Genetic Disorders and Biochemical Pathology — MCQs

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

Question 531: The molecule, which is the initiator of cataract formation in the eye lens and whose 1-phosphate derivative is responsible for liver failure, is:

A. Sorbitol
B. Mannitol
C. Inositol
D. Galacticol (Correct Answer)

Explanation: **Explanation:** The correct answer is **Galacticol** (also known as Dulcitol). This question tests the biochemical pathology of **Galactosemia**, specifically the Classic form (GALT deficiency). **Why Galacticol is correct:** In patients with Galactosemia, galactose levels rise in the blood and tissues. In the lens of the eye, the enzyme **Aldose Reductase** reduces galactose into its sugar alcohol, **Galacticol**. Unlike galactose, galacticol is osmotically active and cannot diffuse out of the lens. It draws water in, causing swelling and denaturation of lens proteins, which is the **initiator of cataract formation**. Furthermore, the second part of the question refers to **Galactose-1-Phosphate**, which accumulates in Classic Galactosemia. This derivative is a potent hepatotoxin that inhibits enzymes like phosphoglucomutase, leading to **liver failure**, jaundice, and cirrhosis. **Why other options are incorrect:** * **Sorbitol:** While sorbitol causes cataracts in diabetic patients (via glucose reduction), its 1-phosphate derivative does not cause liver failure. Fructose-1-phosphate is the metabolite linked to liver issues in Fructose Intolerance. * **Mannitol:** This is an exogenous osmotic diuretic and not a metabolic intermediate that causes cataracts or liver failure. * **Inositol:** A component of phospholipids (PIP2) and a second messenger; it is not associated with these pathologies. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Galactosemia:** Deficiency of Galactose-1-phosphate uridyltransferase (**GALT**). Presents with "Oil-drop" cataracts, hepatomegaly, and *E. coli* sepsis. * **Galactokinase Deficiency:** Only causes cataracts (due to galacticol) without liver or kidney damage. * **Mnemonic:** "GALT" is more severe than "GALK" because the phosphate derivative (1-P) traps energy and causes organ damage.

Question 532: What conditions may develop in a child born to a mother with phenylketonuria?

A. Microcephaly, mental retardation, congenital heart disease (Correct Answer)
B. Mental retardation, cataract, congenital heart disease
C. Hydrocephalus, cataract
D. Microcephaly, cataract, renal dysplasia

Explanation: This question addresses **Maternal Phenylketonuria (PKU) Syndrome**, a critical topic in biochemical genetics. ### **Explanation of the Correct Answer** In a pregnant woman with poorly controlled PKU, high levels of **phenylalanine (Phe)** act as a potent **teratogen** to the developing fetus. Even if the fetus does not have the genetic mutation for PKU, the amino acid crosses the placenta via active transport, reaching concentrations 70–100% higher than maternal levels. High fetal Phe levels disrupt protein synthesis and neurotransmitter development, leading to the classic triad of **Maternal PKU Syndrome**: 1. **Microcephaly:** Due to impaired brain growth. 2. **Mental Retardation (Intellectual Disability):** Resulting from neurotoxicity. 3. **Congenital Heart Disease (CHD):** Most commonly ventricular septal defects or Fallot’s tetralogy. 4. **Intrauterine Growth Retardation (IUGR):** Often seen alongside the triad. ### **Analysis of Incorrect Options** * **Options B, C, and D (Cataract):** Cataracts are a hallmark of **Galactosemia** (due to galactitol accumulation) or congenital Rubella, but are not associated with phenylalanine toxicity. * **Option C (Hydrocephalus):** Maternal PKU causes *Microcephaly* (small brain/head), not Hydrocephalus (excess CSF/enlarged head). * **Option D (Renal Dysplasia):** While various malformations can occur, renal dysplasia is not a characteristic feature of this syndrome. ### **NEET-PG High-Yield Pearls** * **Prevention:** To prevent this syndrome, mothers with PKU must maintain a **Phe-restricted diet** *before* conception and throughout pregnancy (target Phe: 2–6 mg/dL). * **The "Mousy" Odor:** Remember that PKU patients (not necessarily the syndrome infants) often have a "musty" or "mousy" body odor due to **phenylacetate** in sweat and urine. * **Enzyme Defect:** Classic PKU is due to a deficiency of **Phenylalanine Hydroxylase (PAH)**; malignant PKU is due to **Dihydrobiopterin reductase** deficiency.

