Genetic Disorders and Biochemical Pathology — MCQs

A 12-year-old boy rapidly develops hypoglycemia after moderate activity. On physical examination, his kidneys and liver are found to be enlarged. Histopathology of the liver shows deposits of glycogen in excess. Blood examination reveals raised ketone bodies, lactic acid, and triglycerides. What is the diagnosis?

Q406Easy

Niemann-Pick disease is due to deficiency of which enzyme?

Q407Easy

Cleidocranial dysplasia is due to a defect in which gene?

Q408Easy

Beta-galactosidase deficiency is seen in which of the following conditions?

Q409Easy

A failure of synthesis of ceruloplasmin is seen in which of the following conditions?

Q410Easy

Lesh-Nyan syndrome is due to deficiency of which enzyme?

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

Question 401: A 5-year-old child presents with hepatomegaly, hypoglycemia, and growth failure. These signs are pathognomonic of which of the following conditions?

A. Diabetes Mellitus
B. Glycogen Storage Disease (Correct Answer)
C. Lipid Disorder
D. All of the above

Explanation: ### Explanation **1. Why Glycogen Storage Disease (GSD) is Correct:** The triad of **hepatomegaly, hypoglycemia, and growth failure** in a pediatric patient is a classic presentation of Glycogen Storage Diseases, most notably **Type I (von Gierke disease)**. * **Hepatomegaly:** Occurs due to the excessive accumulation of glycogen (which cannot be broken down) or fat in the liver. * **Hypoglycemia:** In GSD Type I, a deficiency in **Glucose-6-Phosphatase** prevents the liver from releasing glucose into the blood during fasting (impairing both glycogenolysis and gluconeogenesis). * **Growth Failure:** Chronic hypoglycemia and the redirection of glucose metabolites into alternative pathways (leading to lactic acidosis) result in stunted growth and a "doll-like" facial appearance. **2. Why Other Options are Incorrect:** * **Diabetes Mellitus:** Typically presents with hyperglycemia, polyuria, and polydipsia. While it can cause growth issues if poorly controlled, it does not present with fasting hypoglycemia or massive hepatomegaly as a primary feature. * **Lipid Disorders:** These usually present with xanthomas, corneal arcus, or pancreatitis (in hypertriglyceridemia). While some fatty acid oxidation disorders can cause hypoglycemia and hepatomegaly, the specific combination with profound growth failure and glycogen accumulation is more characteristic of GSD. **3. NEET-PG High-Yield Pearls:** * **GSD Type I (von Gierke):** Most common; look for **hyperuricemia** (gout), **hyperlipidemia**, and **lactic acidosis** in the clinical stem. * **GSD Type II (Pompe):** "Pompe trashes the Pump." Characterized by **cardiomegaly** and muscle weakness; notably, blood glucose levels are usually normal. * **GSD Type III (Cori):** Similar to Type I but milder, with **normal lactate** levels. * **GSD Type V (McArdle):** Affects skeletal muscle; look for exercise-induced cramps and **myoglobinuria**; no hypoglycemia.

Question 402: Which of the following genetic factors does NOT increase susceptibility and modify the severity of pancreatic injury in acute pancreatitis?

A. Cationic trypsinogen mutations
B. Pancreatic secretory trypsin inhibitor
C. CFTR
D. ATP7a gene mutation (Correct Answer)

