Genetic Disorders and Biochemical Pathology — MCQs

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

Question 521: What is considered the most severe form of G-6-PD deficiency?

A. Typed
B. Type 2
C. Type 3 (Correct Answer)
D. None of the above

Explanation: **Explanation:** Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency is an X-linked recessive disorder characterized by the inability of red blood cells to maintain adequate levels of NADPH, leading to oxidative stress and hemolysis. The World Health Organization (WHO) classifies G6PD variants based on the degree of enzyme deficiency and the severity of hemolysis. **Why Type 1 is the correct answer:** *Note: There appears to be a typographical error in the provided options/key. According to the standard WHO classification:* * **Class I (Type 1):** This is the **most severe form**. It is characterized by extremely low enzyme activity (<10% of normal) and results in **chronic non-spherocytic hemolytic anemia**, even in the absence of oxidative stress. * **Class II (Type 2):** Severe deficiency (<10% activity) but characterized by **intermittent/episodic hemolysis** (e.g., G6PD Mediterranean). * **Class III (Type 3):** Moderate deficiency (10–60% activity) with hemolysis occurring only under significant oxidative stress (e.g., G6PD A-). **Analysis of Options:** * **Option A (Type 1):** Historically and clinically, Class I is the most severe due to chronic hemolysis. * **Option B (Type 2):** Severe, but hemolysis is episodic rather than chronic. * **Option C (Type 3):** This is a **mild to moderate** form. It is common in African populations (G6PD A-) where only older RBCs are deficient, making the condition self-limiting. * **Option D:** Incorrect as a classification system exists. **High-Yield NEET-PG Pearls:** 1. **Genetics:** X-linked recessive; provides a protective advantage against *Plasmodium falciparum* malaria. 2. **Triggers:** Fava beans (Favism), drugs (Primaquine, Sulphonamides, Dapsone), and infections. 3. **Morphology:** **Heinz bodies** (denatured hemoglobin) and **Bite cells** (degmacytes) seen on peripheral smear. 4. **Diagnosis:** Fluorescent spot test is the screening test; enzyme assay is definitive (but may be falsely normal during an acute hemolytic episode).

Question 522: Which of the following chromosomes is involved in Patau syndrome?

A. Chromosome 13 (Correct Answer)
B. Chromosome 18
C. Chromosome 21
D. Chromosome 5

Explanation: **Explanation:** **Patau syndrome** is a severe genetic disorder caused by **Trisomy 13** (the presence of an extra copy of chromosome 13). It is the least common and most severe of the three viable autosomal trisomies. The condition results from meiotic non-disjunction, most commonly associated with advanced maternal age. **Analysis of Options:** * **Option A (Chromosome 13): Correct.** Patau syndrome is characterized by a "triad" of clinical features: **Microphthalmia** (small eyes), **Cleft lip/palate**, and **Polydactyly** (extra fingers/toes). Other features include holoprosencephaly and cutis aplasia (scalp defects). * **Option B (Chromosome 18): Incorrect.** Trisomy 18 is **Edwards syndrome**. Clinical hallmarks include "rocker-bottom feet," clenched fists with overlapping fingers, and micrognathia (small jaw). * **Option C (Chromosome 21): Incorrect.** Trisomy 21 is **Down syndrome**, the most common viable trisomy. It is characterized by intellectual disability, flat facial profile, Simian crease, and an increased risk of early-onset Alzheimer’s and ALL/AML. * **Option D (Chromosome 5): Incorrect.** A terminal deletion of the short arm of chromosome 5 (5p-) leads to **Cri-du-chat syndrome**, characterized by a high-pitched, cat-like cry and microcephaly. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Trisomies:** **P**atau (13 - **P**uberty starts at 13), **E**dwards (18 - **E**lection age is 18), **D**own (21 - **D**rinking age is 21). * **Biochemical Screening:** In Patau syndrome, first-semester screening typically shows **decreased free β-hCG and PAPP-A**. * **Prognosis:** Most infants with Patau syndrome die within the first few days or months of life due to severe neurological or cardiac complications.

Question 523: Abnormality in Vitamin D metabolism leading to rickets is associated with which of the following?

