Genetic Disorders and Biochemical Pathology — MCQs

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?

Q520Easy

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

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

Question 511: Which mucopolysaccharidosis (MPS) typically presents without corneal clouding?

A. Hurler's disease
B. Hunter's disease (Correct Answer)
C. Sly syndrome
D. Maroteaux-Lamy syndrome

Explanation: **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.

Question 512: Mitochondrial DNA (mt-DNA) is known for all except:

A. Maternal inheritance
B. Heteroplasmy
C. Leber hereditary optic neuropathy is the prototype
D. Nemaline myopathy results due to mutations in mt-DNA (Correct Answer)

Explanation: ### 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.

Question 513: Which type of glycogen storage disease predominantly involves muscle?

A. Type I
B. Type III
C. Type IV
D. Type V (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Type V - McArdle Disease)** Type V Glycogen Storage Disease (GSD) is caused by a deficiency of **muscle glycogen phosphorylase** (myophosphorylase). Since this enzyme is specific to skeletal muscle, the pathology is localized entirely to the musculature. Patients cannot break down glycogen into glucose-1-phosphate during exercise, leading to ATP depletion. Clinically, this manifests as exercise intolerance, muscle cramps, and **myoglobinuria** (burgundy-colored urine) after strenuous activity. A hallmark finding is the **"second wind" phenomenon**, where patients improve after a few minutes of exercise once the body switches to using fatty acids as fuel. **Incorrect Options:** * **Type I (von Gierke Disease):** Involves a deficiency of **Glucose-6-Phosphatase**. It primarily affects the **liver and kidneys**. Since muscle lacks this enzyme naturally, it is not the primary site of pathology; instead, patients present with severe fasting hypoglycemia and hepatomegaly. * **Type III (Cori Disease):** Caused by a deficiency of the **Debranching enzyme**. While it can involve muscle (causing mild myopathy), it is primarily characterized by **hepatomegaly** and growth retardation. It is often described as a milder version of Type I but with normal lactate levels. * **Type IV (Andersen Disease):** Caused by a deficiency of the **Branching enzyme**. This leads to the accumulation of abnormal glycogen (polyglucosan) which triggers an immune response, primarily causing **progressive liver cirrhosis** and heart failure, rather than isolated muscle symptoms. **High-Yield NEET-PG Pearls:** * **Ischemic Forearm Exercise Test:** In McArdle disease, this test shows a **failure of blood lactate to rise**, but a significant rise in ammonia. * **Mnemonic:** **M**cArdle = **M**uscle **M**yophosphorylase. * **Type II (Pompe Disease):** The only GSD that is also a **Lysosomal Storage Disease** (Acid Maltase deficiency), affecting the heart (Cardiomegaly).

Question 514: Which of the following is an autosomal dominant metabolic disorder?

A. Hereditary hypercholesterolemia (Correct Answer)
B. Tay-Sachs disease
C. Gaucher's disease
D. Tyrosinemia

Explanation: **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.

Question 515: Mousy odour of urine is characteristically seen in which of the following conditions?

A. Maple syrup urine disease
B. Phenylketonuria (Correct Answer)
C. Isovaleric aciduria
D. Cystinuria

Explanation: **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.

Question 516: Metabolic abnormalities in which jaundice is seen are all of the following except?

A. Von Gierke disease (Correct Answer)
B. Galactosemia
C. Tyrosinemia
D. Hereditary fructose intolerance

Explanation: ### Explanation The key to solving this question lies in distinguishing between metabolic disorders that cause **acute hepatocellular injury** versus those that primarily cause **metabolic storage** without early liver failure. **1. Why Von Gierke Disease (GSD Type I) is the correct answer:** Von Gierke disease is caused by a deficiency of **Glucose-6-Phosphatase**. While it leads to massive hepatomegaly (due to accumulation of glycogen and fat), it **does not typically cause jaundice** or acute liver failure. The primary clinical features are profound fasting hypoglycemia, lactic acidosis, hyperuricemia, and hyperlipidemia. The hepatocytes remain functional in terms of bilirubin conjugation and excretion. **2. Why the other options are incorrect (They DO cause jaundice):** * **Galactosemia (Galactose-1-phosphate uridyltransferase deficiency):** Accumulation of Galactose-1-P is toxic to hepatocytes. It presents early in the neonatal period with jaundice, hepatomegaly, and cataracts upon starting milk feeds. * **Hereditary Fructose Intolerance (Aldolase B deficiency):** Accumulation of Fructose-1-P causes intracellular ATP depletion. This leads to acute liver cell damage, resulting in jaundice, vomiting, and hypoglycemia after ingestion of fructose or sucrose. * **Tyrosinemia (Type I - Fumarylacetoacetate hydrolase deficiency):** The accumulation of toxic metabolites (like succinylacetone) causes severe oxidative damage to the liver, leading to progressive liver failure, cirrhosis, and **jaundice** in infancy. ### NEET-PG High-Yield Pearls * **"Toxic Metabolite" Rule:** In Galactosemia, HFI, and Tyrosinemia, the phosphorylated intermediate (Gal-1-P, Fru-1-P) or organic acid is directly toxic to the liver, causing cell death and jaundice. * **Von Gierke Clinical Triad:** Doll-like facies, massive hepatomegaly, and "hypoglycemic seizures." * **HFI vs. Galactosemia:** HFI symptoms start when **fruit juice/sucrose** is introduced (weaning); Galactosemia starts with **breast milk/formula** (neonatal). * **Tyrosinemia Marker:** Elevated **Succinylacetone** in urine is pathognomonic.

Question 517: Defects in protein folding result in which of the following clinical diseases?

A. Kuru (Correct Answer)
B. Migraine
C. Hypothyroidism
D. Myopia

Explanation: **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.

Question 518: 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?

A. Glucose
B. Fructose
C. Sucrose
D. Galactose (Correct Answer)

Explanation: ### 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.

Question 519: 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?

A. Acute intermittent Porphyria
B. Alkaptonuria
C. Xeroderma Pigmentosa (Correct Answer)
D. Ataxia - Telangiectasia

Explanation: ### 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.

Question 520: Hepatosplenomegaly with foam cells is characteristically seen in a lysosomal disorder. This disorder is due to the deficiency of which enzyme?

A. Sphingomyelinase (Correct Answer)
B. Hexosaminidase
C. Beta-galactosidase
D. Beta-glucosidase

Explanation: **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.

Practice by Chapter

Single Gene Disorders

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Biochemical Diagnosis of Genetic Disorders

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Inborn Errors of Metabolism

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Lysosomal Storage Diseases

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Glycogen Storage Diseases

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Disorders of Lipoprotein Metabolism

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Disorders of Purine and Pyrimidine Metabolism

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Hemoglobinopathies

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Porphyrias

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Biochemical Markers for Disease Diagnosis

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Newborn Screening for Genetic Disorders

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Enzyme Replacement Therapy

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