Genetic Disorders and Biochemical Pathology Practice Questions

Q: Which mucopolysaccharidosis (MPS) typically presents without corneal clouding?

Hunter's disease. **Explanation:** The Mucopolysaccharidoses (MPS) are a group of lysosomal storage disorders caused by the deficiency of enzymes required to break down glycosaminoglycans (GAGs). **1. Why Hunter’s Disease is the Correct Answer:** **Hunter’s disease (MPS II)** is unique among the mucopolysaccharidoses for two primary reasons: * **Inheritance:** It is the only MPS that is **X-linked recessive** (all others are autosomal recessive). * **Clinical Presentation:** It typically presents **without corneal clouding**. Instead, patients may present with clear corneas but may develop retinal degeneration. * **Biochemical Basis:** It is caused by a deficiency of **Iduronate-2-sulfatase**, leading to the accumulation of heparan sulfate and dermatan sulfate. **2. Why the Other Options are Incorrect:** * **Hurler’s Disease (MPS IH):** This is the most severe form (deficiency of $\alpha$-L-iduronidase). Corneal clouding is a hallmark feature and often appears early in life. * **Sly Syndrome (MPS VII):** Caused by $\beta$-glucuronidase deficiency. It presents with a wide clinical spectrum, but corneal clouding is a common finding. * **Maroteaux-Lamy Syndrome (MPS VI):** Caused by Arylsulfatase B deficiency. While intelligence is often normal, physical symptoms are severe, and corneal clouding is prominent. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hunter’s:** "The **Hunter** needs **clear vision** (no corneal clouding) to hit the **X** (X-linked) on the target." * **Aggressive Behavior:** Unlike Hurler’s, Hunter’s syndrome is often associated with aggressive behavior and hyperactivity. * **Pebbly Skin:** Look for "pebbly" skin lesions (papules) over the scapula or upper arms, which are pathognomonic for Hunter’s. * **Diagnosis:** All MPS types show increased urinary excretion of GAGs. Definitive diagnosis is via enzyme assay or genetic testing.

Q: Mitochondrial DNA (mt-DNA) is known for all except:

Nemaline myopathy results due to mutations in mt-DNA. ### Explanation **Why Option D is the Correct Answer:** Nemaline myopathy is **not** a mitochondrial disorder. It is a heterogeneous group of congenital myopathies characterized by the presence of "nemaline rods" (thread-like structures) in muscle fibers. It is caused by mutations in genes encoding proteins of the **skeletal muscle thin filaments** (e.g., *NEB, ACTA1, TPM3*). These mutations follow **Autosomal Dominant or Recessive** inheritance patterns, rather than mitochondrial inheritance. **Analysis of Other Options:** * **A. Maternal inheritance:** This is a hallmark of mt-DNA. During fertilization, the sperm's mitochondria are typically degraded; thus, all mitochondria in the zygote are derived from the oocyte. * **B. Heteroplasmy:** This refers to the presence of a mixture of more than one type of organellar genome (normal and mutated mt-DNA) within a single cell. The severity of mitochondrial diseases often depends on the ratio of mutant to wild-type DNA. * **C. Leber Hereditary Optic Neuropathy (LHON):** This is the classic prototype of mitochondrial diseases. It is characterized by bilateral, painless subacute visual loss due to mutations in genes encoding subunits of Complex I (NADH dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Bottleneck Effect:** Explains the variability in the percentage of mutant mt-DNA passed from mother to offspring. * **Threshold Effect:** Clinical symptoms of mitochondrial diseases appear only when the proportion of mutated mt-DNA exceeds a specific limit. * **Common Mitochondrial Disorders:** MERRF (Myoclonic Epilepsy with Ragged Red Fibers), MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes), and Kearns-Sayre Syndrome. * **Histology:** "Ragged Red Fibers" on Gomori trichrome stain are characteristic of mitochondrial myopathies due to compensatory proliferation of mitochondria.

