Genetic Disorders and Biochemical Pathology — MCQs
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What is the most common group of diseases following Mendelian inheritance?
Which gene is NOT associated with Diabetes Mellitus?
A 2-year-old boy presents with developmental delay, vomiting, irritability, and a skin rash. His mother reports a peculiar 'mousy' odor in his urine. Physical examination reveals an eczema-like rash, hyperreflexia, and increased muscle tone. The child has a notably fair complexion compared to his family. What is the most likely diagnosis?
Sandhoff's disease is due to the absence of which enzyme?
AST/ALT ratio > 2 is seen in deficiency of which enzyme?
In hemochromatosis, in which organ is iron NOT typically deposited?
'I' cells disease is due to a defect in which cellular organelle?
Which anticonvulsant is considered safe in patients with hepatic porphyria?
A child with increased conjugated bilirubin develops seizures and cataract. What is the probable diagnosis?
The gene involved in Rett's syndrome is:
Genetic Disorders and Biochemical Pathology Indian Medical PG Practice Questions and MCQs
Question 391: What is the most common group of diseases following Mendelian inheritance?
- A. Autosomal dominant (Correct Answer)
- B. Autosomal recessive
- C. X-linked dominant
- D. X-linked recessive
Explanation: **Explanation:** The correct answer is **Autosomal dominant (AD)**. In the context of Mendelian (monogenic) inheritance, autosomal dominant disorders represent the most common group of inherited diseases. This is primarily because AD traits are expressed in the heterozygous state; an individual only needs one copy of the mutant allele to manifest the disease. **Why Autosomal Dominant is correct:** Statistically, AD disorders occur more frequently in the general population compared to recessive ones. Many AD conditions involve mutations in genes encoding **structural proteins** (e.g., Collagen in Osteogenesis Imperfecta) or **regulatory proteins/receptors** (e.g., LDL receptor in Familial Hypercholesterolemia). Because these structural defects are often compatible with life until reproductive age, the alleles persist in the gene pool. **Why other options are incorrect:** * **Autosomal recessive (AR):** While AR disorders (like Cystic Fibrosis or Sickle Cell Anemia) are numerous, they require two copies of the mutant allele to manifest. This makes the clinical phenotype less frequent than AD conditions. AR disorders typically involve **enzymatic proteins**. * **X-linked recessive (XLR):** These are less common because they primarily affect males. Females are usually asymptomatic carriers. * **X-linked dominant (XLD):** This is the rarest form of Mendelian inheritance (e.g., Vitamin D-resistant rickets). **High-Yield Clinical Pearls for NEET-PG:** * **Most common AD disorder:** Familial Hypercholesterolemia. * **Most common AR disorder (Global):** Thalassemia. * **Key Concept:** AD disorders often show **pleiotropy** (one gene affecting multiple systems) and **variable expressivity** (varying severity among individuals). * **Rule of Thumb:** Structural protein defect → Autosomal Dominant; Enzyme deficiency → Autosomal Recessive.
Question 392: Which gene is NOT associated with Diabetes Mellitus?
- A. PPARy
- B. KCNJ11
- C. CTLA4
- D. PDGF-R (Correct Answer)
Explanation: **Explanation:** The correct answer is **PDGF-R (Platelet-Derived Growth Factor Receptor)**. This receptor is primarily involved in cell proliferation, wound healing, and angiogenesis. While it plays a role in diabetic complications (like proliferative retinopathy or atherosclerosis), the gene itself is not a primary genetic locus associated with the etiology of Diabetes Mellitus. **Analysis of Options:** * **PPARγ (Peroxisome Proliferator-Activated Receptor gamma):** This is a nuclear receptor that regulates fatty acid storage and glucose metabolism. Mutations or polymorphisms in the *PPARG* gene are strongly linked to Type 2 Diabetes and insulin resistance. It is also the pharmacological target for Thiazolidinediones (e.g., Pioglitazone). * **KCNJ11:** This gene encodes the Kir6.2 subunit of the ATP-sensitive potassium channel in pancreatic beta cells. Mutations in this gene are a well-known cause of **Neonatal Diabetes Mellitus** and are associated with an increased risk of Type 2 Diabetes. * **CTLA4 (Cytotoxic T-Lymphocyte Associated Protein 4):** This is an immune checkpoint molecule. Polymorphisms in the *CTLA4* gene are associated with susceptibility to **Type 1 Diabetes Mellitus**, as it plays a critical role in T-cell mediated autoimmunity. **High-Yield Clinical Pearls for NEET-PG:** * **MODY (Maturity-Onset Diabetes of the Young):** Most common gene involved is **HNF1A** (MODY 3), followed by **Glucokinase** (MODY 2). * **HLA Association:** Type 1 DM is strongly linked to **HLA-DR3 and HLA-DR4**. * **Wolfram Syndrome:** A rare genetic cause of diabetes characterized by **DIDMOAD** (Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy, and Deafness).