Explanation: **Explanation:** The pathogenesis of acute pancreatitis often involves the premature activation of digestive enzymes (especially trypsin) within the pancreatic acini. Genetic factors that regulate trypsin activity or ductal secretion significantly influence disease susceptibility. **Why ATP7a is the correct answer:** The **ATP7a gene** encodes a copper-transporting ATPase. Mutations in this gene lead to **Menkes disease** (kinky hair syndrome), characterized by systemic copper deficiency. It has no established role in pancreatic enzyme regulation or ductal function. In contrast, mutations in **ATP7B** are associated with Wilson disease, which primarily affects the liver and basal ganglia, not the predisposition to acute pancreatitis. **Analysis of Incorrect Options:** * **Cationic trypsinogen (PRSS1) mutations:** These lead to "gain-of-function" mutations where trypsinogen is prematurely converted to trypsin or becomes resistant to inactivation, causing **Hereditary Pancreatitis**. * **Pancreatic secretory trypsin inhibitor (SPINK1):** SPINK1 acts as a "safety valve" by inhibiting small amounts of prematurely activated trypsin. Mutations (e.g., N34S) decrease this protective capacity, increasing injury severity. * **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator):** CFTR mutations result in thick, bicarbonate-poor pancreatic secretions. This leads to ductal obstruction and acinar injury, common in both Cystic Fibrosis and idiopathic chronic pancreatitis. **High-Yield Clinical Pearls for NEET-PG:** * **PRSS1:** Most common cause of autosomal dominant hereditary pancreatitis. * **SPINK1:** Associated with Tropical Calcific Pancreatitis in the Indian subcontinent. * **CASR (Calcium-Sensing Receptor):** Another genetic factor where mutations can predispose to pancreatitis by affecting calcium-mediated trypsinogen activation. * **Menkes Disease (ATP7a):** Look for "steely/kinky hair" and "hypothermia" in clinical vignettes.

Question 403: Which hormone levels are increased in Prader-Willi syndrome?

A. TSH
B. GHRH (Growth Hormone Releasing Hormone) (Correct Answer)
C. Growth hormone
D. Peptide YY

Explanation: **Explanation:** **Prader-Willi Syndrome (PWS)** is a complex genetic disorder caused by the loss of function of genes in the paternal copy of chromosome **15q11-q13**. The core pathophysiology involves **hypothalamic dysfunction**, which leads to multiple endocrine abnormalities. **Why GHRH is increased:** In PWS, there is a functional deficiency of **Growth Hormone (GH)**. This is not due to a primary pituitary defect, but rather a hypothalamic failure to appropriately stimulate the pituitary or a lack of feedback inhibition. Because GH and IGF-1 levels are chronically low, the hypothalamus attempts to compensate by increasing the secretion of **Growth Hormone Releasing Hormone (GHRH)**. Despite high GHRH, the GH pulse frequency and amplitude remain low, leading to short stature and increased fat mass. **Analysis of Incorrect Options:** * **TSH:** While hypothyroidism can occur in PWS due to hypothalamic-pituitary axis dysfunction, TSH levels are typically **low or inappropriately normal** (central hypothyroidism), not increased. * **Growth Hormone:** GH levels are characteristically **decreased** in PWS, which is why recombinant GH therapy is a standard treatment to improve height and body composition. * **Peptide YY:** This is an anorexigenic (appetite-suppressing) hormone. In PWS, Peptide YY levels are typically **decreased**, contributing to the hallmark hyperphagia and lack of satiety. **NEET-PG High-Yield Pearls:** * **Genetics:** Most common cause is **paternal deletion** (70%), followed by **maternal uniparental disomy** (25-30%). * **Ghrelin:** PWS is associated with significantly **elevated Ghrelin** levels (the "hunger hormone"), which drives the insatiable appetite. * **Clinical Triad:** Hypotonia (infancy), hyperphagia/obesity (childhood), and hypogonadism. * **Diagnostic Test:** DNA methylation analysis is the gold standard.

Question 404: What is the most common type of Class II mutation in cystic fibrosis?

A. Missense mutation
B. Frameshift
C. Nonsense
D. Deletion (Correct Answer)

Explanation: **Explanation:** Cystic Fibrosis (CF) is caused by mutations in the **CFTR gene** (Chromosome 7). These mutations are categorized into six classes based on the functional defect of the protein. **Why Deletion is Correct:** The most common mutation worldwide (found in ~70% of cases) is the **ΔF508 mutation**. This is a **three-nucleotide deletion** resulting in the loss of the amino acid phenylalanine at position 508. This specific mutation falls under **Class II**, where the protein is synthesized but fails to fold correctly. Consequently, it is retained in the endoplasmic reticulum and degraded by the proteasome, never reaching the cell membrane. **Analysis of Incorrect Options:** * **Missense Mutation:** While missense mutations occur in CF (e.g., G551D), they typically belong to **Class III** (defective gating/regulation), where the protein reaches the membrane but does not open properly. * **Frameshift & Nonsense Mutations:** These generally belong to **Class I** (defective synthesis). They result in premature stop codons, leading to a complete lack of CFTR protein production. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Diagnosis:** Sweat Chloride Test (>60 mEq/L) is the gold standard. * **Organ Involvement:** Recurrent *Pseudomonas* infections, pancreatic insufficiency (steatorrhea), and bilateral absence of vas deferens (infertility). * **Newer Drugs:** **Lumacaftor** acts as a "chaperone" specifically for Class II (ΔF508) mutations to help the protein reach the surface.