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

Hereditary hypercholesterolemia. **Explanation:** The correct answer is **Hereditary Hypercholesterolemia** (specifically Familial Hypercholesterolemia). **1. Why it is correct:** Most metabolic disorders (inborn errors of metabolism) are inherited in an **autosomal recessive** pattern because a 50% reduction in enzyme activity in heterozygotes is usually sufficient for normal function. However, **Familial Hypercholesterolemia (FH)** is a notable exception. It is an **autosomal dominant** disorder caused by mutations in the **LDL receptor gene**. In this case, a 50% reduction in receptors (heterozygous state) is insufficient to clear LDL from the plasma, leading to premature atherosclerosis and xanthomas. **2. Why the other options are incorrect:** * **Tay-Sachs Disease:** An autosomal recessive lysosomal storage disorder caused by a deficiency of **Hexosaminidase A**, leading to GM2 ganglioside accumulation. * **Gaucher’s Disease:** The most common lysosomal storage disorder, inherited in an **autosomal recessive** manner, caused by a deficiency of **glucocerebrosidase**. * **Tyrosinemia:** A group of **autosomal recessive** metabolic errors in the phenylalanine-tyrosine catabolic pathway (e.g., deficiency of fumarylacetoacetate hydrolase in Type I). **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Autosomal Dominant Metabolic Disorders:** Remember **"H-A-P-P-Y"**: **H**ereditary Spherocytosis/Hypercholesterolemia, **A**cute Intermittent Porphyria, **P**olycystic Kidney Disease (ADPKD), **P**seudohypoparathyroidism, **Y** (and others like Huntington’s). * **Key Clinical Feature of FH:** Tendon xanthomas (especially the Achilles tendon) and xanthelasma. * **Rule of Thumb:** Most enzyme deficiencies are Recessive; most structural protein or receptor defects are Dominant. **Acute Intermittent Porphyria** is another high-yield metabolic exception that is Autosomal Dominant.

Q: Mousy odour of urine is characteristically seen in which of the following conditions?

Phenylketonuria. **Explanation:** **Phenylketonuria (PKU)** is the correct answer. It is an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)** or its cofactor, **tetrahydrobiopterin (BH4)**. This deficiency leads to the accumulation of phenylalanine, which is alternatively metabolized into phenylketones such as **phenylacetate, phenylpyruvate, and phenyllactate**. The characteristic **"mousy" or "musty" odor** of urine is specifically attributed to the presence of **phenylacetate**. **Analysis of Incorrect Options:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the branched-chain alpha-keto acid dehydrogenase complex. It leads to a characteristic **burnt sugar or maple syrup odor** due to the accumulation of isoleucine. * **Isovaleric Aciduria:** A defect in isovaleryl-CoA dehydrogenase leads to the accumulation of isovaleric acid, resulting in a distinct **"sweaty feet" or "cheesy" odor**. * **Cystinuria:** A transport defect of COAL (Cystine, Ornithine, Arginine, Lysine) in the proximal renal tubule. It typically presents with renal stones and does not have a specific diagnostic odor, though some describe a faint **sulfurous** smell. **High-Yield Clinical Pearls for NEET-PG:** * **PKU Triad:** Intellectual disability, "mousy" odor, and hypopigmentation (due to decreased melanin synthesis from tyrosine). * **Guthrie Test:** A classic bacterial inhibition assay used for neonatal screening of PKU. * **Maternal PKU:** If a mother with PKU does not maintain a low-phenylalanine diet during pregnancy, the fetus may develop microcephaly and congenital heart defects (teratogenic effect). * **Other Odors:** * *Tyrosinemia:* Cabbage-like or rancid butter odor. * *Trimethylaminuria:* Fishy odor. * *Multiple Carboxylase Deficiency:* Tomcat urine odor.

Q: Defects in protein folding result in which of the following clinical diseases?

Kuru. **Explanation:** **Correct Option: A. Kuru** Kuru is a human prion disease caused by the **misfolding of proteins**. Prion diseases (Transmissible Spongiform Encephalopathies) occur when the normal cellular prion protein ($PrP^C$), which is rich in alpha-helices, undergoes a conformational change into a pathological, beta-sheet-rich isoform called $PrP^{Sc}$ (Scrapie). This misfolded protein is resistant to proteolysis, aggregates into amyloid plaques, and induces further misfolding of healthy proteins, leading to neurodegeneration. Kuru was historically associated with ritualistic cannibalism among the Fore people of Papua New Guinea. **Incorrect Options:** * **B. Migraine:** This is a complex neurovascular disorder primarily involving trigeminal nerve activation and cortical spreading depression, not protein misfolding. * **C. Hypothyroidism:** Most commonly caused by iodine deficiency or autoimmune destruction (Hashimoto’s thyroiditis), resulting in hormone deficiency rather than proteopathy. * **D. Myopia:** A refractive error of the eye typically caused by an increased axial length of the eyeball or excessive corneal curvature. **High-Yield Clinical Pearls for NEET-PG:** * **Other Protein Misfolding Diseases:** Alzheimer’s (Amyloid-$\beta$ and Tau), Parkinson’s ($\alpha$-synuclein), Huntington’s (Huntingtin), and Creutzfeldt-Jakob Disease (Prion). * **Molecular Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that normally prevent misfolding by assisting in the correct folding of nascent polypeptide chains. * **Prion Characteristics:** They are unique infectious agents because they lack nucleic acids (DNA/RNA) and are highly resistant to standard sterilization methods like boiling or UV radiation.