Question 393: A 2-year-old boy presents with developmental delay, vomiting, irritability, and a skin rash. His mother reports a peculiar 'mousy' odor in his urine. Physical examination reveals an eczema-like rash, hyperreflexia, and increased muscle tone. The child has a notably fair complexion compared to his family. What is the most likely diagnosis?
- A. Tay-Sachs disease
- B. Mcardle disease
- C. Phenylketonuria (Correct Answer)
- D. Pyruvate dehydrogenase deficiency
Explanation: **Explanation:** The clinical presentation is classic for **Phenylketonuria (PKU)**, an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)** or its cofactor **tetrahydrobiopterin (BH4)**. **Why Phenylketonuria is correct:** * **Biochemical Pathogenesis:** The enzyme deficiency leads to the accumulation of phenylalanine and its alternative metabolites (phenylketones like phenylacetate and phenyllactate). * **Mousy Odor:** Phenylacetate excretion in sweat and urine produces the characteristic "mousy" or "musty" odor. * **Fair Complexion:** Phenylalanine is a precursor to tyrosine. Reduced tyrosine levels lead to decreased **melanin** synthesis, resulting in fair skin, blonde hair, and blue eyes. * **Neurological Symptoms:** High phenylalanine levels are neurotoxic, causing developmental delay, seizures, and hyperreflexia. **Why other options are incorrect:** * **Tay-Sachs Disease:** Characterized by a "cherry-red spot" on the macula and progressive neurodegeneration due to Hexosaminidase A deficiency, but lacks the mousy odor and skin hypopigmentation. * **McArdle Disease:** A glycogen storage disease (Type V) affecting muscles. It presents with exercise-induced cramps and myoglobinuria, not developmental delay or skin changes. * **Pyruvate Dehydrogenase Deficiency:** Leads to chronic lactic acidosis and neurological impairment, but does not present with the specific odor or pigmentary changes seen here. **High-Yield Clinical Pearls for NEET-PG:** * **Screening:** Guthrie test (bacterial inhibition assay) is used for neonatal screening. * **Management:** Dietary restriction of phenylalanine and supplementation of tyrosine. * **Maternal PKU:** If a mother with PKU doesn't maintain a strict diet during pregnancy, the fetus may develop microcephaly and congenital heart defects (teratogenic effect).
Question 394: Sandhoff's disease is due to the absence of which enzyme?
- A. Beta-hexosaminidase (Correct Answer)
- B. Beta-glucuronidase
- C. Aryl sulphatase
- D. Alpha galactosidase
Explanation: **Explanation:** **Sandhoff’s Disease** is a lysosomal storage disorder (specifically a GM2 gangliosidosis) caused by a deficiency in **Beta-hexosaminidase**. The enzyme Beta-hexosaminidase exists in two major isoforms: * **Hexosaminidase A:** Composed of $\alpha$ and $\beta$ subunits. * **Hexosaminidase B:** Composed of two $\beta$ subunits. In Sandhoff’s disease, there is a mutation in the **HEXB gene**, which encodes the **$\beta$-subunit**. Consequently, **both Hexosaminidase A and B are deficient**, leading to the accumulation of GM2 gangliosides and globosides in the brain and other visceral organs. This distinguishes it from Tay-Sachs disease, where only Hexosaminidase A is deficient due to an $\alpha$-subunit mutation. **Analysis of Incorrect Options:** * **B. Beta-glucuronidase:** Deficiency leads to **Sly Syndrome** (Mucopolysaccharidosis VII), characterized by hepatosplenomegaly and skeletal deformities. * **C. Aryl sulphatase:** Deficiency of Aryl sulphatase A causes **Metachromatic Leukodystrophy**, leading to the accumulation of cerebroside sulfate. * **D. Alpha-galactosidase:** Deficiency leads to **Fabry Disease**, an X-linked disorder characterized by angiokeratomas, peripheral neuropathy, and renal failure. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Similar to Tay-Sachs (progressive neurodegeneration, **cherry-red spot** on macula, hyperacusis), but Sandhoff’s often presents with **hepatosplenomegaly** (visceral involvement), which is absent in Tay-Sachs. * **Inheritance:** Autosomal Recessive. * **Histology:** "Onion-skin" appearance of lysosomes due to whorled lamellar structures.
