Genetic Disorders and Biochemical Pathology — MCQs

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

Question 231: 17 hydroxylase deficiency causes increased production of which of the following?

A. Aldosterone (Correct Answer)
B. Cortisol
C. Androstenedione
D. All of the above

Explanation: ### Explanation **1. Why Aldosterone is the Correct Answer:** The adrenal cortex utilizes the steroidogenesis pathway to produce mineralocorticoids, glucocorticoids, and sex steroids. The enzyme **17α-hydroxylase** is required to convert Pregnenolone and Progesterone into their 17-hydroxy derivatives (17-OH Pregnenolone and 17-OH Progesterone). In **17α-hydroxylase deficiency**, the pathway toward Cortisol and Sex Steroids is blocked. Consequently, all steroid precursors are shunted toward the **Mineralocorticoid pathway**. This leads to an overproduction of **11-deoxycorticosterone (DOC)** and **Corticosterone**. While actual Aldosterone levels may eventually be suppressed due to feedback inhibition of the Renin-Angiotensin system (caused by high DOC-induced hypertension), the primary biochemical shift is toward the mineralocorticoid line, making it the only viable product among the options. **2. Why the Other Options are Incorrect:** * **B. Cortisol:** 17α-hydroxylase is essential for the synthesis of 11-deoxycortisol, the precursor to Cortisol. Deficiency leads to **hypocortisolism**. * **C. Androstenedione:** The synthesis of adrenal androgens requires 17,20-lyase activity (part of the same enzyme complex). Without it, sex steroid production is abolished, leading to **delayed puberty** and **ambiguous genitalia** in males. * **D. All of the above:** Incorrect because Cortisol and Androstenedione levels are decreased. **3. High-Yield Clinical Pearls for NEET-PG:** * **Presentation:** Hypertension + Hypokalemia (due to excess mineralocorticoids) + Sexual Infantilism (due to lack of androgens). * **Genotype/Phenotype:** Females (46,XX) appear normal at birth but fail to reach puberty; Males (46,XY) often present with female external genitalia (pseudohermaphroditism). * **Mnemonic:** If the enzyme starts with **"1"** (11β or 17α), it causes **Hypertension**. If it ends with **"1"** (11β or 21), it causes **Virilization**. Therefore, 17α-hydroxylase deficiency causes Hypertension but *no* Virilization.

Question 232: Which of the following is an X-linked recessive disorder?

A. Phenylketonuria
B. Mucopolysaccharidosis (Correct Answer)
C. Achondroplasia
D. Marfan's syndrome

Explanation: **Explanation:** The correct answer is **Mucopolysaccharidosis (specifically Hunter Syndrome)**. While the question lists "Mucopolysaccharidosis" generally, it refers to the specific subtype that follows this inheritance pattern. 1. **Why the correct answer is right:** Most Mucopolysaccharidoses (MPS) are Autosomal Recessive. However, **MPS Type II (Hunter Syndrome)** is a classic **X-linked Recessive** disorder. It is caused by a deficiency of the enzyme *Iduronate-2-sulfatase*. Clinically, it presents similarly to Hurler syndrome (MPS I) but is distinguished by the **absence of corneal clouding** and a more aggressive clinical course. In NEET-PG, if "Mucopolysaccharidosis" is listed among autosomal disorders without a subtype, Hunter syndrome is the implied X-linked exception. 2. **Why the other options are incorrect:** * **Phenylketonuria (PKU):** This is a classic **Autosomal Recessive** disorder caused by a deficiency of *Phenylalanine hydroxylase*. * **Achondroplasia:** This is an **Autosomal Dominant** condition (the most common cause of dwarfism), often associated with advanced paternal age. * **Marfan’s Syndrome:** This is an **Autosomal Dominant** connective tissue disorder caused by mutations in the *FBN1* gene (Fibrillin-1). 3. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hunter Syndrome:** "The **Hunter** needs **X**-ray vision (X-linked) to see the **Target** (Clear Cornea/No clouding)." * **Fabry Disease** is the only other common Lysosomal Storage Disease that is X-linked Recessive; all others (Gaucher, Niemann-Pick, etc.) are Autosomal Recessive. * **Enzyme in Hunter:** Iduronate-2-sulfatase. * **Enzyme in Hurler (AR):** Alpha-L-iduronidase.

