Genetics and Disease Practice Questions

Q: Which of the following conditions is not a single gene disorder?

Dilated cardiomyopathy. **Explanation** The correct answer is **Dilated Cardiomyopathy (DCM)**. In the context of genetic inheritance, a **single-gene (monogenic) disorder** is caused by a mutation in one specific gene. While DCM can be familial (occurring in 30-50% of cases), it is predominantly considered a **genetically heterogeneous** and often **multifactorial** condition [1]. It can be triggered by a wide array of non-genetic factors including viral myocarditis, toxins (alcohol, chemotherapy), nutritional deficiencies, and pregnancy [1]. Even when genetic, it often involves complex interactions between multiple loci or environmental triggers, unlike the classic monogenic patterns seen in the other options. **Analysis of Incorrect Options:** * **Hypertrophic Cardiomyopathy (HCM):** This is the quintessential single-gene disorder of the heart, usually inherited in an **Autosomal Dominant** pattern [2]. It is primarily a "disease of the sarcomere," commonly involving mutations in the *MYH7* or *MYBPC3* genes [2]. * **Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC):** This is a monogenic "disease of the desmosome." It is typically Autosomal Dominant, involving genes like *PKP2* (Plakophilin-2). * **Mitral Valve Prolapse (MVP):** While often sporadic, familial MVP is recognized as a monogenic trait (e.g., *DZIP1* gene) or associated with single-gene connective tissue disorders like Marfan Syndrome (*FBN1*). **NEET-PG High-Yield Pearls:** * **HCM:** Most common cause of sudden cardiac death (SCD) in young athletes. * **DCM:** The most common indication for cardiac transplantation. * **ARVC:** Characterized by fibrofatty replacement of the right ventricular myocardium; look for the **Epsilon wave** on ECG. * **Naxos Disease:** An autosomal recessive variant of ARVC associated with woolly hair and palmoplantar keratoderma (mutation in *JUP* gene).

Q: Mutation in Marfan's syndrome is in which gene?

Fibrillin I. **Explanation:** **Marfan Syndrome** is an autosomal dominant connective tissue disorder caused by a mutation in the **FBN1 gene** located on **chromosome 15q21**. This gene encodes **Fibrillin-1**, a large glycoprotein that serves as a structural component of calcium-binding microfibrils. These microfibrils form a scaffold for elastin deposition and regulate **TGF-β (Transforming Growth Factor beta)** signaling. A deficiency in Fibrillin-1 leads to weakened connective tissue and excessive TGF-β activation, resulting in the classic skeletal, ocular, and cardiovascular manifestations [1]. **Analysis of Incorrect Options:** * **Fibrillin II (Option B):** Mutations in the *FBN2* gene (chromosome 5) cause **Congenital Contractural Arachnodactyly (Beals Syndrome)**. While it shares skeletal features with Marfan syndrome (like long limbs), it is characterized by "crumpled" ears and joint contractures rather than aortic involvement. * **Collagen I (Option C):** Mutations here are associated with **Osteogenesis Imperfecta** (brittle bone disease) and certain types of Ehlers-Danlos Syndrome (EDS). * **Collagen IV (Option D):** This is a major component of the basement membrane. Mutations lead to **Alport Syndrome** (hereditary nephritis and sensorineural deafness). **High-Yield Clinical Pearls for NEET-PG:** * **Cardiovascular:** Aortic root dilation and **dissection** (most common cause of death) and Mitral Valve Prolapse (MVP). * **Ocular:** **Ectopia lentis** (dislocation of the lens), typically **upward and outward** (superior-temporal) [1]. * **Skeletal:** Arachnodactyly (positive Wrist and Thumb signs), Pectus excavatum/carinatum, and increased arm span to height ratio (>1.05). * **Differentiating Factor:** Unlike Homocystinuria (which also has Marfanoid habitus), Marfan syndrome patients have **normal intellect** and an upward lens dislocation (Homocystinuria is downward).

Q: Which of the following genes is not associated with Autosomal Dominant Tubulointerstitial Kidney Disease?

