Genetic Disorders and Biochemical Pathology — MCQs

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

Question 571: Which enzyme deficiency is seen in genetic diseases like Tay-Sachs disease?

A. Tay-Sachs disease (Correct Answer)
B. Sickle cell anemia
C. Cystic Fibrosis
D. Wilson's disease

Explanation: **Explanation:** **Tay-Sachs Disease** is a 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 brain and spinal cord. It is inherited in an autosomal recessive pattern and is characterized clinically by progressive neurodegeneration, developmental delay, and a classic **"cherry-red spot"** on the macula. **Analysis of Incorrect Options:** * **Sickle Cell Anemia:** This is a qualitative hemoglobinopathy caused by a point mutation (missense) in the β-globin gene, where glutamic acid is replaced by valine at the 6th position. It is not an enzyme deficiency. * **Cystic Fibrosis:** This is caused by a mutation in the **CFTR gene**, which codes for a chloride channel protein. It leads to thick, viscous secretions in the lungs and pancreas, rather than a primary enzyme deficiency. * **Wilson’s Disease:** This is a disorder of copper metabolism caused by mutations in the **ATP7B gene**. It leads to impaired biliary copper excretion and failure to incorporate copper into ceruloplasmin, resulting in copper deposition in the liver and basal ganglia (Kayser-Fleischer rings). **High-Yield Clinical Pearls for NEET-PG:** * **Tay-Sachs vs. Niemann-Pick:** Both present with a cherry-red spot, but **hepatosplenomegaly** is absent in Tay-Sachs and present in Niemann-Pick (Sphingomyelinase deficiency). * **Histology:** Look for "onion-skin" lysosomes on electron microscopy in Tay-Sachs. * **Carrier Screening:** Highly prevalent in the Ashkenazi Jewish population.

Question 572: A 48-year-old lady presented with hepatosplenomegaly and pancytopenia. On microscopic examination of bone marrow cells, a crumpled tissue paper appearance is seen. Which product is likely to have accumulated?

A. Glucocerebroside (Correct Answer)
B. Sphingomyelin
C. Sulfatide
D. Ganglioside

Explanation: **Explanation:** The clinical presentation of **hepatosplenomegaly**, **pancytopenia** (due to hypersplenism and marrow infiltration), and the pathognomonic **"crumpled tissue paper"** appearance of macrophages (Gaucher cells) is diagnostic of **Gaucher Disease**. **1. Why Glucocerebroside is correct:** Gaucher disease is the most common lysosomal storage disorder, caused by a deficiency of the enzyme **Glucocerebrosidase** (β-glucosidase). This leads to the accumulation of **Glucocerebroside** within the lysosomes of macrophages. These overloaded macrophages develop a fibrillar, striated cytoplasm resembling crumpled silk or tissue paper, primarily infiltrating the bone marrow, liver, and spleen. **2. Why other options are incorrect:** * **Sphingomyelin:** Accumulates in **Niemann-Pick Disease** (deficiency of Sphingomyelinase). Microscopically, it presents as "foam cells" (vacuolated macrophages) rather than crumpled tissue paper. * **Sulfatide:** Accumulates in **Metachromatic Leukodystrophy** (deficiency of Arylsulfatase A). It primarily affects the central and peripheral nervous system, leading to demyelination. * **Ganglioside:** Accumulates in **Tay-Sachs Disease** (GM2 ganglioside) or **GM1 Gangliosidosis**. Tay-Sachs is characterized by a cherry-red spot on the macula and "onion-skin" lysosomes, but lacks hepatosplenomegaly. **Clinical Pearls for NEET-PG:** * **Gaucher Disease** is autosomal recessive. * **Type I (Non-neuronopathic)** is the most common form; it spares the CNS and presents in adulthood (as seen in this 48-year-old patient). * **Radiology Sign:** "Erlenmeyer flask deformity" of the distal femur. * **Biochemical Marker:** Elevated levels of Serum Acid Phosphatase (TRAP) and Angiotensin-Converting Enzyme (ACE) are often seen.

