Genetic Disorders and Biochemical Pathology — MCQs

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

Question 111: A 54-year-old male with acute lymphocytic leukemia develops a blast crisis and is treated with intensive systemic chemotherapy. Following treatment, the patient will be at increased risk for the development of which of the following?

A. Bile pigment gallstones
B. Cholesterol gallstones
C. Cystine kidney stones
D. Uric acid kidney stones (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Uric acid kidney stones.** This patient is experiencing **Tumor Lysis Syndrome (TLS)**, a high-yield oncological emergency in NEET-PG. When intensive chemotherapy is administered for high-grade malignancies like Acute Lymphocytic Leukemia (ALL) or Burkitt lymphoma, there is a massive, rapid breakdown of neoplastic cells. **Pathophysiology:** 1. **Nucleic Acid Breakdown:** As tumor cells lyse, their nuclei release large amounts of purines (adenine and guanine). 2. **Catabolism:** Purines are metabolized by **xanthine oxidase** into **uric acid**. 3. **Hyperuricemia:** The sudden surge in serum uric acid leads to its precipitation in the acidic environment of the renal collecting ducts. 4. **Urate Nephropathy:** This results in obstructive uropathy or the formation of radiolucent uric acid kidney stones. **Analysis of Incorrect Options:** * **A & B (Gallstones):** Bile pigment stones are associated with chronic hemolysis (e.g., Sickle Cell Anemia), while cholesterol stones are linked to obesity, female gender, and rapid weight loss. Chemotherapy-induced cell lysis specifically targets purine metabolism, not biliary components. * **C (Cystine stones):** These are caused by a congenital defect in the renal transport of COLA amino acids (Cystine, Ornithine, Lysine, Arginine), unrelated to malignancy or chemotherapy. **NEET-PG High-Yield Pearls:** * **Prevention of TLS:** Aggressive hydration and **Allopurinol** (xanthine oxidase inhibitor) or **Rasburicase** (recombinant urate oxidase that converts uric acid to soluble allantoin). * **Electrolyte Triad of TLS:** Hyperkalemia, Hyperphosphatemia, and **Hypocalcemia** (due to calcium-phosphate precipitation). * **Uric Acid Stones:** These are **radiolucent** on X-ray but visible on CT scans. They form best in **acidic urine**.

Question 112: All of the following are true about Wilson disease EXCEPT:

A. Autosomal recessive inheritance
B. Kayser-Fleischer rings
C. Raised serum copper level
D. Raised ceruloplasmin level (Correct Answer)

Explanation: **Explanation:** Wilson disease (Hepatolenticular degeneration) 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 incorporating copper into apoceruloplasmin and excreting excess copper into bile. **Why Option D is the Correct Answer (The Exception):** In Wilson disease, the defect in the ATP7B protein leads to a failure in the formation of holoceruloplasmin. Consequently, **serum ceruloplasmin levels are characteristically low** (<20 mg/dL), not raised. This occurs because apoceruloplasmin (the precursor) is unstable and rapidly degraded in the circulation. **Analysis of Other Options:** * **Option A:** It is indeed an **autosomal recessive** condition, making this a true statement. * **Option B:** **Kayser-Fleischer (KF) rings** are pathognomonic findings caused by copper deposition in the Descemet’s membrane of the cornea. They are present in 95% of patients with neurological symptoms. * **Option C:** While total serum copper is usually low (due to low ceruloplasmin), the **free (non-ceruloplasmin bound) serum copper is raised**, leading to tissue deposition in the liver, brain, and kidneys. (Note: In many clinical contexts, "raised serum copper" refers to this toxic free fraction). **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The most sensitive screening test is low serum ceruloplasmin; the gold standard is a **liver biopsy** (showing increased copper content). * **Urinary Findings:** 24-hour urinary copper excretion is **increased** (>100 μg/day). * **Treatment:** Drug of choice is **D-Penicillamine** (copper chelator). Alternatively, Trientine or Zinc (which inhibits intestinal copper absorption) can be used. * **Neuropsychiatric symptoms:** Often presents with tremors, dystonia, or parkinsonian features due to copper deposition in the **basal ganglia** (specifically the putamen).

Question 113: Arthritis occurs in which of the following conditions?