NPHS2. **Autosomal Dominant Tubulointerstitial Kidney Disease (ADTKD)** is a group of rare genetic disorders characterized by progressive interstitial fibrosis, tubular atrophy, and eventual end-stage renal disease (ESRD), typically with a bland urinary sediment and minimal proteinuria. **Why NPHS2 is the correct answer:** **NPHS2** encodes **Podocin**, a critical protein located in the slit diaphragm of the glomerular podocyte. Mutations in *NPHS2* are associated with **Autosomal Recessive Steroid-Resistant Nephrotic Syndrome (SRNS)** and Focal Segmental Glomerulosclerosis (FSGS) [1]. Unlike ADTKD, which is a primary tubulointerstitial pathology, *NPHS2* mutations cause a primary glomerular pathology. **Analysis of Incorrect Options:** * **MUC1 (ADTKD-MUC1):** Caused by a frameshift mutation in the *MUC1* gene. It is a classic cause of ADTKD, often presenting with slowly progressive renal failure. * **HNF1B (ADTKD-HNF1B):** Mutations in *Hepatocyte Nuclear Factor-1β* lead to a multi-system syndrome. It is unique among ADTKD types as it often presents with **renal cysts**, maturity-onset diabetes of the young (**MODY5**), and genitourinary malformations. * **UMOD (ADTKD-UMOD):** The most common cause of ADTKD. It involves mutations in the *Uromodulin* gene (encoding Tamm-Horsfall protein). A key clinical hallmark is **early-onset hyperuricemia and gout**, often preceding the decline in GFR. **High-Yield Clinical Pearls for NEET-PG:** * **ADTKD Hallmark:** Positive family history + Bland urine sediment (no casts/blood) + Normal-sized kidneys on ultrasound (except in HNF1B). * **UMOD Mutation:** Think "Gout in a teenager or young adult with CKD." * **HNF1B Mutation:** Think "Renal cysts + Diabetes (MODY5)." * **REN Mutation:** Another cause of ADTKD, often associated with anemia and mild hyperkalemia in childhood.

Q: Adrenomyeloneuropathy, a variant of adrenoleukodystrophy which manifests as adrenal insufficiency beginning in childhood and later develops into a progressive spastic paraparesis, is transmitted as:

X-linked disorder. **Explanation:** **Adrenoleukodystrophy (ALD)** and its adult-onset variant, **Adrenomyeloneuropathy (AMN)**, are caused by mutations in the ***ABCD1* gene** located on the **X chromosome**. This gene encodes the adrenoleukodystrophy protein (ALDP), a peroxisomal membrane transporter required for the breakdown of **Very Long Chain Fatty Acids (VLCFA)**. 1. **Why X-linked is correct:** The *ABCD1* mutation leads to the accumulation of VLCFAs in the plasma and tissues, specifically the adrenal cortex and the white matter of the CNS. Because the gene is located on the X chromosome, the disease follows an **X-linked recessive** inheritance pattern. It primarily affects males, while female carriers may develop milder neurological symptoms later in life. 2. **Why other options are incorrect:** * **Autosomal Dominant/Recessive:** Peroxisomal biogenesis disorders like Zellweger Syndrome are autosomal recessive, but ALD/AMN is strictly linked to the X chromosome. * **Sporadic:** While *de novo* mutations can occur, the condition is a heritable genetic disorder with a predictable transmission pattern. **Clinical Pearls for NEET-PG:** * **Classic Presentation:** A young boy with behavioral changes, vision loss, and skin hyperpigmentation (due to adrenal insufficiency/Addison’s disease). * **AMN Variant:** Presents in the 20s-40s with progressive spastic paraparesis and sphincter dysfunction. * **Diagnosis:** Elevated plasma levels of **VLCFA** (most specific test). * **MRI Finding:** Symmetrical enhancement of the posterior white matter (occipito-parietal region). * **Treatment:** Dietary restriction is ineffective; Hematopoietic Stem Cell Transplant (HSCT) is the treatment of choice in early stages.

Q: What is the most common cause of death in Menke disease?