Question 395: AST/ALT ratio > 2 is seen in deficiency of which enzyme?
- A. G-6-phosphatase
- B. Branching enzyme
- C. Acid maltase (Correct Answer)
- D. Liver phosphorylase
Explanation: **Explanation:** The **AST/ALT ratio > 2** is a classic biochemical marker for **Pompe Disease (GSD Type II)**, caused by a deficiency of **Acid Maltase (α-1,4-glucosidase)**. **Why Acid Maltase is correct:** Unlike other Glycogen Storage Diseases (GSDs), Pompe disease is a lysosomal storage disorder. Acid maltase deficiency leads to glycogen accumulation in the lysosomes of **skeletal and cardiac muscles**. When these muscle cells are damaged, they release intracellular enzymes. While ALT is primarily liver-specific, **AST is found in high concentrations in both the liver and muscle**. In Pompe disease, the massive release of AST from damaged muscle tissue significantly outweighs the ALT levels, typically resulting in an AST/ALT ratio greater than 2. **Why other options are incorrect:** * **G-6-phosphatase (Von Gierke Disease):** Characterized by severe hypoglycemia, hyperuricemia, and lactic acidosis. While liver enzymes may be elevated, the ratio does not typically exceed 2 as the pathology is primarily hepatic. * **Branching enzyme (Andersen Disease):** Leads to the accumulation of abnormal glycogen (amylopectin-like). It presents with early liver cirrhosis, but the AST/ALT ratio is not a defining feature. * **Liver phosphorylase (Hers Disease):** A milder form of GSD affecting the liver; it does not cause the significant muscle breakdown required to skew the AST/ALT ratio. **High-Yield Clinical Pearls for NEET-PG:** * **Pompe Disease mnemonic:** "Pompe trashes the **Pump** (heart)." It is the only GSD that presents with **Hypertrophic Cardiomyopathy**. * **Biochemical marker:** Elevated **CPK (Creatine Phosphokinase)** and **LDH** are also seen due to muscle involvement. * **Diagnosis:** PAS-positive material in lysosomes; "lace-like" appearance of glycogen on biopsy. * **Note:** Outside of GSDs, an AST/ALT ratio > 2 is most commonly associated with **Alcoholic Liver Disease**.
Question 396: In hemochromatosis, in which organ is iron NOT typically deposited?
- A. Heart
- B. Pituitary
- C. Testis (Correct Answer)
- D. Skin
Explanation: In **Hereditary Hemochromatosis**, an autosomal recessive disorder of iron metabolism, excessive iron is deposited in various parenchymal organs as **hemosiderin**, leading to tissue damage and fibrosis. ### Why Testis is the Correct Answer While hemochromatosis frequently causes **hypogonadotropic hypogonadism**, the primary site of iron deposition is the **Pituitary gland** (specifically the gonadotroph cells), not the testes themselves. Testicular atrophy occurs as a secondary phenomenon due to the lack of FSH and LH stimulation. Therefore, the testis is not a typical site for direct iron deposition. ### Analysis of Incorrect Options * **Heart (A):** Iron deposits in the myocardium, leading to **Restrictive Cardiomyopathy** (early stage) or **Dilated Cardiomyopathy** (late stage) and arrhythmias. * **Pituitary (B):** The anterior pituitary is a major site of deposition. Damage to gonadotrophs leads to decreased libido and impotence, a hallmark of the clinical triad. * **Skin (D):** Iron deposition in the dermis, combined with increased melanin production, results in the characteristic "slate-gray" or metallic pigmentation, contributing to the term **"Bronze Diabetes."** ### High-Yield Clinical Pearls for NEET-PG * **Classic Triad:** Cirrhosis, Diabetes Mellitus ("Bronze Diabetes"), and Skin Pigmentation. * **Most Common Cause:** Mutation in the **HFE gene** (C282Y mutation on Chromosome 6). * **Joint Involvement:** Characterized by **CPPD (Pseudogout)**, typically involving the 2nd and 3rd metacarpophalangeal joints. * **Stain:** **Prussian Blue** stain is used to visualize hemosiderin (iron) in biopsy samples. * **Treatment of Choice:** Therapeutic phlebotomy.