A. Loss of HCO3- (Correct Answer)
B. Loss of Ca++
C. Loss of K+
D. All of the above

Explanation: This question refers to **Renal Tubular Acidosis (RTA) Type 2 (Proximal RTA)**, a condition frequently associated with Vitamin D-resistant rickets (Fanconi Syndrome). ### **Explanation of the Correct Option** **A. Loss of HCO3-:** In Proximal RTA, there is a defect in the proximal convoluted tubule's ability to reabsorb bicarbonate ($HCO_3^-$). This leads to chronic metabolic acidosis. Acidosis interferes with the activity of **1-alpha-hydroxylase** in the kidneys, the enzyme responsible for converting 25-hydroxyvitamin D into its active form, **1,25-dihydroxyvitamin D (Calcitriol)**. Furthermore, chronic acidosis promotes bone demineralization as the body attempts to buffer excess $H^+$ ions using bone carbonate, leading to the clinical presentation of rickets in children. ### **Explanation of Incorrect Options** * **B. Loss of Ca++:** While hypercalciuria can occur in Distal RTA (Type 1) due to bone resorption, the primary biochemical "abnormality in metabolism" triggering the rickets in this specific context is the acidotic environment created by bicarbonate loss. * **C. Loss of K+:** Hypokalemia is a common finding in both Type 1 and Type 2 RTA, but it affects muscle and cardiac function rather than Vitamin D metabolism or bone mineralization. * **D. All of the above:** Incorrect because the specific metabolic trigger for the Vitamin D abnormality is the bicarbonate defect. ### **High-Yield Clinical Pearls for NEET-PG** * **Fanconi Syndrome:** Characterized by the "Global" failure of the PCT, leading to the loss of Glucose, Amino acids, Phosphates, and Bicarbonate (**GAP**). * **Hypophosphatemic Rickets:** Often co-exists with Proximal RTA; low phosphate is a major contributor to poor bone mineralization. * **Distal RTA (Type 1):** Associated with **Kidney Stones** (nephrocalcinosis) due to high urinary pH and hypercalciuria. * **Proximal RTA (Type 2):** Associated with **Rickets/Osteomalacia** but rarely kidney stones, as the urine can still be acidified by the distal tubule.

Question 524: A patient of Mediterranean ancestry was given primaquine to protect against malaria. The patient rapidly developed a hemolytic anemia due to a mostly silent mutation in which one of the following pathways or enzymes?

A. Malic enzyme
B. Glycolysis
C. Hexose monophosphate shunt (Correct Answer)
D. Gluconeogenesis

Explanation: **Explanation:** The clinical presentation describes **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency**, the most common enzymopathy worldwide. G6PD is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). **1. Why the Correct Answer is Right:** The HMP shunt is the sole source of **NADPH** in mature erythrocytes. NADPH is essential for maintaining the pool of **reduced glutathione**, which neutralizes reactive oxygen species (ROS) like hydrogen peroxide. When a patient with G6PD deficiency is exposed to oxidative stress (e.g., **Primaquine**, fava beans, or infections), the lack of NADPH leads to the accumulation of ROS. This causes hemoglobin to denature and precipitate as **Heinz bodies**, leading to membrane damage and subsequent **hemolysis**. **2. Why Other Options are Wrong:** * **Malic Enzyme:** While it produces NADPH in other tissues (like adipose), it is not present in significant amounts in RBCs to compensate for G6PD deficiency. * **Glycolysis:** This pathway provides ATP and NADH. A defect here (e.g., Pyruvate Kinase deficiency) causes chronic non-spherocytic hemolytic anemia but is not typically triggered by oxidative drugs like primaquine. * **Gluconeogenesis:** This pathway occurs in the liver and kidneys to maintain blood glucose; it does not occur in RBCs (as they lack mitochondria) and is unrelated to oxidative stress management. **3. Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked recessive (more common in males). * **Variants:** *G6PD A-* (African, moderate) and *G6PD Mediterranean* (more severe). * **Morphology:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (formed by splenic macrophages removing Heinz bodies). * **Key Trigger Drugs:** S-S-A-P (Sulfonamides, Sulfones/Dapsone, Antimalarials like Primaquine, and Probenecid). * **Diagnosis:** Enzyme assay (Note: Levels may be falsely normal during an acute hemolytic episode as young reticulocytes have higher enzyme levels).