Diaphragmatic paralysis. **Explanation:** **Menkes Disease** (Kinky Hair Syndrome) is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**, which leads to impaired copper absorption and transport. This results in a systemic copper deficiency, affecting copper-dependent enzymes like lysyl oxidase and cytochrome c oxidase. **Why Diaphragmatic Paralysis is Correct:** The most common cause of death in Menkes disease is respiratory failure, specifically due to **diaphragmatic paralysis** or severe pulmonary infections. The underlying mechanism involves progressive neurodegeneration and muscular hypotonia. Copper is a vital cofactor for mitochondrial function; its deficiency leads to ATP depletion in motor neurons and muscle fibers, eventually resulting in the failure of the respiratory muscles (diaphragm). **Analysis of Incorrect Options:** * **A. Cardiac Rupture:** While connective tissue defects occur due to lysyl oxidase deficiency (leading to arterial tortuosity and aneurysms), spontaneous cardiac rupture is not a characteristic feature or the primary cause of death. * **C. Laryngeal Spasm:** This is typically associated with hypocalcemia (tetany) or acute airway obstruction, not the chronic neuro-muscular decline seen in Menkes. * **D. Cyano-methemoglobinemia:** This is a toxicological emergency unrelated to copper metabolism or the ATP7A mutation. **High-Yield Clinical Pearls for NEET-PG:** * **Genetics:** X-linked Recessive; **ATP7A** mutation (vs. ATP7B in Wilson disease). * **Pathognomonic Sign:** **Pili torti** (twisted, brittle, "kinky" hair) seen under light microscopy. * **Biochemical Markers:** Low serum copper and low serum ceruloplasmin. * **Key Enzyme Defect:** **Lysyl oxidase** deficiency leads to defective collagen cross-linking (causing skin laxity and vascular issues). * **Prognosis:** Usually fatal by age 3 without early copper histidine treatment.

Q: Which of the following syndromes is/are associated with mitochondrial inheritance?

All of the above. **Explanation:** Mitochondrial inheritance (maternal inheritance) occurs because mitochondria are transmitted exclusively through the oocyte [1]. These disorders typically affect tissues with high energy requirements, such as the brain, heart, and skeletal muscles [1]. **Why "All of the above" is correct:** * **MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes):** This is the most common mitochondrial disorder. It is characterized by stroke-like episodes before age 40, seizures, and lactic acidosis. * **NARP (Neurogenic muscle weakness, Ataxia, and Retinitis Pigmentosa):** This syndrome is caused by mutations in the *MT-ATP6* gene. It presents with sensory tremors, proximal muscle weakness, and pigmentary retinopathy [1]. * **Leigh’s Syndrome (Subacute Necrotizing Encephalomyelopathy):** This is a severe neurological disorder typically manifesting in infancy. While it can be inherited via nuclear DNA (autosomal recessive), the **mitochondrial DNA (mtDNA) mutation** (often involving the same *MT-ATP6* gene as NARP) is a classic and frequently tested cause. **High-Yield Clinical Pearls for NEET-PG:** 1. **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (mutated vs. normal mtDNA) within a cell. This explains the clinical variability in mitochondrial diseases. 2. **Maternal Inheritance:** An affected mother passes the trait to **all** her children, but an affected father passes it to **none** [1]. 3. **Ragged Red Fibers:** On muscle biopsy (Gomori trichrome stain), these are a hallmark of mitochondrial myopathies (especially MERRF). 4. **Other common examples:** MERRF (Myoclonic Epilepsy with Ragged Red Fibers) and LHON (Leber’s Hereditary Optic Neuropathy).

Q: Which of the following is an autosomal dominant disease?

MEN 2. **Explanation:** The correct answer is **MEN 2 (Multiple Endocrine Neoplasia Type 2)**. **1. Why MEN 2 is Correct:** MEN 2 is an **Autosomal Dominant (AD)** disorder caused by a gain-of-function mutation in the **RET proto-oncogene** on chromosome 10. In AD inheritance, only one copy of the mutated gene is required for the disease to manifest, and it typically appears in every generation. MEN 2 is characterized by Medullary Thyroid Carcinoma (100%), Pheochromocytoma, and Parathyroid hyperplasia (MEN 2A) or Mucosal neuromas/Marfanoid habitus (MEN 2B). **2. Why the other options are incorrect:** * **Wilson’s Disease:** This is an **Autosomal Recessive (AR)** disorder involving a mutation in the *ATP7B* gene on chromosome 13. It leads to impaired biliary copper excretion and toxic accumulation in the liver and basal ganglia. * **Hemochromatosis:** Hereditary Hemochromatosis (most commonly the *HFE* gene mutation, C282Y) is an **Autosomal Recessive** condition. It results in excessive intestinal iron absorption and deposition in organs (liver, heart, pancreas). **3. Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Most structural protein defects and "gain-of-function" mutations (like MEN 2, Achondroplasia, Marfan syndrome, Huntington's) are **Autosomal Dominant**. * **Rule of Thumb:** Most enzyme deficiencies and metabolic disorders (like Wilson’s, Hemochromatosis, Alkaptonuria, Glycogen Storage Diseases) are **Autosomal Recessive**. * **High-Yield Fact:** Prophylactic thyroidectomy is recommended in MEN 2 carriers due to the 100% penetrance of Medullary Thyroid Carcinoma.