Question 233: Which substance accumulates in Tay-Sachs disease?

A. Cerebroside
B. Ganglioside (Correct Answer)
C. Sphingosine
D. Sphingomyelin

Explanation: **Explanation:** **Tay-Sachs Disease** is a lysosomal storage disorder caused by a deficiency of the enzyme **Hexosaminidase A**. This enzyme is responsible for the degradation of GM2 gangliosides. In its absence, **GM2 Ganglioside** (specifically Option B) accumulates within the lysosomes of neurons, leading to progressive neurodegeneration. **Analysis of Options:** * **Option A (Cerebroside):** Glucocerebroside accumulates in **Gaucher disease** (deficiency of Glucocerebrosidase), while Galactocerebroside accumulates in **Krabbe disease**. * **Option C (Sphingosine):** This is the basic organic backbone of all sphingolipids; it does not typically accumulate as a primary metabolite in storage diseases. * **Option D (Sphingomyelin):** This substance accumulates in **Niemann-Pick disease** due to a deficiency of the enzyme Sphingomyelinase. **High-Yield Clinical Pearls for NEET-PG:** * **Cherry-red spot on macula:** A classic finding in Tay-Sachs, but also seen in Niemann-Pick. * **Differentiating Factor:** Tay-Sachs presents **without** hepatosplenomegaly, whereas Niemann-Pick presents **with** hepatosplenomegaly. * **Genetics:** Autosomal Recessive; common in Ashkenazi Jewish populations. * **Histology:** Neurons show "onion-skin" lysosomes under electron microscopy. * **Mnemonic:** Tay-Sa**X** lacks He**X**osaminidase A. (A for **A**bsent hepatosplenomegaly).

Question 234: Regarding severe combined immunodeficiency disease (SCID), which of the following statements is true?

A. Adenosine deaminase deficiency (Correct Answer)
B. Myeloperoxidase deficiency
C. NADPH oxidase deficiency
D. C1 esterase deficiency

Explanation: **Explanation:** **Severe Combined Immunodeficiency (SCID)** is a group of rare disorders characterized by the absence of both humoral and cellular immunity. **1. Why Option A is Correct:** Approximately 15% of SCID cases are caused by a deficiency in **Adenosine Deaminase (ADA)**, an enzyme in the purine salvage pathway. ADA converts adenosine to inosine and deoxyadenosine to deoxyinosine. When ADA is deficient, **deoxyadenosine (dATP)** accumulates. High levels of dATP are toxic to lymphocytes because they inhibit **ribonucleotide reductase**, the enzyme required for DNA synthesis. This leads to a profound decrease in T-cells, B-cells, and NK cells. **2. Why Incorrect Options are Wrong:** * **B. Myeloperoxidase deficiency:** This is a defect in the respiratory burst within neutrophils. It typically presents with recurrent *Candida* infections but does not cause the global immune failure seen in SCID. * **C. NADPH oxidase deficiency:** This causes **Chronic Granulomatous Disease (CGD)**. It results in an inability of phagocytes to produce superoxide radicals, leading to recurrent infections with catalase-positive organisms (e.g., *S. aureus*, *Aspergillus*). * **D. C1 esterase deficiency:** This leads to **Hereditary Angioedema** due to overproduction of bradykinin. It presents with mucosal swelling and airway obstruction, not immunodeficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Cause of SCID:** X-linked SCID (defect in the IL-2 receptor gamma chain). * **ADA Deficiency:** The second most common cause and the first human disease treated with **Gene Therapy**. * **Radiological Sign:** Absence of a thymic shadow on chest X-ray. * **Clinical Presentation:** Failure to thrive, chronic diarrhea, and recurrent "opportunistic" infections (e.g., *Pneumocystis jirovecii*, *Candida*).