Question 397: 'I' cells disease is due to a defect in which cellular organelle?
- A. Peroxisome
- B. Mitochondria
- C. Lysosome
- D. Golgi apparatus (Correct Answer)
Explanation: **Explanation:** **I-cell disease (Inclusion Cell Disease)**, also known as Mucolipidosis II, is a rare lysosomal storage disorder. The primary defect lies in the **Golgi apparatus**, specifically involving the enzyme **UDP-N-acetylglucosamine-1-phosphotransferase**. 1. **Why Golgi apparatus is correct:** In a healthy cell, the Golgi apparatus tags acid hydrolases (lysosomal enzymes) with **Mannose-6-Phosphate (M6P)**. This tag acts as a "postal address," directing enzymes to the lysosomes. In I-cell disease, the deficiency of phosphotransferase means enzymes are not phosphorylated. Consequently, they are constitutively secreted into the extracellular space instead of being sent to lysosomes. This results in empty lysosomes and the accumulation of undigested substrates as "inclusion bodies" (I-cells) within the cytoplasm. 2. **Why other options are incorrect:** * **Peroxisome:** Defects here lead to disorders like Zellweger Syndrome or Adrenoleukodystrophy, involving long-chain fatty acid metabolism. * **Mitochondria:** Defects typically result in energy-failure syndromes (e.g., MELAS, MERFF) due to impaired oxidative phosphorylation. * **Lysosome:** While the *symptoms* manifest in the lysosome (due to lack of enzymes), the *primary defect* is the sorting machinery located in the Golgi apparatus. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmark:** High levels of lysosomal enzymes in the **plasma/serum** but absent within the cells. * **Clinical Presentation:** Coarse facial features, gingival hyperplasia, craniofacial abnormalities, joint contractures, and severe psychomotor retardation. * **Inheritance:** Autosomal Recessive. * **Key Enzyme:** Phosphotransferase (Golgi-resident).
Question 398: Which anticonvulsant is considered safe in patients with hepatic porphyria?
- A. Clonazepam (Correct Answer)
- B. Paraldehyde
- C. Phenytoin
- D. Carbamazepine
Explanation: **Explanation:** The primary biochemical trigger for acute attacks in **Hepatic Porphyrias** (such as Acute Intermittent Porphyria) is the induction of the enzyme **ALA Synthase-1 (ALAS1)**. Most anticonvulsants induce the Cytochrome P450 (CYP450) system, which increases the demand for heme. This depletion of the regulatory heme pool removes the feedback inhibition on ALAS1, leading to the toxic accumulation of porphyrin precursors like ALA and PBG. * **Why Clonazepam is Correct:** Benzodiazepines, specifically **Clonazepam** and **Lorazepam**, are considered safe because they do not significantly induce the CYP450 system or ALAS1. They are the drugs of choice for managing seizures in porphyric patients. * **Why Incorrect Options are Wrong:** * **Phenytoin and Carbamazepine:** These are potent **enzyme inducers**. They significantly stimulate ALAS1 and are notorious for precipitating life-threatening porphyric crises. * **Paraldehyde:** While historically used, it is generally avoided as it is considered potentially porphyrinogenic in various clinical databases. **High-Yield Clinical Pearls for NEET-PG:** * **Safe Anticonvulsants:** Clonazepam, Gabapentin, Levetiracetam, and Magnesium Sulfate (for eclampsia). * **Unsafe Drugs (Porphyria Triggers):** Barbiturates (most common trigger), Sulfonamides, Estrogens, Griseofulvin, and Rifampin. * **Management Tip:** The definitive treatment for an acute porphyria attack is **Intravenous Hematin (Heme arginate)**, which provides exogenous heme to suppress ALAS1 through negative feedback. Glucose loading (high carbohydrate diet) also helps inhibit ALAS1.