Question 573: Which of the following amino acids is excreted in the urine in maple syrup urine disease?

A. Tryptophan
B. Phenylalanine
C. Leucine (Correct Answer)
D. Arginine

Explanation: **Explanation:** **Maple Syrup Urine Disease (MSUD)** is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This enzyme is responsible for the oxidative decarboxylation of the three branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine.** When this enzyme is defective, these amino acids and their corresponding alpha-keto acids accumulate in the blood and are excreted in the urine. **Leucine** is the most toxic of the three and its metabolite, **alpha-ketoisocaproic acid**, is primarily responsible for the neurological damage and the characteristic "maple syrup" or burnt sugar odor of the urine. **Analysis of Incorrect Options:** * **A. Tryptophan:** An aromatic amino acid involved in Hartnup disease (defective transport), not MSUD. * **B. Phenylalanine:** An aromatic amino acid that accumulates in Phenylketonuria (PKU) due to phenylalanine hydroxylase deficiency. * **D. Arginine:** A basic amino acid involved in the urea cycle; its elevation is seen in Argininemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember the BCAAs as **"LIV"** (Leucine, Isoleucine, Valine). * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenate (B5), and **L**ipoic acid (**T**ender **R**omance **N**ever **P**erpetually **L**asts). * **Diagnosis:** Elevated levels of **Alloisoleucine** in the plasma is pathognomonic for MSUD. * **Treatment:** Dietary restriction of BCAAs and, in some cases, high doses of Thiamine (B1-responsive variants).

Question 574: Which one of the following inherited conditions causes direct hyperbilirubinemia?

A. Gilbert syndrome
B. Type II Crigler-Najjar syndrome
C. Rotor syndrome (Correct Answer)
D. Type I Crigler-Najjar syndrome

Explanation: ### Explanation Hyperbilirubinemia is classified based on whether the elevation occurs in **unconjugated (indirect)** or **conjugated (direct)** bilirubin. **Why Rotor Syndrome is Correct:** Rotor syndrome is an autosomal recessive condition characterized by **conjugated (direct) hyperbilirubinemia**. It results from a deficiency in the hepatic uptake and storage of bilirubin, specifically due to mutations in the **OATP1B1 and OATP1B3** transporters. Unlike Dubin-Johnson syndrome (the other common direct hyperbilirubinemia), Rotor syndrome does not feature a black liver on biopsy and has a normal gallbladder visualization on oral cholecystography. **Analysis of Incorrect Options:** * **Gilbert Syndrome (Option A):** A common, benign condition caused by reduced activity of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**. It results in mild **unconjugated** hyperbilirubinemia, often triggered by fasting or stress. * **Crigler-Najjar Syndrome Type I & II (Options B & D):** Both involve a deficiency of UGT1A1. Type I is a total absence (severe, fatal without transplant), while Type II (Arias syndrome) is a partial deficiency. Both cause significant **unconjugated** hyperbilirubinemia. **NEET-PG High-Yield Pearls:** * **Direct Hyperbilirubinemia:** Think **D**ubin-Johnson and **R**otor (Mnemonic: **DR**). * **Dubin-Johnson vs. Rotor:** Dubin-Johnson features a **black liver** (epinephrine metabolite deposition) and high urinary coproporphyrin I isomers. Rotor syndrome has a **normal-colored liver**. * **Unconjugated Hyperbilirubinemia:** Think Gilbert and Crigler-Najjar. * **Phenobarbital Test:** Used to differentiate Crigler-Najjar Type II (bilirubin drops) from Type I (no response).

Question 575: All are in cystinuria except?