Q: A few days after birth, a child becomes lethargic, refuses feeds, and vomits during breastfeeding. On examination, he is pot-bellied. Benedict's test is positive in urine. What is the likely reducing substance present in urine?

Galactose. ### Explanation The clinical presentation describes a classic case of **Classic Galactosemia**, typically caused by a deficiency of the enzyme **Galactose-1-phosphate uridyltransferase (GALT)**. **1. Why Galactose is the correct answer:** Symptoms of galactosemia appear shortly after birth once the infant begins breastfeeding (milk contains **lactose**, which is hydrolyzed into glucose and **galactose**). The accumulation of galactose and its metabolites leads to liver dysfunction (hepatomegaly/pot-bellied appearance), vomiting, and lethargy. Because galactose is a **reducing sugar**, it reacts positively with **Benedict’s reagent**. However, since the defect is in galactose metabolism and not glucose regulation, the urine dipstick (which uses glucose oxidase) would be negative, while the Benedict's test is positive. **2. Why the other options are incorrect:** * **Glucose:** While glucose is a reducing sugar, it is typically associated with Diabetes Mellitus. In this neonatal context, the specific trigger of breastfeeding points toward milk sugar (galactose). * **Fructose:** Fructosemia (Hereditary Fructose Intolerance) presents only after the introduction of weaning foods (fruit juices/sucrose), not during exclusive breastfeeding. * **Sucrose:** Sucrose is a **non-reducing sugar** and would yield a negative Benedict's test. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Most common is GALT (Classic Galactosemia). * **Key Findings:** Hepatomegaly, **Oil-drop cataracts** (due to galactitol accumulation in the lens), and intellectual disability if untreated. * **Infection Risk:** These infants are at a significantly increased risk of **E. coli neonatal sepsis**. * **Diagnosis:** Positive Benedict's test (reducing sugar) + Negative Glucose oxidase test (dipstick). * **Management:** Immediate cessation of breast milk; switch to soy-based or lactose-free formula.

Q: Cells cultured from patients with a certain autosomal recessive genetic disorder exhibit low activity for the nucleotide excision repair process. This disorder is characterized by marked sensitivity to sunlight (ultraviolet light), leading to the formation of multiple skin cancers and premature death. What is this disorder?

Xeroderma Pigmentosa. ### Explanation **Correct Option: C. Xeroderma Pigmentosa (XP)** Xeroderma Pigmentosa is an autosomal recessive disorder caused by a defect in the **Nucleotide Excision Repair (NER)** pathway. * **Mechanism:** UV light causes the formation of **pyrimidine dimers** (usually thymine dimers) in DNA. In healthy individuals, the NER pathway (involving UV-specific endonucleases) identifies and removes these dimers. In XP patients, this repair mechanism is deficient. * **Clinical Presentation:** This leads to extreme photosensitivity, severe sunburn with minimal exposure, and a 1000-fold increased risk of developing cutaneous malignancies (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma) at a very young age. **Why Incorrect Options are Wrong:** * **A. Acute Intermittent Porphyria:** A metabolic disorder of heme biosynthesis due to **Porphobilinogen deaminase** deficiency. It presents with abdominal pain and neuropsychiatric symptoms but, unlike other porphyrias, it does **not** feature photosensitivity. * **B. Alkaptonuria:** A disorder of tyrosine metabolism due to **Homogentisate oxidase** deficiency. It is characterized by ochronosis (darkening of connective tissues) and urine that turns black upon standing, not DNA repair defects. * **D. Ataxia-Telangiectasia:** While this also involves DNA repair, the defect is in the **ATM gene**, which is responsible for repairing **double-stranded DNA breaks** (non-homologous end joining), not NER. It presents with cerebellar ataxia and telangiectasia. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficient in XP:** UV-specific endonuclease (excisionase). * **Inheritance:** Autosomal Recessive. * **Key Association:** "Children of the Night" (must avoid all sunlight). * **Other DNA Repair Disorders:** * **Lynch Syndrome:** Mismatch repair (MSH2, MLH1). * **Bloom Syndrome/Werner Syndrome:** DNA Helicase. * **Fanconi Anemia:** DNA cross-link repair.