Question 235: Blood spot tandem mass spectrometry can be used in the diagnosis of all the following disorders EXCEPT?

A. Lesch Nyhan disease (Correct Answer)
B. Fatty acid oxidation defects
C. Mitochondrial disorders
D. Propionic Acidemia

Explanation: **Explanation:** **Tandem Mass Spectrometry (TMS)** is the gold standard for newborn screening because it can simultaneously detect a wide range of metabolites, specifically **acylcarnitines** and **amino acids**, from a single dried blood spot. 1. **Why Lesch-Nyhan Disease is the correct answer:** Lesch-Nyhan syndrome is an X-linked recessive disorder caused by a deficiency of the enzyme **HGPRT**, leading to the overproduction of uric acid. Diagnosis is primarily based on clinical presentation (self-mutilation, gout, choreoathetosis), hyperuricemia, and confirmed via **enzyme assay** or **molecular genetic testing**. It does not produce the specific acylcarnitine or amino acid profiles detectable by standard TMS screening protocols. 2. **Why the other options are incorrect:** * **Fatty acid oxidation defects (e.g., MCAD deficiency):** These are the hallmark disorders diagnosed by TMS. It detects specific **acylcarnitine** patterns that accumulate when fats cannot be broken down. * **Mitochondrial disorders:** While broad, many primary mitochondrial diseases (like respiratory chain defects) result in secondary elevations of specific metabolites (like C4-DC or C5-OH acylcarnitines) detectable via TMS. * **Propionic Acidemia:** This is an **Organic Acidemia**. TMS detects the elevation of **Propionylcarnitine (C3)**, making it a routine part of the TMS screening panel. **NEET-PG High-Yield Pearls:** * **TMS Utility:** "One test, many diseases." It is used for Aminoacidopathies (PKU, MSUD), Organic Acidemias (MMA, PA), and Fatty Acid Oxidation Disorders (MCAD, VLCAD). * **Lesch-Nyhan Key Feature:** Look for "Orange sand" in the diaper (urates) and self-mutilating behavior. * **Guthrie Test:** An older, bacterial inhibition assay used specifically for PKU; now largely replaced by TMS.

Question 236: Primary Hyperoxaluria occurs due to a defect in the metabolism of which amino acid?

A. Cysteine
B. Tryptophan
C. Tyrosine
D. Glycine (Correct Answer)

Explanation: **Explanation:** **Primary Hyperoxaluria Type 1 (PH1)** is an autosomal recessive disorder caused by a deficiency of the hepatic peroxisomal enzyme **Alanine-Glyoxylate Aminotransferase (AGT)**. 1. **Why Glycine is Correct:** Under normal physiological conditions, **Glycine** is metabolized into **Glyoxylate**. The enzyme AGT (which requires Vitamin B6 as a cofactor) converts glyoxylate back into glycine. When AGT is deficient, glyoxylate cannot be converted back to glycine; instead, it is oxidized by lactate dehydrogenase into **Oxalic acid (Oxalate)**. Excessive oxalate combines with calcium to form insoluble calcium oxalate crystals, leading to nephrocalcinosis and recurrent urolithiasis. 2. **Why Other Options are Incorrect:** * **Cysteine:** Defects in cysteine transport lead to *Cystinuria*, characterized by hexagonal crystals, but not hyperoxaluria. * **Tryptophan:** Metabolism of tryptophan is associated with Hartnup disease and the synthesis of Serotonin, Melatonin, and Niacin. * **Tyrosine:** Disorders of tyrosine metabolism include Alkaptonuria (HGA oxidase deficiency) and Tyrosinemia, which do not involve oxalate production. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Defect:** Alanine-Glyoxylate Aminotransferase (AGT). * **Cofactor:** Pyridoxine (Vitamin B6). High-dose B6 is often used in treatment to enhance residual enzyme activity. * **Clinical Presentation:** Recurrent calcium oxalate stones in a pediatric patient should raise suspicion of Primary Hyperoxaluria. * **Systemic Oxalosis:** In advanced stages, oxalate deposits in bones, joints, and the heart. * **Definitive Treatment:** Combined Liver-Kidney transplantation (since the metabolic defect is in the liver).