Q: Locus heterogeneity is a feature of which of the following conditions?

Osteogenesis imperfecta. **Explanation:** **Locus heterogeneity** occurs when mutations at different genetic loci (different genes) produce the same clinical phenotype. **Why Osteogenesis Imperfecta (OI) is the correct answer:** OI is a classic example of locus heterogeneity. While the majority of cases (Type I-IV) are caused by mutations in the **COL1A1** (Chromosome 17) or **COL1A2** (Chromosome 7) genes, several other types of OI result from mutations in entirely different genes involved in collagen processing, such as **CRTAP** or **P3H1**. Regardless of which gene is mutated, the clinical outcome is brittle bones and skeletal deformities. **Analysis of Incorrect Options:** * **A. Familial Adenomatous Polyposis (FAP):** This condition is primarily caused by mutations in a single gene, the **APC gene** (Chromosome 5q21). It exhibits **allelic heterogeneity** (different mutations within the same gene), but not locus heterogeneity. * **B. MEN (Multiple Endocrine Neoplasia):** MEN syndromes are site-specific. MEN1 is caused by mutations in the *MEN1* gene, and MEN2 is caused by mutations in the *RET* proto-oncogene. Each specific syndrome is linked to a single locus. **High-Yield Clinical Pearls for NEET-PG:** 1. **Locus Heterogeneity Examples:** Osteogenesis Imperfecta, Retinitis Pigmentosa, Albinism, and Sensorineural hearing loss. 2. **Allelic Heterogeneity:** Different mutations in the *same* gene (e.g., Cystic Fibrosis, Beta-thalassemia). 3. **Pleiotropy:** A single gene mutation causing multiple, seemingly unrelated phenotypic effects (e.g., Marfan Syndrome affecting eyes, heart, and skeleton). 4. **Phenocopy:** An environmentally induced phenotype that mimics a genetic condition (e.g., Vitamin D deficiency mimicking Rickets).

Q: Fredrich's ataxia is caused by which type of mutation?

Expanded trinucleotide repeat. **Explanation:** **Friedreich's Ataxia (FRDA)** is an autosomal recessive neurodegenerative disorder. The correct answer is **Expanded Trinucleotide Repeat** because the disease is caused by an unstable expansion of the **GAA** triplet repeat in the first intron of the **FXN gene** on chromosome 9. This expansion leads to transcriptional silencing (epigenetic knockdown) of the gene, resulting in a deficiency of the protein **Frataxin**. Frataxin is essential for mitochondrial iron metabolism; its deficiency leads to iron overload in the mitochondria, causing oxidative stress and cell death, primarily in the dorsal root ganglia, spinal cord, and heart. **Analysis of Incorrect Options:** * **A & C (Point and Missense Mutations):** While point mutations (including missense) can occur in the FXN gene, they account for less than 5% of cases. The hallmark and primary cause (95%+) for NEET-PG purposes is the triplet repeat expansion. * **D (Inversion):** Inversions involve a segment of DNA being reversed end-to-end. This is the classic mechanism for **Hemophilia A** (Factor VIII gene inversion), not Friedreich's Ataxia. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive (unique among most trinucleotide repeat disorders like Huntington’s or Myotonic Dystrophy, which are dominant). * **Repeat Sequence:** **GAA** (Mnemonic: Friedreich is **G**reat **A**t **A**taxia). * **Clinical Triad:** Progressive limb ataxia, loss of position/vibration sense, and absent deep tendon reflexes (especially ankle jerk). * **Most Common Cause of Death:** **Hypertrophic Cardiomyopathy** (HOCM) leading to heart failure or arrhythmias. * **Associated Findings:** Kyphoscoliosis, Pes Cavus (high-arched feet), and Diabetes Mellitus (in ~10-20% of patients).