Question 405: A 12-year-old boy rapidly develops hypoglycemia after moderate activity. On physical examination, his kidneys and liver are found to be enlarged. Histopathology of the liver shows deposits of glycogen in excess. Blood examination reveals raised ketone bodies, lactic acid, and triglycerides. What is the diagnosis?

A. Von Gierke's disease (Correct Answer)
B. Pompe's disease
C. McArdle's disease
D. Cori's disease

Explanation: ### Explanation The clinical presentation of **hepatomegaly, nephromegaly, hypoglycemia, and metabolic derangements** (lactic acidosis, hyperlipidemia, and ketosis) is classic for **Von Gierke’s Disease (GSD Type I)**. **1. Why Von Gierke’s Disease is Correct:** Von Gierke’s is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is the final step for both glycogenolysis and gluconeogenesis. Its absence prevents the liver from releasing glucose into the blood, leading to severe **fasting hypoglycemia**. * **Hepatomegaly/Nephromegaly:** Excess Glucose-6-Phosphate (G6P) cannot be converted to glucose; it is instead shunted into glycogen synthesis, causing massive storage in the liver and kidneys. * **Lactic Acidosis:** Excess G6P enters the glycolytic pathway, producing pyruvate and subsequently lactate. * **Hyperlipidemia/Ketosis:** Hypoglycemia triggers fat mobilization, leading to increased triglycerides and ketone bodies. **2. Why Other Options are Incorrect:** * **Pompe’s Disease (Type II):** Caused by lysosomal acid maltase deficiency. It primarily affects the **heart** (massive cardiomegaly) and muscles. Blood glucose levels are typically normal. * **McArdle’s Disease (Type V):** Caused by skeletal muscle glycogen phosphorylase deficiency. It presents with muscle cramps and myoglobinuria after exercise; it does **not** cause hypoglycemia or hepatomegaly. * **Cori’s Disease (Type III):** Caused by debranching enzyme deficiency. While it presents with hepatomegaly and hypoglycemia, **lactic acid levels are normal** because gluconeogenesis remains intact. **3. NEET-PG High-Yield Pearls:** * **"Doll-like facies"** (fatty cheeks) is a common physical descriptor for Type I GSD. * **Hyperuricemia** is a hallmark of Von Gierke's (due to increased PPP pathway activity and decreased renal clearance of urate), often leading to gout. * **Treatment:** Frequent oral cornstarch (slow-release glucose) and avoidance of fructose/galactose.

Question 406: Niemann-Pick disease is due to deficiency of which enzyme?

A. Sphingomyelinase (Correct Answer)
B. Ceramidase
C. Galactosidase
D. Glucosidase

Explanation: **Explanation:** Niemann-Pick disease (specifically Types A and B) is a lysosomal storage disorder characterized by the deficiency of the enzyme **Acid Sphingomyelinase**. **1. Why Sphingomyelinase is correct:** Acid sphingomyelinase is responsible for the hydrolysis of **sphingomyelin** into ceramide and phosphorylcholine. When this enzyme is deficient, sphingomyelin accumulates within the lysosomes of macrophages (forming characteristic "foam cells") in various organs, primarily the liver, spleen, and brain. This leads to hepatosplenomegaly and progressive neurodegeneration. **2. Why the other options are incorrect:** * **Ceramidase:** Deficiency leads to **Farber disease**, characterized by painful joint swelling, hoarseness (laryngeal nodules), and subcutaneous nodules. * **Galactosidase:** Deficiency of $\beta$-Galactosidase leads to **GM1 Gangliosidosis** or **Krabbé disease** (specifically Galactosylceramidase), while $\alpha$-Galactosidase A deficiency causes **Fabry disease**. * **Glucosidase:** Deficiency of $\beta$-Glucosidase (Glucocerebrosidase) leads to **Gaucher disease**, the most common lysosomal storage disorder. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Pathognomonic Feature:** **"Foam cells"** (lipid-laden macrophages with a "mulberry" or "soap bubble" appearance) in the bone marrow. * **Clinical Triad:** Hepatosplenomegaly, progressive neurodegeneration, and a **Cherry-red spot** on the macula (also seen in Tay-Sachs, but Tay-Sachs lacks hepatosplenomegaly). * **Niemann-Pick Type C:** Unlike A and B, Type C is due to a defect in **cholesterol transport** (NPC1 or NPC2 genes), not a primary sphingomyelinase deficiency.