Question 237: Keratin sulfate in urine is found in which of the following conditions?

A. Hurler's syndrome
B. Hunter's syndrome
C. Morquio's syndrome (Correct Answer)
D. Sanfillipo's syndrome

Explanation: **Explanation:** The question pertains to **Mucopolysaccharidoses (MPS)**, a group of metabolic disorders caused by the deficiency of lysosomal enzymes needed to break down Glycosaminoglycans (GAGs). **1. Why Morquio’s Syndrome is Correct:** Morquio’s syndrome (MPS IV) is uniquely characterized by the accumulation and urinary excretion of **Keratan Sulfate**. It occurs in two forms: Type A (Galactose-6-sulfatase deficiency) and Type B (β-galactosidase deficiency). Unlike other MPS types, Morquio’s presents with severe skeletal dysplasia (spondyloepiphyseal dysplasia) and ligamentous laxity, but notably **spares intelligence**. **2. Analysis of Incorrect Options:** * **Hurler’s Syndrome (MPS IH):** Characterized by the deficiency of α-L-iduronidase. The GAGs found in urine are **Dermatan sulfate** and **Heparan sulfate**. It is the most severe form, presenting with corneal clouding and mental retardation. * **Hunter’s Syndrome (MPS II):** Caused by Iduronate sulfatase deficiency. Like Hurler’s, it involves **Dermatan and Heparan sulfate**, but it is **X-linked recessive** and lacks corneal clouding. * **Sanfilippo’s Syndrome (MPS III):** Primarily involves the accumulation of **Heparan sulfate**. It is clinically distinct due to profound central nervous system involvement (aggressive behavior, dementia) with relatively mild physical symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Keratan Sulfate** is also found in the cornea and cartilage; hence, its accumulation leads to the skeletal features of Morquio’s. * **Mnemonic for GAGs:** "Mor-K" (Morquio = Keratan); "San-H" (Sanfilippo = Heparan). * All MPS are Autosomal Recessive **except Hunter’s**, which is X-linked Recessive (Mnemonic: *The Hunter needs X-ray vision to see the target*). * **Corneal clouding** is absent in Hunter’s and Sanfilippo’s.

Question 238: What is the genetic defect in Dubin-Johnson syndrome?

A. Mutation in the ATP7B gene
B. Mutation in the MRP2 gene (Correct Answer)
C. Mutation in the UGT1A1 gene
D. Mutation in the ATP7A gene

Explanation: **Explanation:** **Dubin-Johnson Syndrome (DJS)** is an autosomal recessive disorder characterized by conjugated hyperbilirubinemia. The correct answer is **Option B (MRP2 gene)**. 1. **Why MRP2 is correct:** The genetic defect lies in the **ABCC2 gene**, which encodes the **Multidrug Resistance-associated Protein 2 (MRP2)**. This protein is an ATP-dependent canalicular transporter responsible for the efflux of conjugated bilirubin from hepatocytes into the bile canaliculi. A mutation leads to the accumulation of conjugated bilirubin in hepatocytes, which then leaks back into the blood. 2. **Why other options are incorrect:** * **Option A (ATP7B):** Mutations here cause **Wilson Disease**, leading to impaired biliary copper excretion and toxic copper accumulation. * **Option C (UGT1A1):** Mutations in this gene cause **Gilbert Syndrome** (mild deficiency) or **Crigler-Najjar Syndrome** (severe/total deficiency), resulting in *unconjugated* hyperbilirubinemia. * **Option D (ATP7A):** Mutations here cause **Menkes Disease**, a disorder of intestinal copper absorption leading to copper deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Gross Pathology:** The hallmark of DJS is a **"Black Liver"** due to the accumulation of epinephrine metabolites (melanin-like pigment) in lysosomes. * **Rotor Syndrome vs. DJS:** Both cause conjugated hyperbilirubinemia, but Rotor Syndrome lacks the black liver and has a different urinary coproporphyrin profile. * **Diagnosis:** In DJS, total urinary coproporphyrin levels are normal, but **Coproporphyrin I** constitutes >80% of the total (normally, Coproporphyrin III predominates). * **Oral Cholecystography:** The gallbladder is typically **not visualized** in DJS.