A. Cystine
B. Ornithine
C. Alanine (Correct Answer)
D. Arginine

Explanation: **Explanation:** **Cystinuria** is an autosomal recessive disorder characterized by a defect in the high-affinity, sodium-independent transport system responsible for reabsorbing specific amino acids in the proximal convoluted tubule of the kidney and the small intestine. **1. Why Alanine is the Correct Answer:** The transport defect in cystinuria is specific to **COLA** amino acids (Cystine, Ornithine, Lysine, and Arginine). **Alanine** is a neutral amino acid that utilizes a different transport system (System N or System A). Therefore, alanine reabsorption remains unaffected, and it is not excreted in excess in the urine of these patients. **2. Why the Other Options are Incorrect:** * **Cystine (A):** This is the primary amino acid involved. Its low solubility in acidic urine leads to the formation of characteristic hexagonal crystals and renal stones. * **Ornithine (B) & Arginine (D):** Along with **Lysine**, these are dibasic amino acids that share the same defective transporter as cystine. Consequently, all three are found in high concentrations in the urine of affected individuals. **NEET-PG High-Yield Pearls:** * **Mnemonic:** Remember **COLA** (Cystine, Ornithine, Lysine, Arginine). * **Clinical Presentation:** Patients typically present with recurrent renal stones (urolithiasis). * **Diagnosis:** The **Cyanide-Nitroprusside test** is the screening test of choice (turns purple/magenta). * **Microscopy:** Pathognomonic **hexagonal (6-sided)** crystals in urine sediment. * **Treatment:** High fluid intake, urinary alkalinization (to increase cystine solubility), and chelating agents like Penicillamine if conservative measures fail.

Question 576: Maple syrup urine disease is due to a defect in which of the following enzymes?

A. a-ketoacid decarboxylase (Correct Answer)
B. Transaminase
C. Isomerase
D. Mutase

Explanation: **Explanation:** **Maple Syrup Urine Disease (MSUD)** is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain α-Keto Acid Dehydrogenase (BCKDH) complex**. This multi-enzyme complex is responsible for the oxidative decarboxylation of α-keto acids derived from the branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. 1. **Why Option A is Correct:** The BCKDH complex functions as an **α-ketoacid decarboxylase**. When this enzyme is defective, the α-keto acid analogs of BCAAs (α-ketoisocaproate, α-keto-β-methylvalerate, and α-ketoisovalerate) accumulate in the blood and spill into the urine, giving it a characteristic burnt sugar or maple syrup odor. 2. **Why Other Options are Incorrect:** * **Transaminase:** This enzyme performs the first step of BCAA metabolism (converting amino acids to α-keto acids). In MSUD, transamination is functional; the block occurs at the subsequent decarboxylation step. * **Isomerase/Mutase:** These enzymes are involved in later stages of metabolism (e.g., Methylmalonyl-CoA mutase in Vitamin B12 metabolism). Defects here lead to organic acidemias (like Methylmalonic acidemia), not MSUD. **Clinical Pearls for NEET-PG:** * **Mnemonic:** "I Love Vermont maple syrup" (**I**soleucine, **L**eucine, **V**aline). * **Key Metabolite:** **Alloisoleucine** is pathognomonic for MSUD. * **Clinical Presentation:** Poor feeding, vomiting, seizures, and "sweet-smelling" urine within the first week of life. * **Cofactors:** The BCKDH complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**T**ender **R**omance **N**ever **P**erpetually **L**asts). * **Treatment:** Dietary restriction of BCAAs and high-dose **Thiamine** supplementation (in thiamine-responsive variants).

Question 577: A patient with raised serum alkaline phosphatase and raised parathormone level along with low calcium and low phosphate level is likely to have which of the following conditions?

