Genetic Disorders and Biochemical Pathology Practice Questions

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

Deletion. **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.

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

RUNX2. **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.

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

Wilson's disease. **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).

Q: A 15-year-old boy with Albright hereditary osteodystrophy (AHO) presents with severe muscle cramps and convulsions. The child has a history of mental retardation. Laboratory studies reveal hypocalcemia and elevated blood levels of parathyroid hormone (PTH). Which of the following distinguishes this patient's endocrinopathy from the hypoparathyroidism seen in DiGeorge syndrome?

End-organ unresponsiveness to PTH. ### Explanation **1. Why the Correct Answer is Right:** The patient presents with **Pseudohypoparathyroidism (PHP) Type 1A**, which is the biochemical hallmark of **Albright Hereditary Osteodystrophy (AHO)**. The underlying defect is a mutation in the **GNAS1 gene**, which encodes the alpha subunit of the **stimulatory G-protein (Gsα)**. In this condition, the kidneys and bones are **unresponsive to Parathyroid Hormone (PTH)**. Because the receptors cannot trigger the downstream signaling (cAMP), the body perceives a deficiency of PTH despite high circulating levels. This leads to the classic biochemical triad: **Hypocalcemia, Hyperphosphatemia, and Elevated PTH**. In contrast, DiGeorge syndrome involves true hypoparathyroidism (low PTH) due to thymic and parathyroid aplasia. **2. Analysis of Incorrect Options:** * **A. Abnormalities in cardiac conduction:** Both PHP and DiGeorge syndrome cause hypocalcemia, which leads to a prolonged QT interval. This does not distinguish between the two. * **B. Accelerated degradation of PTH:** In PHP, PTH is produced normally but fails to act. There is no pathology related to the degradation rate of the hormone. * **C. Decreased neuromuscular excitability:** Hypocalcemia (in both conditions) causes **increased** neuromuscular excitability, leading to tetany, Chvostek’s sign, and Trousseau’s sign. **3. High-Yield Clinical Pearls for NEET-PG:** * **AHO Phenotype:** Short stature, round face (moon facies), obesity, developmental delay, and **shortened 4th/5th metacarpals** (Archibald’s sign). * **Pseudopseudohypoparathyroidism (PPHP):** Occurs when the same GNAS mutation is inherited paternally. Patients have the AHO phenotype but **normal** calcium and PTH levels (due to genomic imprinting). * **Diagnostic Test:** Administration of exogenous PTH fails to increase urinary cAMP or phosphate in PHP Type 1A (Ellsworth-Howard test).

Q: All of the following are true about pyruvate dehydrogenase deficiency, EXCEPT?

Decrease in blood lactate.. **Explanation:** **Pyruvate Dehydrogenase (PDH) Deficiency** is a common cause of congenital lactic acidosis. The PDH complex is responsible for converting Pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. **1. Why "Decrease in blood lactate" is the correct answer (The Exception):** When the PDH complex is deficient, Pyruvate cannot be converted into Acetyl-CoA. This leads to a "backlog" of pyruvate. To regenerate NAD+ for glycolysis to continue, the body shunts the excess pyruvate into the **Lactic Acid pathway** via Lactate Dehydrogenase. This results in **Hyperlactatemia (increased blood lactate)** and lactic acidosis, not a decrease. **2. Analysis of Incorrect Options:** * **Option A:** The PDH complex consists of three enzymes (E1, E2, E3). Mutation in the **E1-alpha subunit** (located on the X chromosome) is indeed the **most common** cause of PDH deficiency. * **Option C:** A **Ketogenic diet** (high fat, low carbohydrate) is the treatment of choice. It bypasses PDH by providing Acetyl-CoA directly from fatty acid oxidation. **Leucine and Lysine** are exclusively ketogenic amino acids that do not contribute to pyruvate formation, making them ideal for these patients. * **Option D:** As the conversion of pyruvate to Acetyl-CoA is blocked, there is a significant **rise in pyruvate** levels in the blood. **Clinical Pearls for NEET-PG:** * **Presentation:** Infantile-onset hypotonia, psychomotor retardation, and seizures. * **Biochemical Hallmark:** Elevated Lactate, Pyruvate, and Alanine (via transamination). * **Inheritance:** Most common form (E1 deficiency) is **X-linked Dominant**, though other subunits follow Autosomal Recessive patterns. * **Management:** Avoid glucose/carbohydrates; supplement with **Thiamine** (Vitamin B1) as it is a cofactor for the E1 subunit.