Question 239: Gaucher disease is inherited in which pattern?

A. Autosomal recessive (AR) (Correct Answer)
B. Autosomal dominant (AD)
C. X-linked recessive (XLR)
D. X-linked dominant (XLD)

Explanation: **Explanation:** **Gaucher disease** is the most common lysosomal storage disorder. It is caused by a deficiency of the enzyme **glucocerebrosidase** (acid β-glucosidase), leading to the accumulation of glucocerebroside in the macrophages (Gaucher cells). 1. **Why Autosomal Recessive (AR) is correct:** The gene encoding glucocerebrosidase (*GBA1*) is located on chromosome 1q21. Like the vast majority of enzyme deficiencies in inborn errors of metabolism, Gaucher disease follows an **autosomal recessive** inheritance pattern. This means an individual must inherit two defective alleles (one from each parent) to manifest the disease. 2. **Why other options are incorrect:** * **Autosomal Dominant (AD):** AD disorders usually involve structural proteins or receptors (e.g., Marfan syndrome, Familial Hypercholesterolemia), not metabolic enzymes. * **X-linked Recessive (XLR):** While some lysosomal storage diseases are XLR (notably **Fabry disease** and **Hunter syndrome**), Gaucher is not among them. * **X-linked Dominant (XLD):** These are rare (e.g., Alport syndrome, Vitamin D resistant rickets) and do not include the common sphingolipidoses. **NEET-PG Clinical Pearls:** * **Pathognomonic Feature:** "Crumpled tissue paper" appearance of the macrophage cytoplasm (Gaucher cells). * **Clinical Triad:** Hepatosplenomegaly (most common), bone involvement (Erlenmeyer flask deformity of the femur, bone crises), and pancytopenia. * **Biomarker:** Elevated serum **Chitotriosidase** levels are used for monitoring. * **Treatment:** Enzyme Replacement Therapy (ERT) with Recombinant Glucocerebrosidase (Imiglucerase).

Question 240: Which of the following genetic disorders exclusively affects males?

A. Scheie's syndrome
B. Hunter's syndrome (Correct Answer)
C. Hurler's syndrome
D. Gaucher's disease

Explanation: **Explanation:** The correct answer is **Hunter’s syndrome** because it is the only Mucopolysaccharidosis (MPS) that follows an **X-linked recessive** inheritance pattern. Since males have only one X chromosome, they are exclusively affected by the clinical phenotype, while females are typically asymptomatic carriers. * **Hunter’s Syndrome (MPS II):** Caused by a deficiency of the enzyme **Iduronate-2-sulfatase**. It is characterized by the accumulation of heparan and dermatan sulfate. Clinically, it presents similarly to Hurler’s but is distinguished by the **absence of corneal clouding** and the presence of aggressive behavior. * **Hurler’s Syndrome (MPS IH) & Scheie’s Syndrome (MPS IS):** Both are caused by a deficiency of **Alpha-L-iduronidase**. They follow an **Autosomal Recessive (AR)** inheritance pattern, meaning they affect males and females equally. Hurler’s is the severe form, while Scheie’s is the mild, adult-onset form. * **Gaucher’s Disease:** This is a sphingolipidosis caused by a deficiency of **Glucocerebrosidase**. It also follows an **Autosomal Recessive** pattern and affects both sexes. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hunter’s:** "The **Hunter** needs **X**-ray vision (X-linked) to see the **Clear** sky (No corneal clouding) and aim at the **Target** (Pebbly skin lesions/Papules)." * All Mucopolysaccharidoses are Autosomal Recessive **EXCEPT** Hunter’s (X-linked). * All Sphingolipidoses are Autosomal Recessive **EXCEPT** Fabry’s disease (X-linked). * **Enzyme Diagnosis:** Gold standard for diagnosis is the assay of specific lysosomal enzymatic activity in leukocytes or fibroblasts.

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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