A. Primary hyperparathyroidism
B. Paget's disease
C. Osteoporosis
D. Vitamin D deficiency (Correct Answer)

Explanation: ### Explanation The biochemical profile described—**low calcium, low phosphate, elevated Parathyroid Hormone (PTH), and elevated Alkaline Phosphatase (ALP)**—is the classic presentation of **Vitamin D deficiency** (Osteomalacia in adults or Rickets in children). #### 1. Why Vitamin D Deficiency is Correct * **Low Calcium & Phosphate:** Vitamin D is essential for the intestinal absorption of both calcium and phosphorus. Its deficiency leads to a decrease in both. * **Secondary Hyperparathyroidism:** In response to low serum calcium, the parathyroid glands secrete more PTH to mobilize calcium from bones and increase renal calcium reabsorption. This explains the **raised PTH**. * **Raised ALP:** Increased osteoblastic activity (an attempt to remodel bone despite poor mineralization) leads to an elevation in serum Alkaline Phosphatase. #### 2. Why Other Options are Incorrect * **Primary Hyperparathyroidism:** Characterized by **high calcium** and low phosphate. The pathology lies in the gland itself (e.g., adenoma), not as a reaction to low calcium. * **Paget’s Disease:** Typically presents with **isolated markedly elevated ALP**. Serum calcium, phosphate, and PTH levels are usually **normal**. * **Osteoporosis:** This is a quantitative decrease in bone mass, but the quality of mineralization is normal. Consequently, serum calcium, phosphate, PTH, and ALP are all typically **normal**. #### 3. NEET-PG High-Yield Pearls * **PTH Effect on Kidney:** PTH increases calcium reabsorption but **decreases** phosphate reabsorption (phosphaturic effect) in the proximal tubule. This further contributes to the low phosphate seen in Vitamin D deficiency. * **ALP as a Marker:** ALP is a marker of **osteoblastic activity**. It is elevated whenever there is high bone turnover or compensatory bone formation (Rickets, Paget’s, Bone metastasis). * **Differential Diagnosis:** If a question mentions low calcium but **high phosphate** with high PTH, think of **Chronic Kidney Disease (CKD)** or **Pseudohypoparathyroidism**.

Question 578: The syndrome of apparent mineralocorticoid excess is due to deficiency of which enzyme?

A. 11α-hydroxysteroid dehydrogenase
B. 11β-hydroxysteroid dehydrogenase (Correct Answer)
C. 12α-hydroxysteroid dehydrogenase
D. 12β-hydroxysteroid dehydrogenase

Explanation: **Explanation:** **Syndrome of Apparent Mineralocorticoid Excess (SAME)** is an autosomal recessive disorder caused by a deficiency of the enzyme **11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2)**. **1. Why the Correct Answer is Right:** In normal physiology, the mineralocorticoid receptor (MR) in the kidneys has an equal affinity for both aldosterone and cortisol. Since circulating cortisol levels are much higher than aldosterone, the enzyme **11β-HSD2** protects the receptor by converting active **cortisol** into inactive **cortisone**. When this enzyme is deficient, cortisol remains active, binds to the MR, and mimics the action of aldosterone. This leads to the clinical triad of **hypertension, hypokalemia, and metabolic alkalosis**, despite low levels of plasma renin and aldosterone. **2. Why Incorrect Options are Wrong:** * **11α-hydroxysteroid dehydrogenase:** This is a stereoisomer that does not play a role in human steroid metabolism related to mineralocorticoid escape. * **12α and 12β-hydroxysteroid dehydrogenase:** These enzymes are primarily involved in bile acid metabolism (e.g., converting deoxycholic acid), not in the regulation of cortisol or mineralocorticoid receptors. **3. High-Yield Clinical Pearls for NEET-PG:** * **Licorice Connection:** Consumption of Glycyrrhetinic acid (found in natural licorice) inhibits 11β-HSD2, causing an acquired form of SAME. * **Diagnosis:** Look for a high ratio of urinary cortisol to cortisone metabolites. * **Treatment:** Spironolactone (MR antagonist) or potassium-sparing diuretics like Amiloride. * **Key Distinction:** Unlike Conn’s syndrome (Primary Hyperaldosteronism), SAME presents with **low/suppressed aldosterone levels**.

Question 579: A 6-month-old baby presents with recurrent seizures, developmental delay, alopecia, and scaly skin rashes. Investigations reveal metabolic acidosis, elevated lactates, and ketonuria. What is the most likely underlying enzyme deficiency?

