Genetic Disorders and Biochemical Pathology — MCQs

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

Question 201: Enzyme deficiency in Natowicz syndrome is?

A. Iduronate sulfatase
B. Hyaluronidase (Correct Answer)
C. b-Glucuronidase
D. Galactosamine 6-sulfatase

Explanation: **Explanation:** **Natowicz syndrome**, also known as **Mucopolysaccharidosis type IX (MPS IX)**, is an extremely rare autosomal recessive lysosomal storage disorder. It is caused by a deficiency of the enzyme **Hyaluronidase-1 (HYAL1)**. This enzyme is responsible for the degradation of hyaluronic acid (hyaluronan), a major component of the extracellular matrix. Its deficiency leads to the accumulation of hyaluronan within lysosomes, primarily manifesting as soft tissue masses (nodular periarticular masses), joint pain, and short stature, but notably lacking the severe skeletal dysplasia or intellectual disability seen in other MPS types. **Analysis of Incorrect Options:** * **Option A: Iduronate sulfatase** – Deficiency causes **Hunter Syndrome (MPS II)**, characterized by X-linked inheritance, coarse facies, and clear corneas. * **Option C: β-Glucuronidase** – Deficiency causes **Sly Syndrome (MPS VII)**, which presents with hepatosplenomegaly, skeletal deformities, and hydrops fetalis in severe cases. * **Option D: Galactosamine 6-sulfatase** – Deficiency causes **Morquio Syndrome Type A (MPS IVA)**, distinguished by severe skeletal dysplasia (dysostosis multiplex) and ligamentous laxity. **High-Yield Clinical Pearls for NEET-PG:** * **MPS IX (Natowicz)** is the only MPS involving hyaluronic acid metabolism. * **Inheritance:** All Mucopolysaccharidoses are Autosomal Recessive **except** Hunter Syndrome (MPS II), which is X-linked Recessive. * **Corneal Clouding:** Present in Hurler (MPS I) and Morquio (MPS IV); **absent** in Hunter (MPS II) and Natowicz (MPS IX). * **Key Diagnostic:** Elevated serum hyaluronic acid levels are a hallmark of Natowicz syndrome.

Question 202: Patients with axonal neuropathy, unexplained abdominal pain, and a history of psychiatric illness may be suffering from which condition?

A. Porphyria (Correct Answer)
B. Paroxysmal nocturnal hemoglobinuria (PNH)
C. Arsenic toxicity
D. Hypothyroidism

Explanation: ### Explanation The correct answer is **Porphyria**, specifically **Acute Intermittent Porphyria (AIP)**. **1. Why Porphyria is Correct:** AIP is an autosomal dominant metabolic disorder caused by a deficiency in the enzyme **Porphobilinogen (PBG) deaminase**. This leads to the accumulation of toxic heme precursors, namely **delta-aminolevulinic acid (ALA)** and **porphobilinogen (PBG)**. The clinical presentation is classically described by the "triad" seen in this question: * **Abdominal Pain:** Severe, poorly localized, and "out of proportion" to physical findings (neurogenic in origin). * **Neuropsychiatric Symptoms:** Anxiety, psychosis, or confusion. * **Peripheral Neuropathy:** Primarily motor axonal neuropathy, which can progress to respiratory paralysis. **2. Why Incorrect Options are Wrong:** * **Paroxysmal Nocturnal Hemoglobinuria (PNH):** Presents with hemolytic anemia, pancytopenia, and venous thrombosis (e.g., Budd-Chiari syndrome), but not typically axonal neuropathy. * **Arsenic Toxicity:** While it causes abdominal pain and neuropathy, it is usually associated with skin changes (Mees' lines, hyperkeratosis) and is less likely to present with the specific psychiatric profile of AIP. * **Hypothyroidism:** Can cause "pseudomyotonia" or entrapment neuropathies (Carpal Tunnel), but the abdominal symptom is typically constipation, not acute crisis-like pain. **3. NEET-PG High-Yield Pearls:** * **The 5 P's of AIP:** **P**ainful abdomen, **P**ort-wine colored urine (on standing), **P**olyneuropathy, **P**sychological disturbances, **P**recipitated by drugs (Barbiturates, Cytochrome P450 inducers). * **Diagnosis:** Elevated urinary PBG and ALA during attacks. * **Management:** Intravenous **Hematin** or **Heme arginate** (inhibits ALA synthase via negative feedback) and high-dose glucose.