Q: Hepatosplenomegaly with foam cells is characteristically seen in a lysosomal disorder. This disorder is due to the deficiency of which enzyme?

Sphingomyelinase. **Explanation:** The clinical presentation of **hepatosplenomegaly** combined with the presence of **foam cells** (lipid-laden macrophages) is the hallmark of **Niemann-Pick Disease (Types A and B)**. This autosomal recessive lysosomal storage disorder is caused by a deficiency of the enzyme **Sphingomyelinase**, leading to the accumulation of sphingomyelin within the lysosomes of the reticuloendothelial system. **Analysis of Options:** * **A. Sphingomyelinase (Correct):** Its deficiency leads to Niemann-Pick Disease. The characteristic "foam cells" are macrophages with a "mulberry" or soap-bubble appearance due to accumulated sphingomyelin. * **B. Hexosaminidase A:** Deficiency causes **Tay-Sachs Disease**. While it features a cherry-red spot on the macula, it is distinguished by the **absence** of hepatosplenomegaly. * **C. Beta-galactosidase:** Deficiency causes **Krabbé disease** (characterized by globoid cells and demyelination) or **GM1 gangliosidosis**. * **D. Beta-glucosidase (Glucocerebrosidase):** Deficiency causes **Gaucher Disease**. While it also presents with hepatosplenomegaly, the characteristic cells are **Gaucher cells** (described as "wrinkled tissue paper" or "crumpled silk" appearance), not foam cells. **NEET-PG High-Yield Pearls:** * **Niemann-Pick vs. Tay-Sachs:** Both can have a **cherry-red spot**, but only Niemann-Pick has **hepatosplenomegaly**. * **Mnemonic:** "No-man Picks (Niemann-Pick) his nose with his **Foamy** finger." * **Zebra Bodies:** Electron microscopy in Niemann-Pick shows lamellated cytosolic inclusions called zebra bodies. * **Type A vs. B:** Type A is the infantile, neuropathic form; Type B presents later with no CNS involvement.

Question 1

Which mucopolysaccharidosis (MPS) typically presents without corneal clouding?

Accepted Answer

Hunter's disease

Answer

Hurler's disease

Answer

Sly syndrome

Answer

Maroteaux-Lamy syndrome

Question 2

Mitochondrial DNA (mt-DNA) is known for all except:

Accepted Answer

Nemaline myopathy results due to mutations in mt-DNA

Answer

Maternal inheritance

Answer

Heteroplasmy

Answer

Leber hereditary optic neuropathy is the prototype

Question 3

Which type of glycogen storage disease predominantly involves muscle?

Accepted Answer

Type V

Answer

Type I

Answer

Type III

Answer

Type IV

Question 4

Which of the following is an autosomal dominant metabolic disorder?

Accepted Answer

Hereditary hypercholesterolemia

Answer

Tay-Sachs disease

Answer

Gaucher's disease

Answer

Tyrosinemia

Question 5

Mousy odour of urine is characteristically seen in which of the following conditions?

Accepted Answer

Phenylketonuria

Answer

Maple syrup urine disease

Answer

Isovaleric aciduria

Answer

Cystinuria

Question 6

Metabolic abnormalities in which jaundice is seen are all of the following except?

Accepted Answer

Von Gierke disease

Answer

Galactosemia

Answer

Tyrosinemia

Answer

Hereditary fructose intolerance

Question 7

Defects in protein folding result in which of the following clinical diseases?

Accepted Answer

Kuru

Answer

Migraine

Answer

Hypothyroidism

Answer

Myopia

Question 8

A few days after birth, a child becomes lethargic, refuses feeds, and vomits during breastfeeding. On examination, he is pot-bellied. Benedict's test is positive in urine. What is the likely reducing substance present in urine?

Accepted Answer

Galactose

Answer

Glucose

Answer

Fructose

Answer

Sucrose

Question 9

Cells cultured from patients with a certain autosomal recessive genetic disorder exhibit low activity for the nucleotide excision repair process. This disorder is characterized by marked sensitivity to sunlight (ultraviolet light), leading to the formation of multiple skin cancers and premature death. What is this disorder?

Accepted Answer

Xeroderma Pigmentosa

Answer

Acute intermittent Porphyria

Answer

Alkaptonuria

Answer

Ataxia - Telangiectasia

Question 10

Hepatosplenomegaly with foam cells is characteristically seen in a lysosomal disorder. This disorder is due to the deficiency of which enzyme?

Accepted Answer

Sphingomyelinase

Answer

Hexosaminidase

Answer

Beta-galactosidase

Answer

Beta-glucosidase

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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