Q: Non-frameshift mutation of the dystrophin gene causes which of the following conditions?

Becker's Muscular Dystrophy. **Explanation:** The correct answer is **Becker’s Muscular Dystrophy (BMD)**. The underlying medical concept is the **"Reading Frame Hypothesis."** 1. **Why BMD is correct:** Becker’s Muscular Dystrophy is caused by **non-frameshift mutations** (typically in-frame deletions). Because the reading frame is preserved, the cell can still produce a truncated but **partially functional** dystrophin protein. [1] This results in a milder clinical phenotype, later onset (usually >15 years), and a longer life expectancy compared to Duchenne. 2. **Why other options are incorrect:** * **Duchenne Muscular Dystrophy (DMD):** Caused by **frameshift mutations** (or nonsense mutations) that completely disrupt the reading frame. [1] This leads to a total absence of functional dystrophin, resulting in severe, progressive muscle wasting starting in early childhood. * **Myotonic Dystrophy:** This is an autosomal dominant condition caused by **triplet repeat expansions** (CTG repeats in the *DMPK* gene for Type 1), not mutations in the dystrophin gene. * **Limb Girdle Dystrophy (LGMD):** This is a heterogeneous group of disorders caused by mutations in genes encoding **sarcoglycans** [2] or other structural proteins, rather than the dystrophin gene itself. **High-Yield Clinical Pearls for NEET-PG:** * **Dystrophin Gene:** The largest known human gene (located on Xp21), making it highly susceptible to spontaneous mutations. [1] * **Gower’s Sign:** Classic clinical finding in DMD/BMD due to proximal muscle weakness. * **Diagnosis:** Gold standard is genetic testing; Muscle biopsy shows "patchy" dystrophin staining in BMD and "absent" staining in DMD. * **Cause of Death:** Most commonly due to dilated cardiomyopathy or respiratory failure.

Question 1

Which of the following conditions is not a single gene disorder?

Accepted Answer

Dilated cardiomyopathy

Answer

Arrhythmogenic right ventricular cardiomyopathy

Answer

Hypertrophic cardiomyopathy

Answer

Mitral valve prolapse

Question 2

Mutation in Marfan's syndrome is in which gene?

Accepted Answer

Fibrillin I

Answer

Fibrillin II

Answer

Collagen I

Answer

Collagen IV

Question 3

Which of the following genes is not associated with Autosomal Dominant Tubulointerstitial Kidney Disease?

Accepted Answer

NPHS2

Answer

MUC1

Answer

HNF1b

Answer

UMOD

Question 4

Adrenomyeloneuropathy, a variant of adrenoleukodystrophy which manifests as adrenal insufficiency beginning in childhood and later develops into a progressive spastic paraparesis, is transmitted as:

Accepted Answer

X-linked disorder

Answer

Autosomal dominant

Answer

Autosomal recessive

Answer

Sporadic

Question 5

What is the most common cause of death in Menke disease?

Accepted Answer

Diaphragmatic paralysis

Answer

Cardiac rupture

Answer

Laryngeal spasm

Answer

Cyano-methemoglobinemia

Question 6

Which of the following syndromes is/are associated with mitochondrial inheritance?

Accepted Answer

All of the above

Answer

Leigh's syndrome

Answer

NARP syndrome

Answer

MELAS syndrome

Question 7

Which of the following is an autosomal dominant disease?

Accepted Answer

MEN 2

Answer

Wilson's disease

Answer

Hemochromatosis

Answer

All of the above

Question 8

Locus heterogeneity is a feature of which of the following conditions?

Accepted Answer

Osteogenesis imperfecta

Answer

Familial adenomatous polyposis

Answer

MENU

Answer

All of the above

Question 9

Fredrich's ataxia is caused by which type of mutation?

Accepted Answer

Expanded trinucleotide repeat

Answer

Point mutation

Answer

Missense mutation

Answer

Inversion

Question 10

Non-frameshift mutation of the dystrophin gene causes which of the following conditions?

Accepted Answer

Becker's Muscular Dystrophy

Answer

Duchenne Muscular Dystrophy

Answer

Myotonic Dystrophy

Answer

Limb Girdle Dystrophy

Genetics and Disease — MCQs

Genetics and Disease — MCQs

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