Question 203: Which of the following lysosomal storage disorders is NOT inherited as an autosomal recessive trait?

A. Fabry disease (Correct Answer)
B. Krabbe disease
C. Gaucher disease
D. Tay-Sachs disease

Explanation: **Explanation:** The inheritance pattern of Lysosomal Storage Disorders (LSDs) is a high-yield topic for NEET-PG. Most LSDs follow an **Autosomal Recessive (AR)** inheritance pattern. However, there are two notable exceptions that are **X-linked Recessive (XLR)**: **Fabry disease** and **Hunter syndrome**. 1. **Why Fabry Disease is Correct:** Fabry disease is caused by a deficiency of the enzyme **$\alpha$-galactosidase A**, leading to the accumulation of ceramide trihexoside. The gene responsible is located on the X chromosome, making it an **X-linked Recessive** disorder. Clinical hallmarks include angiokeratomas, peripheral neuropathy (acroparesthesia), and late-stage renal/cardiac failure. 2. **Why Other Options are Incorrect:** * **Krabbe Disease:** An **AR** disorder caused by galactocerebrosidase deficiency. It is characterized by "globoid cells" and severe demyelination. * **Gaucher Disease:** The most common LSD, inherited as **AR**. It results from glucocerebrosidase deficiency. Look for "crinkled paper" cytoplasm in macrophages and hepatosplenomegaly. * **Tay-Sachs Disease:** An **AR** disorder caused by Hexosaminidase A deficiency. It presents with a cherry-red spot on the macula and progressive neurodegeneration, but notably lacks hepatosplenomegaly (unlike Niemann-Pick). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for XLR LSDs:** *"The **Hunter** shot a **Fabry** bird with his **X**-bow."* (Hunter syndrome and Fabry disease are X-linked). * **Hunter vs. Hurler:** Both are Mucopolysaccharidoses, but Hunter is XLR and lacks corneal clouding, while Hurler is AR and has corneal clouding. * **Enzyme Replacement Therapy (ERT)** is now available for Gaucher, Fabry, and Hunter/Hurler diseases.

Question 204: In a patient with G6PD deficiency, hemolysis is due to a decrease in which of the following?

A. H+
B. Thiamine pyrophosphate (TPP)
C. Nicotinamide adenine dinucleotide (NADH)
D. Nicotinamide adenine dinucleotide phosphate (NADPH) (Correct Answer)

Explanation: **Explanation** **1. Why NADPH is the Correct Answer:** Glucose-6-Phosphate Dehydrogenase (G6PD) is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary role in red blood cells (RBCs) is the production of **NADPH**. In RBCs, NADPH is essential for maintaining a pool of **reduced glutathione**. Reduced glutathione acts as an antioxidant that neutralizes Reactive Oxygen Species (ROS) like hydrogen peroxide ($H_2O_2$). In G6PD deficiency, the lack of NADPH leads to a depletion of reduced glutathione. Consequently, oxidative stress causes hemoglobin to denature and precipitate (forming **Heinz bodies**), damaging the RBC membrane and leading to hemolysis. **2. Why Other Options are Incorrect:** * **A. H+:** Hydrogen ion concentration relates to pH balance. While acidosis can shift the oxygen dissociation curve, it is not the primary biochemical deficit in G6PD deficiency. * **B. Thiamine pyrophosphate (TPP):** TPP is a derivative of Vitamin B1. While it is a cofactor for Transketolase (another HMP shunt enzyme), its deficiency leads to Beriberi or Wernicke-Korsakoff syndrome, not acute hemolysis. * **C. NADH:** NADH is primarily generated in glycolysis (Embden-Meyerhof pathway) and the TCA cycle. It is used by Methemoglobin Reductase to reduce Fe3+ to Fe2+, but it cannot substitute for NADPH in the glutathione reductase reaction. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** X-linked recessive (more common in males). * **Triggers:** Fava beans, infections, and drugs (Sulfa drugs, Primaquine, Nitrofurantoin). * **Morphology:** **Heinz bodies** (denatured Hb) and **Bite cells** (formed when splenic macrophages remove Heinz bodies). * **Protective Effect:** G6PD deficiency provides a selective advantage against *Plasmodium falciparum* malaria.

