions like cystic fibrosis (misfolding) or osteogenesis imperfecta, but not Zellweger syndrome. * **B. Lysosome:** Defects in lysosomal enzymes lead to **Lysosomal Storage Diseases** (e.g., Gaucher, Tay-Sachs, or I-cell disease). While both involve organelle dysfunction, lysosomes primarily degrade complex carbohydrates and sphingolipids. * **C. Mitochondria:** Mitochondrial disorders (e.g., MELAS, MERFF) typically involve defects in the electron transport chain or oxidative phosphorylation, leading to lactic acidosis and muscle weakness [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Craniofacial dysmorphism (high forehead, wide fontanelles), hepatomegaly, and severe neurological dysfunction (hypotonia, seizures). * **Biochemical Marker:** Elevated levels of **VLCFAs** in the blood is the diagnostic hallmark [2]. * **Prognosis:** It is the most severe of the Peroxisome Biogenesis Disorders (PBD), usually fatal within the first year of life [2]. * **Related Condition:** **Adrenoleukodystrophy (X-linked)** is also a peroxisomal disorder, but it specifically affects the transport of VLCFAs into the peroxisome (ABCD1 mutation), rather than organelle biogenesis.

Question 298: Conjugated hyperbilirubinemia is seen in which of the following conditions?

A. Gilbert syndrome
B. Crigler-Najjar syndrome
C. Breast milk jaundice
D. Dubin-Johnson syndrome (Correct Answer)

Explanation: Hyperbilirubinemia is classified based on whether the elevation is in the **unconjugated (indirect)** or **conjugated (direct)** fraction of bilirubin. ### **Explanation of the Correct Option** **D. Dubin-Johnson syndrome:** This is an autosomal recessive disorder caused by a mutation in the **MRP2 (ABCC2) gene**, which encodes a canalicular multispecific organic anion transporter. While the liver can conjugate bilirubin normally, it cannot transport the conjugated bilirubin into the bile canaliculi. This leads to a reflux of conjugated bilirubin into the blood. A classic clinical feature is a **grossly black liver** due to the accumulation of epinephrine metabolites in lysosomes. ### **Explanation of Incorrect Options** * **A. Gilbert syndrome:** Caused by a mild reduction in the activity of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**. It results in mild, fluctuating **unconjugated** hyperbilirubinemia, often triggered by stress or fasting. * **B. Crigler-Najjar syndrome:** Caused by a severe deficiency (Type II) or total absence (Type I) of **UGT1A1**. This leads to severe **unconjugated** hyperbilirubinemia, with Type I often resulting in kernicterus. * **C. Breast milk jaundice:** Occurs due to substances in breast milk (like beta-glucuronidase or pregnane-3-alpha, 20-beta diol) that inhibit UGT1A1 or increase enterohepatic circulation, leading to **unconjugated** hyperbilirubinemia. ### **NEET-PG High-Yield Pearls** * **Conjugated Hyperbilirubinemia:** Think **Dubin-Johnson** (Black liver, normal gallbladder visualization) and **Rotor Syndrome** (No black liver, gallbladder not visualized on oral cholecystography). * **Urinary Coproporphyrin:** In Dubin-Johnson, total urinary coproporphyrin is normal, but **>80% is Coproporphyrin I** (vs. the normal Coproporphyrin III). * **Mnemonic:** "Dubin-Johnson has a **D**efect in **D**irect bilirubin secretion and a **D**ark liver."

Question 299: All the following are inherited disorders of connective tissue EXCEPT?

A. Alport syndrome
B. Ehlers-Danlos syndrome
C. Marfan syndrome
D. McArdle's disease (Correct Answer)

Explanation: **Explanation:** The correct answer is **McArdle's disease** because it is a **Glycogen Storage Disease (Type V)**, not a primary disorder of connective tissue. It is caused by a deficiency of the enzyme **myophosphorylase**, leading to an inability to break down glycogen in muscle cells. Clinically, it presents with exercise intolerance, muscle cramps, and myoglobinuria, but it does not involve defects in collagen or elastin synthesis. **Analysis of Incorrect Options:** * **Alport Syndrome:** An inherited disorder caused by mutations in genes (COL4A3, COL4A4, COL4A5) encoding **Type IV collagen**. It primarily affects the basement membranes of the kidney (glomerulonephritis), ears (sensorineural deafness), and eyes. * **Ehlers-Danlos Syndrome (EDS):** A heterogeneous group of disorders characterized by defects in the synthesis or structure of **fibrillar collagen**. Common features include skin hyperextensibility, joint hypermobility, and tissue fragility. * **Marfan Syndrome:** An autosomal dominant disorder caused by a mutation in the **FBN1 gene**, which encodes **Fibrillin-1** (a glycoprotein essential for the formation of elastic fibers). It affects the skeletal, ocular, and cardiovascular systems (e.g., aortic root aneurysm). **High-Yield Clinical Pearls for NEET-PG:** * **McArdle’s Sign:** "Second wind phenomenon" (improvement in exercise tolerance after a few minutes of activity). * **Ischemic Forearm Test:** In McArdle’s, there is a failure of blood lactate levels to rise after strenuous exercise. * **Collagen Types:** Remember **Type I** (Bone/Tendon), **Type II** (Cartilage), **Type III** (Reticulin/Blood vessels), and **Type IV** (Basement membrane - "Under the floor").

Question 300: An infant presents with a history of seizures and skin rashes. Investigations reveal metabolic acidosis and increased blood ketone levels. Which of the following conditions is this child most likely suffering from?

A. Propionic acidemia
B. Urea cycle disorder
C. Phenylketonuria
D. Multiple carboxylase deficiency (Correct Answer)

Explanation: **Explanation:** **Multiple Carboxylase Deficiency (MCD)** is the correct diagnosis because it uniquely accounts for the triad of **seizures, skin rashes (alopecia/dermatitis), and metabolic acidosis with ketosis.** MCD occurs due to a defect in either **Holocarboxylase synthetase** (neonatal onset) or **Biotinidase** (late-onset). Biotin is a mandatory cofactor for four essential enzymes: 1. **Pyruvate carboxylase:** Deficiency leads to lactic acidosis. 2. **Propionyl-CoA carboxylase:** Deficiency leads to propionic acidemia and ketosis. 3. **Acetyl-CoA carboxylase:** Impairs fatty acid synthesis. 4. **3-Methylcrotonyl-CoA carboxylase:** Leads to characteristic organic aciduria. The skin rash and neurological symptoms are hallmark features of biotin-related disorders. **Why other options are incorrect:** * **Propionic Acidemia:** While it causes metabolic acidosis and ketosis, it typically lacks the characteristic **skin rashes** seen in MCD. * **Urea Cycle Disorders:** These typically present with **hyperammonemia and respiratory alkalosis**, not metabolic acidosis or ketosis. * **Phenylketonuria (PKU):** Presents with intellectual disability and a "mousy odor," but does not cause acute metabolic acidosis or ketosis. **High-Yield Clinical Pearls for NEET-PG:** * **Biotinidase deficiency** is often called the "late-onset" form of MCD and is treatable with oral biotin supplementation. * **The "Skin + Acidosis" Clue:** In any biochemistry question, the combination of organic acidemia (acidosis/ketosis) plus dermatological findings (rash/alopecia) should immediately point toward **Biotin/Multiple Carboxylase Deficiency.** * **Key Enzyme:** Biotinidase is required to release biotin from biocytin (derived from dietary proteins).

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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