Question 533: In Wilson disease, which element is chelated?

A. Iron
B. Zinc (Correct Answer)
C. Copper
D. Selenium

Explanation: **Explanation:** In the context of **Wilson Disease** (Hepatolenticular degeneration), the question asks which element is **chelated** as a therapeutic intervention. While Wilson disease is a disorder of **copper** metabolism, the correct answer is **Zinc** because it acts as a pharmacological chelator/antagonist used in long-term maintenance therapy. **Why Zinc is Correct:** Zinc (usually as Zinc Acetate) induces the synthesis of **Metallothionein** in the intestinal mucosal cells. Metallothionein is an endogenous chelator that has a high affinity for copper. It binds dietary copper within the enterocytes, preventing its absorption into the portal circulation. The bound copper is then excreted safely in the feces as the intestinal cells are sloughed off. **Analysis of Incorrect Options:** * **Option A (Iron):** Iron chelation (e.g., Deferoxamine) is used in Hemochromatosis or Thalassemia, not Wilson disease. * **Option C (Copper):** Copper is the element that *accumulates* pathologically due to a mutation in the **ATP7B gene**. While drugs like D-Penicillamine and Trientine are used to chelate and excrete copper via urine, Zinc is the specific element among the options that functions as an indirect chelator/blocker of absorption. * **Option D (Selenium):** Selenium is a cofactor for Glutathione Peroxidase and has no role in the chelation therapy of Wilson disease. **NEET-PG High-Yield Pearls:** * **Gold Standard Diagnosis:** Liver biopsy (increased hepatic copper >250 μg/g dry weight). * **Screening:** Low serum Ceruloplasmin and presence of **Kayser-Fleischer (KF) rings** on slit-lamp exam. * **Treatment Triad:** 1. **D-Penicillamine:** First-line chelator (Side effect: Nephrotic syndrome, B6 deficiency). 2. **Trientine:** Preferred if Penicillamine is not tolerated. 3. **Zinc:** Best for maintenance and asymptomatic patients.

Question 534: What is the cause of alpha thalassemia?

A. Deletion of alpha genes (Correct Answer)
B. Deletion of beta genes
C. Excess of alpha genes
D. Single amino acid substitution in alpha chain

Explanation: ### Explanation **Correct Option: A. Deletion of alpha genes** Alpha-thalassemia is primarily caused by the **deletion** of one or more of the four alpha-globin genes located on **chromosome 16**. Unlike beta-thalassemia, which usually results from point mutations, the physical loss of the gene loci via unequal crossing over during meiosis is the hallmark of this condition. The severity of the disease depends directly on the number of genes deleted (1 to 4). **Analysis of Incorrect Options:** * **B. Deletion of beta genes:** This would lead to beta-thalassemia; however, beta-thalassemia is more commonly caused by point mutations in the promoter or splicing sites rather than large deletions. * **C. Excess of alpha genes:** An excess of alpha chains occurs in **beta-thalassemia**, where the lack of beta chains leaves alpha chains unpaired. These excess alpha chains precipitate, causing hemolysis. * **D. Single amino acid substitution in alpha chain:** This describes a **hemoglobin variant** (hemoglobinopathy) rather than thalassemia. Thalassemia is a *quantitative* defect (reduced production), whereas substitutions represent a *qualitative* defect (e.g., HbS or HbC). **High-Yield Clinical Pearls for NEET-PG:** * **Genetics:** Alpha-globin cluster is on **Chromosome 16**; Beta-globin cluster is on **Chromosome 11**. * **Genotypes:** * **1 gene deletion:** Silent carrier. * **2 gene deletions:** Alpha-thalassemia trait (Microcytic anemia). * **3 gene deletions:** **HbH Disease** (Beta-tetramers, $\beta_4$). HbH forms "golf ball" inclusions on brilliant cresyl blue stain. * **4 gene deletions:** **Hb Barts** (Gamma-tetramers, $\gamma_4$). Leads to **Hydrops Fetalis**, as Hb Barts has an extremely high oxygen affinity and cannot deliver $O_2$ to tissues. * **Cis vs. Trans:** The *cis* deletion (both deletions on the same chromosome) is common in Asians and increases the risk of Hydrops Fetalis in offspring.