A. Alkaptonuria (Correct Answer)
B. Cystinosis
C. Maple syrup urine disease
D. Homocystinuria

Explanation: ### Explanation **Correct Answer: A. Alkaptonuria** **Why Alkaptonuria is correct:** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**. This leads to the accumulation of **Homogentisic Acid (HGA)**. Over time, HGA undergoes oxidation and polymerization to form a brownish-black pigment that deposits in connective tissues, a process known as **Ochronosis**. When this pigment deposits in the large joints (especially the spine, hips, and knees), it causes inflammation and cartilage destruction, leading to **Ochronotic Arthritis**. This typically manifests in the 3rd or 4th decade of life. **Why the other options are incorrect:** * **B. Cystinosis:** This is a lysosomal storage disorder characterized by the accumulation of cystine crystals within various organs. It primarily affects the kidneys (Fanconi syndrome) and eyes (corneal crystals), but does not typically present with arthritis. * **C. Maple Syrup Urine Disease (MSUD):** Caused by a deficiency of the **Branched-chain alpha-keto acid dehydrogenase** complex. It presents with neurological deterioration, seizures, and a "maple syrup" odor in urine during infancy, not joint pathology. * **D. Homocystinuria:** Caused by a deficiency of **Cystathionine beta-synthase**. While it involves the skeletal system, it presents with a Marfanoid habitus, osteoporosis, and kyphoscoliosis, but the hallmark clinical features are thromboembolism and ectopia lentis (downward dislocation), not arthritis. **High-Yield NEET-PG Pearls:** * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (blue-black pigmentation of sclera and ear cartilage), 3. Arthritis. * **Diagnostic Test:** Ferric chloride test (yields a transient deep blue color). * **Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and **Nitisinone**, which inhibits 4-hydroxyphenylpyruvate dioxygenase to reduce HGA production.

Question 114: All of the following are true about Sickle cell disease, EXCEPT:

A. A single nucleotide change results in the substitution of Glutamine with Valine.
B. Restriction Fragment Length Polymorphism (RFLP) is a result of a single base change.
C. A 'sticky patch' is generated due to the replacement of a nonpolar residue with a polar residue. (Correct Answer)
D. Hemoglobin S (HbS) confers resistance against malaria in heterozygotes.

Explanation: ### Explanation **1. Why Option C is the Correct Answer (The False Statement):** In Sickle Cell Disease (SCD), the mutation involves the replacement of **Glutamic acid** (a polar, negatively charged amino acid) with **Valine** (a nonpolar, hydrophobic amino acid) at the 6th position of the β-globin chain. This substitution creates a **hydrophobic "sticky patch"** on the surface of the hemoglobin molecule. In the deoxygenated state, these hydrophobic patches interact with other HbS molecules, leading to polymerization and "sickling." The option incorrectly states that a nonpolar residue is replaced by a polar one; it is actually the reverse. **2. Analysis of Other Options:** * **Option A (True):** SCD is a classic example of a **point mutation** (missense mutation). A single nucleotide change (GAG to GTG) in the DNA results in the substitution of Glutamate with Valine. * **Option B (True):** The single base change (A to T) in the β-globin gene destroys a specific recognition site for the restriction enzyme **MstII**. This change in DNA fragment length during electrophoresis is a classic example of **RFLP**, used for prenatal diagnosis. * **Option D (True):** Heterozygotes (HbAS/Sickle cell trait) exhibit a survival advantage against **Plasmodium falciparum** malaria. The premature clearance of sickled RBCs reduces the parasite load, a phenomenon known as "balanced polymorphism." **3. Clinical Pearls for NEET-PG:** * **Mutation:** β6 Glu → Val (Substitution of a polar residue with a nonpolar one). * **HbC Disease:** β6 Glu → Lys (Glutamate is replaced by Lysine). * **Electrophoresis:** On alkaline electrophoresis (pH 8.6), the speed of migration is **HbA > HbF > HbS > HbC** (mnemonic: **A** Fat **S**low **C**at). * **Precipitating Factors:** Hypoxia, acidosis, and dehydration increase the formation of deoxy-HbS, promoting sickling.

Question 115: Menkes kinky hair disease is due to a deficiency of which element?