Question 525: Dystrophic gene mutation leads to which of the following conditions?

A. Myasthenia gravis
B. Motor neuron disease
C. Poliomyelitis
D. Duchenne muscular dystrophy (Correct Answer)

Explanation: **Explanation:** **Duchenne Muscular Dystrophy (DMD)** is caused by a mutation in the **DMD gene**, located on the short arm of the **X chromosome (Xp21)**. This gene is the largest known human gene, making it highly susceptible to spontaneous mutations. It encodes **dystrophin**, a critical cytoplasmic protein that links the intracellular actin cytoskeleton of the muscle fiber to the extracellular matrix via the dystroglycan complex. In DMD, a "frameshift" mutation leads to a near-complete absence of dystrophin, resulting in membrane instability, calcium influx, and progressive myofiber necrosis. **Analysis of Incorrect Options:** * **Myasthenia Gravis:** An autoimmune channelopathy caused by antibodies against the **postsynaptic acetylcholine receptors (AChR)** at the neuromuscular junction, not a genetic mutation of structural proteins. * **Motor Neuron Disease (MND):** A neurodegenerative disorder affecting upper and lower motor neurons. While some forms (like ALS) have genetic links (e.g., SOD1 mutation), it is not related to the dystrophin gene. * **Poliomyelitis:** An infectious disease caused by the **Poliovirus**, which selectively destroys the anterior horn cells of the spinal cord. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked recessive (primarily affects males). * **Becker Muscular Dystrophy (BMD):** Also involves the DMD gene but results from "in-frame" mutations, leading to truncated but partially functional dystrophin (milder phenotype). * **Clinical Signs:** Gower’s sign (using hands to "climb up" the body to stand) and pseudohypertrophy of calves (fatty replacement of muscle). * **Biochemical Marker:** Significantly elevated **Creatine Kinase (CK-MM)** levels are seen even in the preclinical stage. * **Death:** Usually occurs due to respiratory failure or dilated cardiomyopathy.

Question 526: Defective branched-chain ketoacid decarboxylation is characteristic of which of the following genetic disorders?

A. Maple syrup urine disease (Correct Answer)
B. Hartnup disease
C. Alkaptonuria
D. GM-1 gangliosidosis

Explanation: ### Explanation **1. Why Maple Syrup Urine Disease (MSUD) is Correct:** MSUD is caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This multi-enzyme complex is responsible for the oxidative decarboxylation of the keto-acid derivatives of the three branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. When this enzyme is defective, these keto-acids accumulate in the blood and spill into the urine, giving it a characteristic sweet, maple syrup-like odor. **2. Analysis of Incorrect Options:** * **Hartnup Disease:** This is a transport defect involving a neutral amino acid transporter in the kidneys and intestine. It primarily leads to a deficiency of **Tryptophan**, resulting in pellagra-like symptoms. * **Alkaptonuria:** This is caused by a deficiency of **Homogentisate oxidase** in the tyrosine catabolic pathway. It leads to the accumulation of homogentisic acid, causing dark urine and ochronosis. * **GM-1 Gangliosidosis:** This is a lysosomal storage disorder caused by a deficiency of **Beta-galactosidase**, leading to the accumulation of GM1 gangliosides in the brain and viscera. **3. NEET-PG High-Yield Clinical Pearls:** * **The "I" in Isoleucine:** Isoleucine is specifically responsible for the characteristic **maple syrup smell**. * **Cofactors:** The BCKAD complex requires five cofactors (similar to Pyruvate Dehydrogenase): **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**T**ender **R**oving **N**ights **P**lease **L**ove). * **Management:** Treatment involves a diet restricted in Leucine, Isoleucine, and Valine. Some patients respond to high doses of **Thiamine** (Thiamine-responsive MSUD). * **Diagnosis:** Elevated levels of BCAAs in plasma and the presence of **alloisoleucine** (pathognomonic).