Question 399: A child with increased conjugated bilirubin develops seizures and cataract. What is the probable diagnosis?
- A. Tyrosinemia
- B. Fructosemia
- C. Galactosemia (Correct Answer)
- D. Glycogen storage disorder
Explanation: **Explanation:** The clinical triad of **conjugated hyperbilirubinemia (jaundice), cataracts, and seizures** in an infant is a classic presentation of **Classic Galactosemia**, caused by a deficiency of the enzyme **Galactose-1-phosphate uridyltransferase (GALT)**. 1. **Why Galactosemia is correct:** * **Cataracts:** Excess galactose is diverted to the polyol pathway, where **aldose reductase** converts it into **galactitol**. Galactitol is osmotically active, causing water to enter the lens, leading to cataracts (often "oil-drop" appearance). * **Jaundice:** Accumulation of Galactose-1-phosphate is toxic to hepatocytes, leading to liver dysfunction and conjugated hyperbilirubinemia. * **Seizures:** These occur due to secondary **hypoglycemia**, as accumulated galactose-1-phosphate inhibits phosphoglucomutase, impairing glycogenolysis. 2. **Why other options are incorrect:** * **Tyrosinemia:** Presents with liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor, but cataracts are not a feature. * **Fructosemia (Hereditary Fructose Intolerance):** Presents with jaundice and hypoglycemia after introducing fruit juice/sucrose, but **does not cause cataracts** because fructose is not converted to a polyol in the lens. * **Glycogen Storage Disorders (GSD):** Primarily present with hepatomegaly and severe hypoglycemia (e.g., Von Gierke’s), but do not typically present with cataracts. **NEET-PG High-Yield Pearls:** * **Screening Test:** Benedict’s test is positive for reducing sugars in urine (but glucose oxidase dipstick is negative). * **Increased Risk:** Patients are highly susceptible to **E. coli neonatal sepsis**. * **Management:** Immediate withdrawal of lactose/galactose from the diet (switch to soy milk). * **Galactokinase deficiency:** Causes cataracts only, without the systemic "sick baby" features (no jaundice or seizures).
Question 400: The gene involved in Rett's syndrome is:
- A. P53
- B. Rb
- C. MeCP2 (Correct Answer)
- D. FGFR3
Explanation: ### Explanation **Correct Answer: C. MeCP2** **Mechanism:** Rett syndrome is an X-linked dominant neurodevelopmental disorder caused by mutations in the **MECP2 gene** (Methyl-CpG-binding protein 2), located on the X chromosome (Xq28). The MeCP2 protein is a transcriptional repressor that binds to methylated CpG islands in DNA. It recruits histone deacetylases (HDACs) to condense chromatin, effectively "silencing" specific genes. In Rett syndrome, the absence of functional MeCP2 leads to the failure of gene silencing, causing inappropriate expression of genes during brain development, which results in neuronal dysfunction. **Analysis of Incorrect Options:** * **A. P53:** Known as the "Guardian of the Genome," this is a tumor suppressor gene. Mutations are associated with **Li-Fraumeni syndrome** and various cancers, not neurodevelopmental disorders. * **B. Rb:** The Retinoblastoma gene is a tumor suppressor that regulates the G1-S phase transition of the cell cycle. Mutations lead to **Retinoblastoma** and osteosarcoma. * **D. FGFR3:** Fibroblast Growth Factor Receptor 3 mutations lead to **Achondroplasia** (the most common form of dwarfism) due to constitutive activation that inhibits chondrocyte proliferation. **High-Yield Clinical Pearls for NEET-PG:** * **Demographics:** Almost exclusively affects **females**; it is usually lethal in hemizygous males in utero. * **Clinical Presentation:** Characterized by a period of normal development (6–18 months) followed by regression of language and motor skills. * **Pathognomonic Sign:** Repetitive, **stereotypical hand-wringing** or "hand-washing" movements. * **Other Features:** Microcephaly, seizures, ataxia, and sighing/hyperventilation.
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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