Question 205: All of the following are examples of liver glycogenosis except?

A. Von Gierke disease
B. Hers disease
C. Type III glycogenosis
D. Pompe disease (Correct Answer)

Explanation: **Explanation:** Glycogen Storage Diseases (GSDs) are traditionally categorized based on the primary organ system affected: **Hepatic (Liver)**, **Muscular**, or **Generalized**. **Why Pompe Disease (Type II) is the correct answer:** Pompe disease is unique among GSDs because it is a **lysosomal storage disorder** caused by a deficiency of **α-1,4-glucosidase (Acid Maltase)**. Unlike other GSDs, it does not primarily manifest as a metabolic liver disorder (hypoglycemia). Instead, it is a **generalized/systemic** disease that predominantly affects the **heart and skeletal muscles**. The hallmark clinical feature is massive cardiomegaly and hypertrophic cardiomyopathy, leading to early death in the infantile form. **Why the other options are incorrect:** * **Von Gierke Disease (Type I):** The prototype of hepatic GSD. Deficiency of Glucose-6-Phosphatase leads to severe fasting hypoglycemia, hepatomegaly, and lactic acidosis. * **Hers Disease (Type VI):** Caused by a deficiency of Liver Phosphorylase. It presents as a milder form of hepatic GSD with hepatomegaly and growth retardation. * **Type III Glycogenosis (Cori/Forbes Disease):** Caused by Debranching enzyme deficiency. It affects both the liver and muscle, but it is a major cause of liver glycogenosis presenting with hepatomegaly and hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Pompe = Pump:** Remember "Pompe affects the Pump (Heart)." It is the only GSD that is also a Lysosomal Storage Disease. * **Type I vs. Type III:** Both have hepatomegaly, but **Type I has lactic acidosis and hyperuricemia**, whereas Type III typically does not. * **Anderson Disease (Type IV):** Characterized by "Abnormal" glycogen (long outer branches) due to branching enzyme deficiency, leading to early liver cirrhosis.

Question 206: In Hartnup disorder, metabolism of which of the following amino acids is affected?

A. Tyrosine
B. Tryptophan (Correct Answer)
C. Phenylalanine
D. Homocysteine

Explanation: **Explanation:** **Hartnup disease** is an autosomal recessive disorder characterized by a defect in the **SLC6A19 gene**, which encodes a sodium-dependent neutral amino acid transporter. This transporter is primarily located in the proximal renal tubules and the brush border of the intestinal mucosa. **Why Tryptophan is the Correct Answer:** The primary biochemical defect is the impaired transport of **neutral amino acids**, most notably **Tryptophan**, in both the gut and kidneys. Tryptophan is a crucial precursor for the synthesis of **Niacin (Vitamin B3)** via the kynurenine pathway. A deficiency in Tryptophan leads to a secondary deficiency of Niacin, resulting in **Pellagra-like symptoms** (Dermatitis, Diarrhea, and Dementia). **Why Other Options are Incorrect:** * **Tyrosine & Phenylalanine:** While these are neutral amino acids, their clinical significance in Hartnup is secondary to Tryptophan. Deficiencies in these typically point toward Phenylketonuria or Albinism. * **Homocysteine:** This is a sulfur-containing amino acid. Its metabolism is affected in Homocystinuria (due to Cystathionine beta-synthase deficiency), not Hartnup disease. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Characterized by a photosensitive pellagra-like skin rash, cerebellar ataxia, and emotional lability. * **Diagnostic Marker:** **Neutral Aminoaciduria** (presence of neutral amino acids in urine, while dibasic/acidic amino acids are spared). * **Indicanuria:** Unabsorbed tryptophan in the gut is converted by bacteria into indoles, which are excreted in the urine (turning it blue upon oxidation). * **Treatment:** High-protein diet and **Nicotinamide (Niacin) supplementation**.