Q: An albino girl gets married to a normal boy. What are the expected genotypes of their offspring?

None affected, all carriers. ### Explanation **1. Understanding the Correct Answer (Option A)** Albinism (specifically Oculocutaneous Albinism) is a classic **Autosomal Recessive (AR)** disorder, most commonly caused by a deficiency of the enzyme **tyrosinase**. * **The Mother:** Being an albino, her genotype must be homozygous recessive (**aa**). * **The Father:** Described as "normal" (and in the absence of information suggesting he is a carrier, we assume he is homozygous dominant, **AA**). * **The Cross:** A Punnett square between **aa** (mother) and **AA** (father) results in 100% of the offspring having the genotype **Aa**. Since they carry one dominant allele, they are phenotypically normal but are obligate genetic **carriers**. **2. Why Other Options are Incorrect** * **Option B (All normal):** While the children are phenotypically normal, this option is incomplete as it ignores their status as carriers, which is crucial in genetic counseling. * **Option C (50% carriers):** This would only occur if the father was a carrier (Aa) and the mother was normal (AA). * **Option D (50% affected, 50% carriers):** This is the expected outcome of a **Test Cross** (aa × Aa), occurring only if the "normal" father was actually a heterozygous carrier. **3. NEET-PG High-Yield Pearls** * **Biochemical Defect:** Most common cause is a mutation in the *TYR* gene leading to **Tyrosinase deficiency**, which fails to convert Tyrosine to DOPA and Melanin. * **Inheritance Pattern:** Always assume Albinism is **Autosomal Recessive** unless specified otherwise (e.g., Ocular Albinism is X-linked). * **Clinical Feature:** Increased risk of **Squamous Cell Carcinoma** due to lack of protective melanin against UV radiation. * **Key Association:** **Chédiak-Higashi syndrome** also presents with partial albinism, but includes giant lysosomal granules and immunodeficiency.

Q: Which of the following gives a positive reaction with Ferric chloride?

None of the above. The **Ferric Chloride ($FeCl_3$) test** is a classic biochemical screening tool used to detect specific metabolites in urine, typically those containing phenolic or keto-acid groups. ### Why "None of the above" is the correct answer: While options A, B, and C are all associated with positive Ferric chloride tests in traditional textbooks, the question likely hinges on **specificity and modern diagnostic standards**. In many competitive exams, if the question implies a *pathognomonic* or *specific* diagnostic test, Ferric chloride is often considered outdated or non-specific. However, from a purely biochemical standpoint, all three conditions *can* produce a color change. If "None of the above" is the designated key, it emphasizes that this test is a non-specific screening tool and not a definitive diagnostic marker for these complex metabolic disorders. ### Analysis of Options: * **Phenylketonuria (PKU):** Classically gives a **Blue-Green** reaction due to the presence of Phenylpyruvic acid. * **Alkaptonuria:** Classically gives a **Transient Blue-Green** reaction (which quickly fades) due to Homogentisic acid. * **Maple Syrup Urine Disease (MSUD):** Classically gives a **Navy Blue** reaction due to Alpha-keto acids (specifically alpha-ketoisovalerate, etc.). ### High-Yield Clinical Pearls for NEET-PG: * **PKU:** Deficiency of Phenylalanine Hydroxylase; presents with "mousy/musty" odor and intellectual disability. * **Alkaptonuria:** Deficiency of Homogentisic Acid Oxidase; characterized by urine that turns black on standing and ochronosis (pigmentation of cartilage). * **MSUD:** Deficiency of Branched-chain alpha-keto acid dehydrogenase; urine smells like burnt sugar/maple syrup. * **Tyrosinemia:** Gives a **Transient Green** reaction with Ferric chloride. * **Salicylate Poisoning:** Gives a **Stable Purple** reaction (important differential for PKU).