Question 527: A 5-year-old child presents with abdominal distention and an enlarged liver on examination. The child has also experienced episodes of uncontrolled hypoglycemia and ketosis. What is the most probable diagnosis?

A. Diabetes mellitus
B. Glycogen storage diseases (Correct Answer)
C. Lipid storage disorders
D. Mucopolysaccharidoses

Explanation: ### Explanation The clinical presentation of **hepatomegaly (enlarged liver)** combined with **fasting hypoglycemia and ketosis** is a classic triad for **Glycogen Storage Diseases (GSDs)**, specifically Type I (von Gierke disease) or Type III (Cori disease). **1. Why Glycogen Storage Diseases (GSD) is correct:** In GSDs, the body cannot mobilize glucose from glycogen stores in the liver. In **Type I GSD**, the deficiency of *Glucose-6-phosphatase* prevents the final step of both glycogenolysis and gluconeogenesis. This leads to: * **Hepatomegaly:** Excessive accumulation of glycogen in hepatocytes. * **Hypoglycemia:** Inability to maintain blood glucose during fasting. * **Ketosis:** The body shifts to fat oxidation to provide alternative fuel (ketone bodies) because glucose is unavailable. **2. Why other options are incorrect:** * **Diabetes Mellitus:** While it presents with ketosis (DKA), it is characterized by **hyperglycemia**, not hypoglycemia. Hepatomegaly is not a primary feature. * **Lipid Storage Disorders (e.g., Gaucher disease):** These present with massive hepatosplenomegaly, but they **do not** typically cause hypoglycemia or ketosis, as glucose metabolism remains intact. * **Mucopolysaccharidoses (e.g., Hurler syndrome):** These involve coarse facial features, skeletal deformities, and organomegaly, but metabolic crises like hypoglycemia are absent. **3. NEET-PG High-Yield Pearls:** * **Von Gierke (Type I):** Associated with hyperuricemia (gout), hyperlipidemia, and lactic acidosis. "Doll-like" facies is a common descriptor. * **Cori Disease (Type III):** Deficiency of Debranching enzyme; similar to Type I but with **normal lactate** levels. * **Pompe Disease (Type II):** "Acid maltase" deficiency; primarily affects the **heart** (cardiomegaly) rather than causing hypoglycemia. * **Key differentiator:** If a question mentions hypoglycemia **without** ketosis, think of Fatty Acid Oxidation defects (like MCAD deficiency).

Question 528: What is the most common type of Porphyria?

A. Variegate porphyria
B. Porphyria cutanea tarda (Correct Answer)
C. Congenital erythropoietic porphyria
D. Acute intermittent porphyria

Explanation: **Explanation:** **Porphyria Cutanea Tarda (PCT)** is the most common type of porphyria worldwide. It results from a deficiency of the enzyme **Uroporphyrinogen decarboxylase (UROD)**. Unlike many other porphyrias, PCT is unique because it is most frequently **acquired (Type I)**, often triggered by factors like alcohol consumption, Hepatitis C, smoking, or iron overload, though a familial form (Type II) also exists. Clinically, it presents with photosensitivity and fluid-filled blisters on sun-exposed areas (hands and face). **Analysis of Incorrect Options:** * **Acute Intermittent Porphyria (AIP):** This is the most common **acute** (neurovisceral) porphyria, but not the most common overall. It is caused by a deficiency of Porphobilinogen deaminase. * **Variegate Porphyria:** This is a mixed porphyria (cutaneous and neurological symptoms) caused by Protoporphyrinogen oxidase deficiency. It is rare globally but has a high prevalence in South Africa due to a founder effect. * **Congenital Erythropoietic Porphyria (Gunther disease):** An extremely rare autosomal recessive disorder caused by Uroporphyrinogen III synthase deficiency, characterized by severe mutilating photosensitivity and erythrodontia (red teeth). **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Defect in PCT:** Uroporphyrinogen decarboxylase. * **Urine Finding:** Urine shows "tea-colored" or "port-wine" discoloration and fluoresces **coral pink** under Wood’s lamp due to uroporphyrins. * **Key Association:** Strongly associated with **Hepatitis C** and **Iron Overload** (Hereditary Hemochromatosis). * **Treatment:** Low-dose chloroquine/hydroxychloroquine or therapeutic phlebotomy to reduce iron stores.