Question 535: In sickle cell anemia, what is the amino acid replacement at the beta-globin chain?

A. Valine by Glutamic acid in beta-6
B. Glutamic acid by Valine in beta-6 (Correct Answer)
C. Histidine and Valine in beta-8
D. Amino acid at alpha-6

Explanation: ### Explanation **1. Understanding the Correct Answer (Option B):** Sickle cell anemia is caused by a **point mutation** (specifically a transversion) in the DNA sequence of the $\beta$-globin gene. At the **6th position** of the $\beta$-globin chain, the codon GAG (which codes for **Glutamic acid**) is mutated to GTG (which codes for **Valine**). * **Biochemical Impact:** Glutamic acid is a polar, negatively charged amino acid, whereas Valine is non-polar and hydrophobic. This substitution creates a "sticky" hydrophobic patch on the surface of the hemoglobin molecule (HbS). Under deoxygenated conditions, these patches cause HbS molecules to polymerize into long fibers, distorting the RBC into a sickle shape. **2. Analysis of Incorrect Options:** * **Option A:** This is a common distractor. It reverses the order. In HbS, Glutamic acid is the *original* amino acid being replaced, not the replacement itself. * **Option C:** Histidine and Valine at position 8 are not involved in the primary pathology of Sickle Cell Anemia. * **Option D:** The mutation in Sickle Cell Anemia occurs exclusively in the **$\beta$-globin chain**, not the $\alpha$-chain. **3. NEET-PG High-Yield Clinical Pearls:** * **Genetics:** It is an autosomal recessive disorder. The mutation is a **missense mutation**. * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** toward the anode than HbA because it has lost two negative charges (one per $\beta$-chain). * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Diagnosis:** The "Gold Standard" for diagnosis is **Hb Electrophoresis** or **HPLC**. Screening is done via the Solubility test or Sodium Metabisulfite test.

Question 536: Which of the following is an autosomal dominant metabolic disorder?

A. Cystic fibrosis
B. Phenylketonuria
C. Alpha-1-antitrypsin deficiency
D. Familial hypercholesterolemia (Correct Answer)

Explanation: **Explanation:** The correct answer is **Familial Hypercholesterolemia (FH)**. **1. Why Familial Hypercholesterolemia is correct:** Most metabolic disorders (enzymopathies) follow an autosomal recessive (AR) inheritance pattern because a 50% reduction in enzyme activity in heterozygotes is usually sufficient for normal function. However, **Familial Hypercholesterolemia** is a notable exception. It is an **Autosomal Dominant (AD)** disorder caused by mutations in the **LDL receptor gene**. This leads to a "dosage effect" where even a 50% reduction in functional receptors (heterozygotes) causes significant elevation in plasma LDL, leading to premature atherosclerosis and xanthomas. **2. Why other options are incorrect:** * **Cystic Fibrosis:** This is the most common lethal **Autosomal Recessive** disorder in Caucasians, caused by a mutation in the CFTR gene on Chromosome 7. * **Phenylketonuria (PKU):** This is a classic **Autosomal Recessive** metabolic disorder caused by a deficiency of Phenylalanine Hydroxylase. * **Alpha-1-antitrypsin deficiency:** This follows an **Autosomal Recessive** (specifically co-dominant) inheritance pattern, leading to emphysema and liver cirrhosis. **3. NEET-PG Clinical Pearls:** * **Mnemonic for AD Metabolic Disorders:** Most AD disorders are structural (e.g., Marfan) or regulatory. Metabolic exceptions include **FH**, **Acute Intermittent Porphyria (AIP)**, and **Hereditary Spherocytosis**. * **FH Clinical Markers:** Look for **Tendon Xanthomas** (especially Achilles) and **Corneal Arcus** in a young patient with a family history of early Myocardial Infarction. * **Genetics:** Homozygous FH is much more severe, often presenting with MI before age 20.

Question 537: Which of the following diseases has an autosomal recessive inheritance pattern?

