Genetic Disorders and Biochemical Pathology Practice Questions

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

Chromosome 13. **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.

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

Duchenne muscular dystrophy. **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.

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

Maple syrup urine disease. ### 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).

Q: What is the most common type of Porphyria?

Porphyria cutanea tarda. **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.

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

All of the above. **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.

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

Xeroderma pigmentosum. **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).

Question 1

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

Accepted Answer

Type 3

Answer

Typed

Answer

Type 2

Answer

None of the above

Question 2

Which of the following chromosomes is involved in Patau syndrome?

Accepted Answer

Chromosome 13

Answer

Chromosome 18

Answer

Chromosome 21

Answer

Chromosome 5

Question 3

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

Accepted Answer

Loss of HCO3-

Answer

Loss of Ca++

Answer

Loss of K+

Answer

All of the above

Question 4

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?

Accepted Answer

Hexose monophosphate shunt

Answer

Malic enzyme

Answer

Glycolysis

Answer

Gluconeogenesis

Question 5

Dystrophic gene mutation leads to which of the following conditions?

Accepted Answer

Duchenne muscular dystrophy

Answer

Myasthenia gravis

Answer

Motor neuron disease

Answer

Poliomyelitis

Question 6

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

Accepted Answer

Maple syrup urine disease

Answer

Hartnup disease

Answer

Alkaptonuria

Answer

GM-1 gangliosidosis

Question 7

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?

Accepted Answer

Glycogen storage diseases

Answer

Diabetes mellitus

Answer

Lipid storage disorders

Answer

Mucopolysaccharidoses

Question 8

What is the most common type of Porphyria?

Accepted Answer

Porphyria cutanea tarda

Answer

Variegate porphyria

Answer

Congenital erythropoietic porphyria

Answer

Acute intermittent porphyria

Question 9

Which of the following statements is true regarding Gilbert syndrome?

Accepted Answer

All of the above

Answer

Reduced activity of glucoronyl transferase

Answer

Causes indirect hyperbilirubinemia

Answer

Does not require any treatment

Question 10

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

Accepted Answer

Xeroderma pigmentosum

Answer

Li-Fraumeni syndrome

Answer

Retinoblastoma

Answer

None of the above

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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