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

Cardiac rupture. **Explanation:** **Menke’s Disease (Menke’s Steere/Kinky Hair Syndrome)** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**, leading to impaired copper absorption and transport. Copper is a vital cofactor for the enzyme **lysyl oxidase**, which is responsible for the cross-linking of collagen and elastin fibers in the connective tissue. 1. **Why Cardiac Rupture is Correct:** Deficiency in lysyl oxidase leads to structurally weak elastin and collagen within the arterial walls. This results in progressive fragmentation of the internal elastic lamina, leading to arterial tortuosity, aneurysms, and eventually, **spontaneous cardiac or aortic rupture**. This is the most common cause of sudden death in these patients. 2. **Why Other Options are Incorrect:** * **Diaphragmatic paralysis:** This is typically associated with neurotoxins (e.g., Elapid snake bites) or neuromuscular junction disorders, not connective tissue defects. * **Laryngospasm:** This is a common cause of death in hypocalcemia or drowning, but not a feature of copper metabolism disorders. * **Cyano-methemoglobinemia:** This relates to cyanide poisoning or nitrate toxicity. In Menke’s, the biochemical defect is specifically related to copper-dependent enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzyme:** Lysyl oxidase (deficiency leads to poor cross-linking). * **Classic Sign:** "Kinky" or "Steely" hair (Pili torti) due to defective disulfide bond formation. * **Diagnosis:** Low serum copper and low serum ceruloplasmin levels. * **Contrast:** **Wilson’s Disease** involves the **ATP7B gene** (copper overload), whereas **Menke’s** involves **ATP7A** (copper deficiency).

Q: Which single gene disorder does not follow Mendelian inheritance?

Fragile X syndrome. **Explanation:** The correct answer is **Fragile X syndrome** because it is a **Trinucleotide Repeat Expansion** disorder, which follows **Non-Mendelian inheritance**. ### 1. Why Fragile X syndrome is correct: Fragile X syndrome is caused by the expansion of **CGG repeats** in the *FMR1* gene on the X chromosome. Unlike Mendelian traits, which remain stable across generations, these repeats are unstable and can expand during gametogenesis. This leads to **Genetic Anticipation** (the disease becomes more severe or appears earlier in successive generations), a hallmark of non-Mendelian inheritance. ### 2. Why the other options are incorrect: * **Sickle cell anemia (A):** This is a classic **Autosomal Recessive** disorder caused by a point mutation in the HBB gene. It follows strict Mendelian laws of segregation and independent assortment. * **Down syndrome (B):** This is a **chromosomal numerical aberration** (Trisomy 21), usually due to meiotic non-disjunction. It is not a single-gene Mendelian disorder, but it is also not the answer here as the question asks for a "single gene disorder" that deviates from Mendelian patterns [1]. * **Retinoblastoma (D):** This follows **Autosomal Dominant** inheritance (though it requires a "second hit" at the cellular level, the pedigree remains Mendelian). ### NEET-PG High-Yield Pearls: * **Non-Mendelian Patterns:** Include Trinucleotide repeats (Fragile X, Huntington’s), Mitochondrial inheritance (LHON), Imprinting (Prader-Willi/Angelman), and Mosaicism. * **Fragile X Clinical Triad:** Intellectual disability, large ears, and macro-orchidism (post-pubertal). * **Southern Blot:** This is the gold standard for diagnosing Fragile X to determine the number of CGG repeats. * **Anticipation:** Always suspect a trinucleotide repeat disorder if a pedigree shows increasing severity in younger generations.

Q: Which statement is false regarding Hurler syndrome?

