Genetic Disorders and Biochemical Pathology Practice Questions

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

Tay-Sachs disease. **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.

Q: 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?

Glucocerebroside. **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.

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

Rotor syndrome. ### 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).

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

11β-hydroxysteroid dehydrogenase. **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**.

Q: 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?

Multiple carboxylase. ### 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.

Q: Analyze the following pedigree and determine the mode of inheritance.

Mitochondrial inheritance. ***Mitochondrial inheritance*** - Shows **maternal-only transmission** where all children of affected mothers inherit the condition, regardless of sex. - **No paternal transmission** occurs because mitochondria are inherited exclusively from the mother through the egg cytoplasm. *Autosomal recessive* - Requires **both parents to be carriers** and typically shows a **horizontal pattern** affecting siblings in the same generation. - Affected individuals usually have **unaffected parents**, and consanguinity increases risk. *Autosomal dominant* - Shows **vertical transmission** through generations with **affected individuals having at least one affected parent**. - Demonstrates **50% risk** to offspring and affects both males and females equally with paternal transmission possible. *X-linked dominant* - **Affected males** cannot pass the condition to their sons, only to daughters who would all be affected. - Shows **criss-cross inheritance pattern** and typically has more affected females than males in the pedigree.

Q: Hypoceruloplasminemia is associated with which abnormality?

Menkes disease. **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).

Q: Tomcat urine odor is characteristic of which of the following conditions?

Multiple carboxylase deficiency. **Explanation:** **Multiple Carboxylase Deficiency (MCD)** is the correct answer. This condition results from a deficiency in **Holocarboxylase synthetase** or **Biotinidase**, leading to the dysfunction of four biotin-dependent enzymes: Pyruvate carboxylase, Acetyl-CoA carboxylase, Propionyl-CoA carboxylase, and 3-Methylcrotonyl-CoA carboxylase. The "tomcat urine" odor is specifically attributed to the accumulation of **3-hydroxyisovaleric acid** and **3-methylcrotonylglycine**, which are metabolites of leucine that cannot be properly processed due to the deficiency of 3-Methylcrotonyl-CoA carboxylase. **Analysis of Incorrect Options:** * **Phenylketonuria (PKU):** Characterized by a **mousy or musty odor** due to the accumulation of phenylacetic acid. * **Hawkinuria:** A rare defect in tyrosine metabolism (4-hydroxyphenylpyruvate dioxygenase deficiency) characterized by a **swimming pool or chlorine-like odor**. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex, leading to a **burnt sugar or maple syrup odor**. **High-Yield Clinical Pearls for NEET-PG:** * **Isovaleric Acidemia:** Characterized by a **"sweaty feet"** or "cheese-like" odor. * **Trimethylaminuria:** Known as **"Fish odor syndrome."** * **Tyrosinemia Type I:** Characterized by a **boiled cabbage or rancid butter** odor. * **Oasthouse Urine Disease:** Characterized by a **dried malt or hops** odor. * **MCD Clinical Presentation:** Often presents with metabolic acidosis, skin rash (alopecia/dermatitis), and neurological symptoms; it is highly responsive to **Biotin supplementation**.

Q: All of the following are autosomal dominant disorders except?

Duchenne muscular dystrophy. **Explanation:** The correct answer is **C. Duchenne muscular dystrophy (DMD)**. **1. Why Duchenne Muscular Dystrophy is the correct answer:** DMD is an **X-linked recessive (XLR)** disorder, not autosomal dominant. It is caused by a mutation in the *DMD* gene located on the X chromosome (Xp21), which encodes the protein **dystrophin**. Since it is X-linked, it primarily affects males, while females are typically asymptomatic carriers. **2. Analysis of Incorrect Options (Autosomal Dominant Disorders):** * **A. Von Hippel-Lindau (VHL) disease:** An autosomal dominant (AD) condition caused by a mutation in the *VHL* tumor suppressor gene on chromosome 3. It is characterized by hemangioblastomas, renal cell carcinoma, and pheochromocytoma. * **B. Achondroplasia:** The most common form of dwarfism, inherited in an AD pattern. It results from a gain-of-function mutation in the **FGFR3** gene. Note: 80% of cases arise from *de novo* mutations. * **D. Multiple Endocrine Neoplasia (MEN):** Both MEN1 and MEN2 (2A and 2B) are inherited in an AD fashion. They involve a predisposition to tumors in various endocrine glands (e.g., parathyroid, pancreas, thyroid). **3. High-Yield Clinical Pearls for NEET-PG:** * **DMD Hallmark:** Gower’s sign (using hands to "climb up" the legs to stand) and pseudohypertrophy of the calves (fatty replacement of muscle). * **Biochemical Marker:** Significantly elevated **Creatine Kinase (CK-MM)** levels are seen in DMD even before clinical symptoms appear. * **Rule of Thumb:** Most structural protein defects (e.g., Achondroplasia, Marfan) are **Autosomal Dominant**, while most enzyme deficiencies are **Autosomal Recessive**. DMD is a notable X-linked exception involving a structural protein (dystrophin).

Question 1

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

Accepted Answer

Tay-Sachs disease

Answer

Sickle cell anemia

Answer

Cystic Fibrosis

Answer

Wilson's disease

Question 2

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?

Accepted Answer

Glucocerebroside

Answer

Sphingomyelin

Answer

Sulfatide

Answer

Ganglioside

Question 3

Which one of the following inherited conditions causes direct hyperbilirubinemia?

Accepted Answer

Rotor syndrome

Answer

Gilbert syndrome

Answer

Type II Crigler-Najjar syndrome

Answer

Type I Crigler-Najjar syndrome

Question 4

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

Accepted Answer

11β-hydroxysteroid dehydrogenase

Answer

11α-hydroxysteroid dehydrogenase

Answer

12α-hydroxysteroid dehydrogenase

Answer

12β-hydroxysteroid dehydrogenase

Question 5

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?

Accepted Answer

Multiple carboxylase

Answer

Phenylalanine hydroxylase

Answer

Epimerase

Answer

Glucose-6-phosphatase

Question 6

Analyze the following pedigree and determine the mode of inheritance.

Accepted Answer

Mitochondrial inheritance

Answer

Autosomal recessive

Answer

Autosomal dominant

Answer

X-linked dominant

Question 7

Hypoceruloplasminemia is associated with which abnormality?

Accepted Answer

Menkes disease

Answer

Alzheimer's disease

Answer

Schizophrenia

Answer

Obsessive-compulsive disorder

Question 8

Tomcat urine odor is characteristic of which of the following conditions?

Accepted Answer

Multiple carboxylase deficiency

Answer

Phenylketonuria

Answer

Hawkinuria

Answer

Maple syrup urine disease

Question 9

All of the following are autosomal dominant disorders except?

Accepted Answer

Duchenne muscular dystrophy

Answer

Von Hippel-Lindau disease

Answer

Achondroplasia

Answer

Multiple endocrine neoplasia

Question 10

Which of the following statements is FALSE about Wolman disease?

Accepted Answer

It is characterized by adrenal calcification and corneal clouding.

Answer

It is a lysosomal storage disorder.

Answer

It is characterized by deficient acid lysosomal lipase.

Answer

It shows an autosomal recessive inheritance pattern.

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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