Question 529: Which of the following statements is true regarding Gilbert syndrome?

A. Reduced activity of glucoronyl transferase
B. Causes indirect hyperbilirubinemia
C. Does not require any treatment
D. All of the above (Correct Answer)

Explanation: **Explanation:** Gilbert syndrome is a common, benign autosomal recessive hereditary disorder of bilirubin metabolism. It is characterized by intermittent episodes of mild jaundice, typically triggered by stress, fasting, or illness. **Why Option D is correct:** * **Reduced activity of glucuronyl transferase (Option A):** The primary defect is a mutation in the promoter region of the **UGT1A1 gene**, leading to a reduction (approximately 70–80% decrease) in the activity of the enzyme **UDP-glucuronosyltransferase**. * **Causes indirect hyperbilirubinemia (Option B):** Because the enzyme responsible for conjugating bilirubin with glucuronic acid is deficient, unconjugated (indirect) bilirubin accumulates in the blood. There is no hemolysis or underlying liver disease. * **Does not require any treatment (Option C):** Gilbert syndrome is a harmless condition. The bilirubin levels rarely exceed 3–4 mg/dL, and it does not lead to liver damage or fibrosis. Therefore, reassurance is the management of choice. **Clinical Pearls for NEET-PG:** 1. **Diagnosis:** It is often a diagnosis of exclusion. A key diagnostic feature is the **"Fasting Test"**—bilirubin levels rise significantly after a 24-hour fast. 2. **Phenobarbital:** This drug can be used to induce the UGT1A1 enzyme and lower bilirubin levels, though it is rarely clinically necessary. 3. **Crigler-Najjar Syndrome:** Contrast this with Crigler-Najjar Type I (total absence of enzyme, fatal) and Type II (severe deficiency, <10% activity). 4. **Protective Effect:** Interestingly, mild hyperbilirubinemia in Gilbert syndrome may provide a reduced risk of cardiovascular diseases due to the antioxidant properties of bilirubin.

Question 530: DNA repair defect is seen in which of the following conditions?

A. Xeroderma pigmentosum (Correct Answer)
B. Li-Fraumeni syndrome
C. Retinoblastoma
D. None of the above

Explanation: **Explanation:** **1. Why Xeroderma Pigmentosum (XP) is correct:** Xeroderma pigmentosum is the classic example of a **DNA repair defect**. It is an autosomal recessive disorder caused by a deficiency in **Nucleotide Excision Repair (NER)**. Under normal conditions, NER enzymes (specifically UV-specific endonucleases) identify and excise **pyrimidine dimers** (usually thymine dimers) formed by exposure to ultraviolet (UV) radiation. In XP patients, these dimers cannot be repaired, leading to extreme photosensitivity, hyperpigmentation, and a significantly increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). **2. Why the other options are incorrect:** * **Li-Fraumeni Syndrome:** This is caused by a germline mutation in the **TP53 gene** (a tumor suppressor gene). While TP53 is involved in the cell cycle and apoptosis, the primary defect is in the "guardian of the genome" signaling rather than the direct enzymatic machinery of DNA repair. * **Retinoblastoma:** This is caused by a mutation in the **RB1 gene** on chromosome 13. It is a cell cycle regulation defect (specifically at the G1/S checkpoint) rather than a DNA repair defect. **Clinical Pearls for NEET-PG:** * **Other DNA Repair Defects:** * **Mismatch Repair (MMR):** Lynch Syndrome (HNPCC). * **Homologous Recombination:** BRCA 1/2 (Breast/Ovarian cancer) and Fanconi Anemia. * **Double-Strand Break Repair:** Ataxia-telangiectasia (ATM gene). * **High-Yield Fact:** XP patients are often referred to as "Children of the Night" because they must avoid all sunlight. * **Enzyme involved in XP:** UV-specific endonuclease (excisionase).

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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