Question 207: All of the following are recognized features of Wilson's disease except?

A. Psychological disturbances
B. Increased ceruloplasmin levels (Correct Answer)
C. Increased copper content of the liver
D. All

Explanation: **Explanation:** Wilson’s disease (Hepatolenticular degeneration) is an autosomal recessive disorder caused by a mutation in the **ATP7B gene** on chromosome 13. This defect impairs the biliary excretion of copper and its incorporation into apo-ceruloplasmin. **1. Why Option B is the Correct Answer (The "Except"):** In Wilson’s disease, **ceruloplasmin levels are decreased** (typically <20 mg/dL), not increased. The failure to incorporate copper into apo-ceruloplasmin leads to the secretion of an unstable molecule that is rapidly degraded in the plasma. This is a hallmark diagnostic finding. **2. Analysis of Incorrect Options:** * **Option A (Psychological disturbances):** This is a recognized feature. Copper deposition in the basal ganglia (specifically the putamen) leads to neuropsychiatric symptoms, including psychosis, depression, personality changes, and tremors. * **Option C (Increased copper content of the liver):** This is the primary pathology. Defective biliary excretion causes toxic copper accumulation in hepatocytes, leading to cirrhosis or acute liver failure. **High-Yield Clinical Pearls for NEET-PG:** * **Kayser-Fleischer (KF) Rings:** Copper deposition in the **Descemet’s membrane** of the cornea (best seen on slit-lamp exam). * **Diagnosis:** Low serum ceruloplasmin, **increased 24-hour urinary copper excretion** (>100 μg/day), and increased hepatic copper content on biopsy (Gold Standard). * **Treatment:** Copper chelators like **D-Penicillamine** (first-line) or Trientine. Zinc is used for maintenance as it interferes with intestinal copper absorption. * **Sunflower Cataract:** Another ocular finding due to copper deposition in the lens.

Question 208: Ochronosis is found in which of the following conditions?

A. Alkaptonuria (Correct Answer)
B. Tyrosinemia
C. Phenylketonuria
D. Homocystinuria

Explanation: **Explanation:** **Alkaptonuria** is the correct answer. It is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate Oxidase** in the tyrosine catabolic pathway. This deficiency leads to the accumulation of **Homogentisic Acid (HGA)**. When HGA is excreted in urine, it oxidizes upon exposure to air, turning the urine black. In the body, HGA undergoes oxidation and polymerization to form a brownish-black pigment. This pigment deposits in connective tissues, cartilages (like the pinna of the ear), and joints—a clinical phenomenon known as **Ochronosis**. Long-term deposition leads to ochronotic arthritis, typically affecting large weight-bearing joints. **Why other options are incorrect:** * **Tyrosinemia:** Caused by deficiencies in enzymes like Fumarylacetoacetate hydrolase (Type I). It presents with liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor, but not ochronosis. * **Phenylketonuria (PKU):** Due to Phenylalanine Hydroxylase deficiency. It presents with intellectual disability, seizures, and a "mousy" odor. Patients often have hypopigmentation (fair skin/blue eyes) rather than dark pigment deposition. * **Homocystinuria:** Caused by Cystathionine β-synthase deficiency. It is characterized by ectopia lentis (downward dislocation), marfanoid habitus, and thromboembolic events. **NEET-PG High-Yield Pearls:** * **Triad of Alkaptonuria:** Homogentisic aciduria (black urine), Ochronosis (pigmentation), and Arthritis. * **Diagnostic Test:** Ferric chloride test (yields a transient deep blue color). * **Dietary Management:** Restriction of Phenylalanine and Tyrosine; **Nitisinone** can be used to inhibit HGA production.