Question 407: Cleidocranial dysplasia is due to a defect in which gene?

A. RANKL
B. RUNX2 (Correct Answer)
C. HOXD13
D. COL2A1

Explanation: **Explanation:** **Cleidocranial Dysplasia (CCD)** is an autosomal dominant skeletal disorder characterized by the triad of delayed closure of cranial sutures (fontanelles), absent or hypoplastic clavicles, and multiple supernumerary teeth. 1. **Why RUNX2 is correct:** The condition is caused by a mutation in the **RUNX2 gene** (located on chromosome 6p21). RUNX2 (Runt-related transcription factor 2) is a master transcription factor essential for **osteoblast differentiation** and chondrocyte maturation. Without functional RUNX2, the precursor mesenchymal cells cannot differentiate into osteoblasts, leading to defective intramembranous and endochondral ossification. 2. **Why the other options are incorrect:** * **RANKL (Option A):** This ligand is involved in osteoclast activation. Mutations or imbalances in the RANK/RANKL pathway are associated with Osteopetrosis or Paget’s disease, not CCD. * **HOXD13 (Option C):** Mutations in this homeobox gene lead to **Synpolydactyly** (fusion and extra digits). It governs limb pattern formation rather than generalized bone ossification. * **COL2A1 (Option D):** This gene encodes Type II collagen. Mutations result in **Type II Collagenopathies**, such as Stickler syndrome and Achondrogenesis Type II, characterized by joint problems and vitreoretinal degeneration. **High-Yield Clinical Pearls for NEET-PG:** * **Dental Findings:** CCD is a classic cause of **delayed eruption of permanent teeth** and the presence of multiple **supernumerary teeth**. * **Physical Sign:** Patients can often "bring their shoulders together" in the midline due to the absence of clavicles. * **Stature:** Patients typically have short stature and a "brachycephalic" (wide) skull.

Question 408: Beta-galactosidase deficiency is seen in which of the following conditions?

A. Tay-Sach disease
B. Niemann-Pick disease
C. Gaucher's disease
D. Krabbe's disease (Correct Answer)

Explanation: **Explanation:** The correct answer is **Krabbe’s disease**. This condition is an autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **Galactocerebrosidase** (also known as **Galactosylceramidase** or **$\beta$-galactosidase**). ### 1. Why Krabbe’s Disease is Correct In Krabbe’s disease, the inability to break down galactocerebroside leads to the accumulation of **psychosine**, which is toxic to oligodendrocytes. This results in the destruction of the myelin sheath (demyelination). A hallmark histological finding is the presence of **Globoid cells** (multinucleated macrophages) in the brain white matter. ### 2. Analysis of Incorrect Options * **Tay-Sachs Disease:** Caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides. Key features include a cherry-red spot on the macula and NO hepatosplenomegaly. * **Niemann-Pick Disease:** Caused by a deficiency of **Sphingomyelinase**, leading to sphingomyelin accumulation. It features a cherry-red spot AND hepatosplenomegaly (foam cells). * **Gaucher’s Disease:** The most common lysosomal storage disorder, caused by a deficiency of **$\beta$-Glucocerebrosidase**. It is characterized by "wrinkled tissue paper" appearing Gaucher cells and bone crises. ### 3. High-Yield Clinical Pearls for NEET-PG * **Krabbe’s Mnemonic:** "The **Glob**e (**Globoid cells**) is a **Crab** (**Krabbe**) in **Space** (**Galactose/Galactocerebrosidase**)." * **Enzyme Distinction:** Do not confuse $\beta$-galactosidase (Krabbe) with $\alpha$-galactosidase A, which is deficient in **Fabry disease**. * **Clinical Presentation:** Krabbe’s typically presents in infancy with irritability, developmental delay, limb stiffness, and optic atrophy.