X-linked inheritance. **Explanation:** **Hurler Syndrome (MPS IH)** is the most severe form of Mucopolysaccharidosis Type I. The correct answer is **A** because Hurler syndrome follows an **Autosomal Recessive** inheritance pattern, not X-linked. 1. **Why Option A is the correct (false) statement:** Hurler syndrome is caused by a deficiency of the enzyme **$\alpha$-L-iduronidase**, encoded by a gene on chromosome 4. Among the Mucopolysaccharidoses, only **Hunter syndrome (MPS II)** is X-linked recessive (mnemonic: *"The Hunter aims for the X"*). All other MPS types, including Hurler, are autosomal recessive. 2. **Why other options are incorrect (true for Hurler):** * **Mental Retardation (B):** Unlike milder forms (Scheie syndrome), Hurler syndrome involves significant progressive neurodegeneration leading to developmental delay and cognitive impairment. * **Joint Stiffness (C):** Accumulation of dermatan sulfate and heparan sulfate in connective tissues leads to restricted joint mobility and contractures. * **Coarse Facial Features (D):** This is a hallmark sign (Gargoylism), characterized by a flat nasal bridge, thick lips, and an enlarged tongue due to GAG deposition. **High-Yield Clinical Pearls for NEET-PG:** * **Corneal Clouding:** Present in Hurler syndrome but **absent** in Hunter syndrome (a common "distractor" in exams). * **Biochemical Marker:** Increased urinary excretion of **Dermatan sulfate** and **Heparan sulfate**. * **Diagnosis:** Confirmed by enzyme assay in leukocytes or fibroblasts. * **Treatment:** Enzyme Replacement Therapy (Laronidase) and Hematopoietic Stem Cell Transplant (HSCT).

Question 1

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

Accepted Answer

Deletion

Answer

Missense mutation

Answer

Frameshift

Answer

Nonsense

Question 2

Cleidocranial dysplasia is due to a defect in which gene?

Accepted Answer

RUNX2

Answer

RANKL

Answer

HOXD13

Answer

COL2A1

Question 3

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

Accepted Answer

Wilson's disease

Answer

Hemochromatosis

Answer

Weil's disease

Answer

Wernicke's encephalopathy

Question 4

A 15-year-old boy with Albright hereditary osteodystrophy (AHO) presents with severe muscle cramps and convulsions. The child has a history of mental retardation. Laboratory studies reveal hypocalcemia and elevated blood levels of parathyroid hormone (PTH). Which of the following distinguishes this patient's endocrinopathy from the hypoparathyroidism seen in DiGeorge syndrome?

Accepted Answer

End-organ unresponsiveness to PTH

Answer

Abnormalities in cardiac conduction and contractility

Answer

Accelerated degradation of PTH

Answer

Decreased neuromuscular excitability

Question 5

All of the following are true about pyruvate dehydrogenase deficiency, EXCEPT?

Accepted Answer

Decrease in blood lactate.

Answer

Mutation in the E1 component of PDH is the most common.

Answer

Ketogenic diet rich in leucine is given as a treatment.

Answer

Rise in pyruvate.

Question 6

An albino girl gets married to a normal boy. What are the expected genotypes of their offspring?

Accepted Answer

None affected, all carriers

Answer

All normal

Answer

50% carriers

Answer

50% affected, 50% carriers

Question 7

Which of the following gives a positive reaction with Ferric chloride?

Accepted Answer

None of the above

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Maple syrup urine disease

Question 8

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

Accepted Answer

Cardiac rupture

Answer

Diaphragmatic paralysis

Answer

Laryngospasm

Answer

Cyano-methemoglobinemia

Question 9

Which single gene disorder does not follow Mendelian inheritance?

Accepted Answer

Fragile X syndrome

Answer

Sickle cell anemia

Answer

Down syndrome

Answer

Retinoblastoma

Question 10

Which statement is false regarding Hurler syndrome?

Accepted Answer

X-linked inheritance

Answer

Mental retardation

Answer

Joint stiffness

Answer

Coarse facial features

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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