A. Phenylalanine hydroxylase
B. Epimerase
C. Multiple carboxylase (Correct Answer)
D. Glucose-6-phosphatase

Explanation: ### Explanation The clinical presentation of seizures, developmental delay, and metabolic acidosis, combined with the characteristic dermatological triad of **alopecia and scaly skin rashes**, is highly suggestive of **Multiple Carboxylase Deficiency (MCD)**. **1. Why "Multiple Carboxylase" is correct:** MCD occurs due to a deficiency in either **Holocarboxylase synthetase** (neonatal onset) or **Biotinidase** (late-onset). Biotin is a mandatory cofactor for four essential carboxylase enzymes: * **Pyruvate carboxylase:** Conversion of pyruvate to oxaloacetate (deficiency leads to **lactic acidosis**). * **Acetyl-CoA carboxylase:** Fatty acid synthesis (deficiency leads to **skin rashes/alopecia**). * **Propionyl-CoA carboxylase** and **3-methylcrotonyl-CoA carboxylase:** Amino acid catabolism (deficiency leads to **organic aciduria and ketonuria**). The inability to activate these enzymes simultaneously results in the multisystemic metabolic crisis seen in this infant. **2. Why the other options are incorrect:** * **Phenylalanine hydroxylase:** Deficiency causes Phenylketonuria (PKU). While it presents with developmental delay and seizures, it is characterized by a "mousy odor" and hypopigmentation, not metabolic acidosis or alopecia. * **Epimerase:** Galactose-4-epimerase deficiency is a rare form of Galactosemia. It typically presents with cataracts, liver failure, and jaundice. * **Glucose-6-phosphatase:** Deficiency causes Von Gierke Disease (GSD Type I). It presents with severe fasting hypoglycemia, hepatomegaly, and doll-like facies, but not alopecia or scaly rashes. **Clinical Pearls for NEET-PG:** * **Biotinidase deficiency** is often called the "late-onset" form and is easily treatable with oral biotin supplementation. * **High-Yield Triad:** Alopecia + Periorificial dermatitis + Metabolic acidosis = Biotin-related disorder. * **Diagnostic marker:** Elevated 3-hydroxyisovaleric acid in urine.

Question 580: Hypoceruloplasminemia is associated with which abnormality?

A. Menkes disease (Correct Answer)
B. Alzheimer's disease
C. Schizophrenia
D. Obsessive-compulsive disorder

Explanation: **Explanation:** **Hypoceruloplasminemia** refers to low levels of ceruloplasmin, the primary copper-carrying protein in the blood. **1. Why Menkes Disease is Correct:** Menkes disease (kinky hair syndrome) is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This gene encodes a copper-transporting ATPase responsible for absorbing copper from the GI tract and transporting it across the blood-brain barrier. In Menkes disease, copper is trapped within intestinal mucosal cells and cannot reach the liver. Since the liver requires copper to synthesize **holoceruloplasmin** (the functional form of ceruloplasmin), the lack of available copper leads to the production of unstable apoceruloplasmin, which is rapidly degraded. This results in the characteristic **hypoceruloplasminemia** and low serum copper levels. **2. Why the Other Options are Incorrect:** * **Alzheimer’s Disease:** While copper dysregulation is studied in neurodegeneration, there is no consistent association with hypoceruloplasminemia. * **Schizophrenia & OCD:** These are psychiatric disorders primarily linked to neurotransmitter imbalances (dopamine, serotonin, glutamate) rather than systemic copper transport defects. **3. High-Yield Clinical Pearls for NEET-PG:** * **Menkes vs. Wilson:** Both show low ceruloplasmin. However, **Menkes (ATP7A)** involves systemic copper **deficiency**, while **Wilson (ATP7B)** involves copper **overload** (toxic accumulation in liver/brain). * **Clinical Triad of Menkes:** Steely/kinky hair (pili torti), growth retardation, and progressive neurological deterioration. * **Enzyme Deficits:** Low copper in Menkes leads to secondary deficiency of copper-dependent enzymes like **Lysyl oxidase** (causing connective tissue defects) and **Tyrosinase** (causing hypopigmentation).

Practice by Chapter

Single Gene Disorders

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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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Enzyme Replacement Therapy

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