A. Cystic fibrosis (Correct Answer)
B. Hydrocephalus
C. Duchene muscular dystrophy
D. None of the above

Explanation: **Explanation:** **Cystic Fibrosis (Option A)** is the correct answer. It is one of the most common **autosomal recessive** disorders, particularly in Caucasian populations. It is caused by a mutation in the **CFTR gene** located on **chromosome 7**. This mutation leads to defective chloride ion transport across epithelial membranes, resulting in thick, dehydrated secretions in the lungs, pancreas, and reproductive tract. **Analysis of Incorrect Options:** * **Hydrocephalus (Option B):** This is a clinical sign (excess CSF accumulation) rather than a single genetic disease. While it can be part of genetic syndromes, the most common inherited form (X-linked aqueductal stenosis) follows an **X-linked recessive** pattern. Most cases are sporadic or acquired (e.g., post-meningitic). * **Duchenne Muscular Dystrophy (Option C):** This is a classic example of an **X-linked recessive** disorder. It involves a mutation in the *Dystrophin* gene (the largest known human gene), primarily affecting males. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Autosomal Recessive (AR) Disorders:** "ABCDE PQRST" – **A**lbinism, **B**artter syndrome, **C**ystic fibrosis/CAH, **D**eafness (sensorineural), **E**nzyme deficiencies (most), **P**KU, **Q**-fever (not genetic, but used for flow), **R**enal tubular acidosis, **S**ickle cell anemia/Thalassemia, **T**ay-Sachs. * **CFTR Mutation:** The most common mutation is **ΔF508** (deletion of phenylalanine at position 508). * **Diagnostic Gold Standard:** Sweat Chloride Test (Chloride levels >60 mmol/L). * **Associated Finding:** Congenital Bilateral Absence of the Vas Deferens (CBAVD) leading to infertility in males.

Question 538: Adrenogenital syndrome is most commonly caused by which of the following?

A. 21-hydroxylase deficiency (Correct Answer)
B. 17-alpha-hydroxylase deficiency
C. 3-beta-hydroxylase deficiency
D. Steroid sulfatase deficiency

Explanation: **Explanation:** **Adrenogenital Syndrome (Congenital Adrenal Hyperplasia - CAH)** refers to a group of autosomal recessive disorders characterized by defects in the enzymes required for cortisol synthesis. **1. Why 21-Hydroxylase deficiency is correct:** This is the **most common cause**, accounting for approximately **90-95% of all CAH cases**. A deficiency in 21-hydroxylase prevents the conversion of Progesterone to 11-deoxycorticosterone (mineralocorticoid pathway) and 17-OH Progesterone to 11-deoxycortisol (glucocorticoid pathway). This leads to: * **Cortisol deficiency:** Triggers increased ACTH secretion via negative feedback. * **Adrenal Hyperplasia:** ACTH overstimulates the adrenal cortex. * **Androgen Excess:** Precursors are shunted toward the androgen pathway, causing virilization/ambiguous genitalia in females and precocious puberty in males. **2. Why the other options are incorrect:** * **17-alpha-hydroxylase deficiency:** Rare. It leads to decreased sex hormones and cortisol but **increased mineralocorticoids**, resulting in hypertension and hypokalemia. * **3-beta-hydroxysteroid dehydrogenase deficiency:** Rare. It blocks all three pathways (mineralocorticoids, glucocorticoids, and sex steroids), leading to salt wasting and ambiguous genitalia in males. * **Steroid sulfatase deficiency:** This is associated with **X-linked Ichthyosis**, not CAH. It affects placental estrogen production and skin desquamation. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Marker:** Elevated levels of **17-Hydroxyprogesterone (17-OHP)** are pathognomonic for 21-hydroxylase deficiency. * **Salt-Wasting:** Severe 21-hydroxylase deficiency leads to "salt-wasting" (hyponatremia, hyperkalemia, and hypotension) due to aldosterone deficiency. * **The "1" Rule:** If the enzyme ends in 1 (21, 11), it causes virilization. If it starts with 1 (17, 11), it causes hypertension. (Note: 11-beta-hydroxylase deficiency causes both).

Question 539: Patients with G6PD deficiency often have an increased susceptibility to infection, which in turn may precipitate a hemolytic episode. What is the most likely underlying mechanism for the increased risk of infection?