A. Copper (Correct Answer)
B. Iodine
C. Iron
D. Selenium

Explanation: **Explanation:** **Menkes Kinky Hair Disease** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This gene encodes a P-type ATPase responsible for the absorption of dietary copper from the intestine and its transport into the bloodstream. In this condition, copper is trapped within intestinal mucosal cells, leading to severe systemic **copper deficiency**. Copper is a vital cofactor for several enzymes. Its deficiency results in: * **Lysyl oxidase failure:** Leads to defective collagen cross-linking (causing arterial tortuosity and skeletal issues). * **Tyrosinase failure:** Leads to hypopigmentation. * **Cytochrome c oxidase failure:** Leads to neurodegeneration and hypotonia. * **Characteristic finding:** "Steely" or "kinky" hair (pili torti) due to defective keratin disulfide bond formation. **Analysis of Incorrect Options:** * **B. Iodine:** Deficiency leads to Goiter and Hypothyroidism (Cretinism in infants). * **C. Iron:** Deficiency causes Microcytic Hypochromic Anemia, not structural hair defects. * **D. Selenium:** Deficiency is associated with **Keshan disease** (cardiomyopathy) and Kashin-Beck disease (osteoarthropathy). **High-Yield Pearls for NEET-PG:** 1. **ATP7A vs. ATP7B:** Remember **A**TP7**A** is for **A**bsorption (Menkes - deficiency), while **ATP7B** is for **B**iliary excretion (Wilson’s disease - toxicity). 2. **Diagnosis:** Low serum copper and low serum ceruloplasmin levels. 3. **Inheritance:** X-linked Recessive (primarily affects males). 4. **Hair Microscopy:** Shows *Pili torti* (twisted hair).

Question 116: Which of the following is NOT a mitochondrial disorder?

A. Pearson Syndrome
B. MERRF (Myoclonic Epilepsy Ragged Red Fibre)
C. MELAS (Mitochondrial Encephalopathy Lactic Acidosis and Stroke-like episode)
D. Fragile X Syndrome (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Fragile X Syndrome** **1. Why Fragile X Syndrome is the Correct Answer:** Fragile X Syndrome is not a mitochondrial disorder; it is an **X-linked dominant** disorder caused by a **trinucleotide repeat expansion (CGG)** in the *FMR1* gene on the X chromosome. This expansion leads to hypermethylation and silencing of the gene, resulting in intellectual disability, macroorchidism, and distinct facial features. It follows Mendelian (X-linked) inheritance patterns, unlike mitochondrial disorders which exhibit maternal inheritance. **2. Analysis of Incorrect Options (Mitochondrial Disorders):** Mitochondrial DNA (mtDNA) mutations typically affect tissues with high energy demands (brain, muscle, heart). * **Pearson Syndrome:** A severe mitochondrial condition caused by large deletions in mtDNA. It primarily affects the bone marrow (sideroblastic anemia, pancytopenia) and the exocrine pancreas. * **MERRF (Myoclonic Epilepsy Ragged Red Fibres):** Caused by a point mutation in the mitochondrial tRNA-Lys gene. It is characterized by myoclonus, ataxia, and the presence of "ragged red fibres" (clumps of diseased mitochondria) on muscle biopsy using Gömöri trichrome stain. * **MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes):** Usually caused by a mutation in the mitochondrial tRNA-Leu gene. It presents with recurrent stroke-like episodes and lactic acidosis due to impaired oxidative phosphorylation. **3. NEET-PG High-Yield Pearls:** * **Maternal Inheritance:** Mitochondrial disorders are transmitted only by females to all offspring (both males and females). Affected males do not pass the disease to their children. * **Heteroplasmy:** The coexistence of mutated and normal mtDNA within a single cell, which explains the variable clinical severity of mitochondrial diseases. * **Trinucleotide Repeat Disorders:** Remember the mnemonic **"CGG"** for Fragile X (**C**hin (protruding), **G**iant **G**onads). * **Ragged Red Fibres:** A classic histological hallmark of mitochondrial myopathies seen on muscle biopsy.

Question 117: A 30-year-old female presents with bluish-black discoloration of the sclera and pinna for the last 4 months. Her urine turns black on standing. Which of the following tests is not used for the diagnosis of this condition?