Question 209: What is true about Hurler's syndrome?

A. Corneal clouding is seen.
B. Abnormal deposits are found in fibroblasts.
C. Reilly bodies are the characteristic feature.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Hurler’s Syndrome (Mucopolysaccharidosis Type IH) is an autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **$\alpha$-L-iduronidase**. This leads to the systemic accumulation of glycosaminoglycans (GAGs), specifically **dermatan sulfate and heparan sulfate**. * **Option A (Corneal Clouding):** This is a hallmark clinical feature of Hurler’s syndrome, distinguishing it from Hunter’s syndrome (MPS II), where the cornea remains clear. The accumulation of GAGs in the corneal stroma leads to progressive opacification. * **Option B (Abnormal deposits in fibroblasts):** Due to the enzyme deficiency, undegraded GAGs accumulate within the lysosomes of various cells. In fibroblasts, these appear as "clear" or "vacuolated" areas under microscopy, often referred to as **Hurler cells** or gargoyle cells. * **Option C (Reilly Bodies):** These are characteristic large, purple-to-blue azurophilic granules found in the cytoplasm of **polymorphonuclear leukocytes** (neutrophils) in patients with mucopolysaccharidosis. They represent lysosomal aggregates of GAGs. Since all three statements accurately describe the clinical and pathological features of the disease, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Defect:** $\alpha$-L-iduronidase. * **Genetics:** Autosomal Recessive (Note: Hunter’s is X-linked Recessive). * **Clinical Features:** Coarse facial features (Gargoylism), hepatosplenomegaly, skeletal deformities (dysostosis multiplex), and mental retardation. * **Diagnosis:** Increased urinary excretion of dermatan and heparan sulfate; definitive diagnosis via enzyme assay in leukocytes or fibroblasts.

Question 210: What is the toxin found in unripe apple fruit that causes Jamaican vomiting sickness?

A. Ricin
B. Muscarine
C. Amaydin
D. Hypoglycin A (Correct Answer)

Explanation: **Explanation:** **Jamaican Vomiting Sickness** is caused by the ingestion of the unripe fruit of the **Ackee tree** (*Blighia sapida*), which contains the potent toxin **Hypoglycin A**. **Why Hypoglycin A is the correct answer:** Hypoglycin A is a non-proteinogenic amino acid that is metabolized into **methylenecyclopropylacetyl-CoA (MCPA-CoA)**. This metabolite acts as a suicide inhibitor of **Acyl-CoA Dehydrogenase**, the first enzyme in the **$\beta$-oxidation of fatty acids**. By blocking fatty acid oxidation, the body cannot generate acetyl-CoA or NADH required for gluconeogenesis. This leads to profound **non-ketotic hypoglycemia**, as the liver cannot produce glucose or ketone bodies to meet metabolic demands. **Analysis of Incorrect Options:** * **Ricin (A):** A potent cytotoxin found in **castor beans** (*Ricinus communis*). It acts by inhibiting the 60S ribosomal subunit, halting protein synthesis. * **Muscarine (B):** A toxin found in certain **mushrooms** (*Amanita muscaria*). it acts as a parasympathomimetic, causing cholinergic crisis (SLUDGE syndrome). * **Amygdalin (C):** A cyanogenic glycoside found in **apricot kernels** and apple seeds. It is metabolized into hydrogen cyanide, which inhibits cytochrome c oxidase in the electron transport chain. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** Severe vomiting, altered consciousness, and seizures due to hypoglycemia, occurring 2–6 hours after ingestion. * **Biochemical Hallmark:** **Non-ketotic hypoglycemia** (unlike starvation, where ketones are elevated). * **Enzyme Inhibited:** Primarily **Short-chain and Medium-chain Acyl-CoA Dehydrogenases (MCAD)**. * **Treatment:** Aggressive IV glucose administration and supportive care.

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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