A. Decreased amount of glutathione in the white blood cells
B. Decreased amount of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in the white blood cells (Correct Answer)
C. Defective phagocytosis
D. Deficiency of myeloperoxidase

Explanation: ### Explanation **Correct Option: B (Decreased amount of NADPH in the white blood cells)** Glucose-6-Phosphate Dehydrogenase (G6PD) is the rate-limiting enzyme of the Hexose Monophosphate (HMP) shunt, which is the primary source of **NADPH** in cells. In white blood cells (specifically neutrophils and macrophages), NADPH is a critical substrate for the enzyme **NADPH oxidase**. This enzyme initiates the "Respiratory Burst" by converting molecular oxygen into superoxide radicals to kill phagocytosed bacteria. In G6PD deficiency, the lack of NADPH impairs this microbicidal activity, leading to increased susceptibility to infections. **Analysis of Incorrect Options:** * **A. Decreased amount of glutathione:** While G6PD deficiency leads to low reduced glutathione in RBCs (causing hemolysis due to oxidative stress), the primary cause of *impaired immunity* is the lack of NADPH for the oxidative burst, not glutathione levels. * **C. Defective phagocytosis:** Phagocytosis (the ingestion of bacteria) is usually normal in G6PD deficiency; the defect lies in the subsequent **intracellular killing** mechanism. * **D. Deficiency of myeloperoxidase:** Myeloperoxidase (MPO) deficiency is a distinct genetic disorder. While it also affects bacterial killing, it is not caused by G6PD deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Granulomatous Disease (CGD):** Severe G6PD deficiency can clinically mimic CGD because both result in a defective respiratory burst (CGD is a direct defect in NADPH oxidase). * **Inheritance:** G6PD deficiency is an **X-linked recessive** disorder. * **Precipitants:** Hemolysis is typically triggered by **oxidative stress** (e.g., Fava beans, infections, or drugs like Primaquine, Sulphonamides, and Nitrofurantoin). * **Morphology:** Look for **Heinz bodies** (denatured hemoglobin) and **Bite cells** (deformed RBCs after splenic macrophage action) on a peripheral smear.

Question 540: A patient is on treatment for his abnormal lipid profile blood tests. All of the following statements are true about familial hypercholesterolemia, except?

A. Patients with receptor-defective LDL receptor defects have the worst prognosis. (Correct Answer)
B. Cutaneous xanthomas on the hands, wrists, elbows present.
C. Total cholesterol levels can be higher than 1000 mg/dL.
D. Accelerated atherosclerosis is a devastating complication.

Explanation: **Explanation** **Familial Hypercholesterolemia (FH)** is an autosomal dominant disorder primarily caused by mutations in the **LDL receptor (LDLR) gene**, leading to elevated LDL-C levels and premature cardiovascular disease. **Why Option A is the correct answer (The False Statement):** The prognosis of FH depends on the residual activity of the LDL receptors. **Receptor-negative** (null) mutations, where there is <2% of normal receptor activity, result in the **worst prognosis** and most severe clinical presentation. In contrast, **receptor-defective** mutations (2–25% activity) allow for some clearance of LDL, leading to a relatively milder phenotype compared to the null variant. **Analysis of Other Options:** * **Option B:** Physical findings are hallmark features. **Tendinous xanthomas** (most commonly on the Achilles tendon and extensor tendons of the hands) and **tuberous xanthomas** (elbows/knees) are classic signs of cholesterol deposition. * **Option C:** In **Homozygous FH**, total cholesterol levels are extremely high, often ranging from **600 to 1000+ mg/dL**. Heterozygotes typically show levels between 300–600 mg/dL. * **Option D:** Accelerated atherosclerosis is the primary cause of morbidity. Without treatment, homozygous patients often suffer from myocardial infarction or sudden death before the age of 20. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Dominant (LDLR gene on Chromosome 19). * **Other Mutations:** Mutations in **ApoB-100** (Ligand-defective) or **PCSK9** (Gain-of-function) can also cause FH. * **Clinical Signs:** Xanthelasma (eyelids) and Corneal Arcus (arcus senilis) in young patients are highly suggestive. * **Treatment:** Statins are first-line; however, homozygous patients often require **LDL apheresis** or newer agents like **Evolocumab** (PCSK9 inhibitor) or **Lomitapide**.

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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Newborn Screening for Genetic Disorders

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Enzyme Replacement Therapy

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