Question 409: A failure of synthesis of ceruloplasmin is seen in which of the following conditions?

A. Hemochromatosis
B. Weil's disease
C. Wilson's disease (Correct Answer)
D. Wernicke's encephalopathy

Explanation: **Explanation:** **Wilson’s Disease (Hepatolenticular Degeneration)** is the correct answer. It is an autosomal recessive disorder caused by a mutation in the **ATP7B gene** on chromosome 13. This gene encodes a copper-transporting ATPase responsible for two critical functions in the liver: 1. Incorporating copper into **apo-ceruloplasmin** to form functional **ceruloplasmin**. 2. Excreting excess copper into the bile. A defect in ATP7B leads to a failure in ceruloplasmin synthesis and impaired biliary copper excretion, resulting in toxic copper accumulation in the liver, brain (basal ganglia), and cornea. **Analysis of Incorrect Options:** * **Hemochromatosis:** A disorder of **iron** metabolism (not copper) characterized by excessive iron absorption and deposition (hemosiderin) in organs, leading to the "bronze diabetes" triad. * **Weil’s Disease:** The severe form of **Leptospirosis** (a bacterial infection). It presents with jaundice, renal failure, and hemorrhage, but has no primary association with ceruloplasmin synthesis. * **Wernicke’s Encephalopathy:** A neurological condition caused by **Thiamine (Vitamin B1) deficiency**, typically seen in chronic alcoholism. It is characterized by the triad of ophthalmoplegia, ataxia, and confusion. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Markers:** Low serum ceruloplasmin (<20 mg/dL), increased urinary copper excretion, and increased hepatic copper content. * **Kayser-Fleischer (KF) Rings:** Copper deposition in the **Descemet’s membrane** of the cornea (pathognomonic). * **Treatment:** Copper chelators like **D-Penicillamine** (first-line) or Trientine, and Zinc (which inhibits intestinal copper absorption).

Question 410: Lesh-Nyan syndrome is due to deficiency of which enzyme?

A. Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) (Correct Answer)
B. Carbamoyl phosphate synthetase I (CPS I)
C. Carbamoyl phosphate synthetase II (CPS II)
D. Phosphoribosyl pyrophosphate (PRPP) synthetase

Explanation: **Lesch-Nyhan Syndrome** is an X-linked recessive disorder characterized by a complete deficiency of the enzyme **Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase)**. ### 1. Why HGPRTase is the Correct Answer HGPRTase is a key enzyme in the **Purine Salvage Pathway**. It converts hypoxanthine to IMP and guanine to GMP by utilizing PRPP. When HGPRTase is deficient: * **Purine Salvage fails:** Hypoxanthine and guanine cannot be recycled. * **Uric Acid Overproduction:** Excess purines are instead shunted into the degradation pathway, leading to severe hyperuricemia. * **PRPP Accumulation:** Since PRPP is not consumed by the salvage pathway, its levels rise, further stimulating *de novo* purine synthesis, creating a vicious cycle of uric acid production. ### 2. Explanation of Incorrect Options * **B. CPS I:** This is the rate-limiting enzyme of the **Urea Cycle** (located in mitochondria). Deficiency leads to Hyperammonemia Type I, not purine disorders. * **C. CPS II:** This is the rate-limiting enzyme of **Pyrimidine synthesis** (located in cytosol). It is inhibited by UTP and activated by PRPP. * **D. PRPP Synthetase:** Overactivity (not deficiency) of this enzyme leads to increased purine production and gout, but it does not present with the specific neurodevelopmental profile of Lesch-Nyhan. ### 3. High-Yield Clinical Pearls for NEET-PG * **Classic Triad:** Hyperuricemia (Orange sand in diapers/Urate stones), Intellectual disability, and **Self-mutilation** (biting lips and fingers). * **Inheritance:** X-linked recessive (Affects males). * **Biochemical Hallmark:** Increased PRPP levels and decreased IMP/GMP levels. * **Treatment:** Allopurinol or Febuxostat (manages uric acid but does not reverse neurological symptoms).

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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