A. Benedict's test
B. Ferric chloride test
C. Silver nitrate test
D. Guthrie test (Correct Answer)

Explanation: ### Explanation The clinical presentation of **bluish-black discoloration of the sclera (ochronosis)** and **urine turning black on standing** is characteristic of **Alkaptonuria**. This autosomal recessive disorder is caused by a deficiency of the enzyme **homogentisate oxidase**, leading to the accumulation of homogentisic acid (HGA). **Why Option D is the Correct Answer:** The **Guthrie test** is a semi-quantitative bacterial inhibition assay used for the neonatal screening of **Phenylketonuria (PKU)**, not Alkaptonuria. It detects elevated levels of phenylalanine in the blood using *Bacillus subtilis*. **Analysis of Other Options (Diagnostic Tests for Alkaptonuria):** * **Benedict’s Test (A):** Homogentisic acid is a strong reducing agent. It gives a positive result (yellow/orange precipitate) and a characteristic dark supernatant. * **Ferric Chloride Test (B):** When added to urine containing HGA, it produces a **transient deep blue/green color**. * **Silver Nitrate Test (C):** HGA reduces silver nitrate to metallic silver in an alkaline medium, causing the solution to turn black (Ammoniacal Silver Nitrate test). ### High-Yield Clinical Pearls for NEET-PG: * **Enzyme Defect:** Homogentisate 1,2-dioxygenase (HGO). * **Pathophysiology:** Accumulation of HGA leads to the formation of **alkapton bodies** (polymers) that deposit in connective tissues (Ochronosis). * **Clinical Triad:** Alkaptonuria (black urine), Ochronosis (pigmentation), and Arthritis (large joint involvement). * **Gold Standard Diagnosis:** Quantitative measurement of HGA in urine via **Gas Chromatography-Mass Spectrometry (GC-MS)**. * **Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase).

Question 118: What is the most likely diagnosis if a urine sample collected for routine testing darkens on standing?

A. Phenylketonuria
B. Alkaptonuria (Correct Answer)
C. Maple syrup disease
D. Tyrosinemia

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**. This enzyme is essential in the catabolic pathway of tyrosine and phenylalanine. Its absence leads to the accumulation of **Homogentisic Acid (HGA)**. When urine containing high levels of HGA is left standing or is exposed to an alkaline environment, the HGA undergoes oxidation and polymerization to form a melanin-like pigment, causing the urine to turn **brownish-black**. **Analysis of Incorrect Options:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. Urine typically has a characteristic **"mousy" or "musty" odor** due to phenylacetate, but it does not darken on standing. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. The urine has a distinct **burnt sugar or maple syrup smell**, not a color change upon standing. * **Tyrosinemia:** Characterized by a **"boiled cabbage-like" or "rancid" odor** (especially in Type I) due to the accumulation of methionine metabolites and succinylacetone. **High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The deposition of black pigment in connective tissues (cartilage, sclera, ears), often leading to severe arthritis in later life. * **Diagnostic Test:** Addition of Benedict’s reagent to urine gives a **strongly positive (yellow-orange) result** for reducing sugars, but the supernatant turns black. Ferric chloride test also gives a transient deep blue/green color. * **Dietary Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and high doses of Vitamin C (to prevent HGA oxidation). **Nitisinone** is a newer drug used to inhibit HGA production.

Question 119: A young adult presents with progressive intellectual deterioration, weakness, ataxia, and seizures. Laboratory tests demonstrate an abnormality of an important mitochondrial enzyme. Which of the following conditions is this person most likely suffering from?

A. Adrenoleukodystrophy
B. Central pontine myelinolysis
C. Krabbe's disease
D. Leigh's disease (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Leigh’s Disease** **1. Why Leigh’s Disease is Correct:** Leigh’s disease (Subacute Necrotizing Encephalomyelopathy) is a rare, progressive neurodegenerative disorder typically caused by **mitochondrial dysfunction**. The underlying pathology involves defects in oxidative phosphorylation, most commonly affecting **Pyruvate Dehydrogenase (PDH) complex**, **Cytochrome c oxidase (Complex IV)**, or other components of the Electron Transport Chain. * **Clinical Presentation:** It manifests as progressive psychomotor regression, ataxia, seizures, and hypotonia. * **Biochemical Hallmark:** Elevated lactate levels in blood and CSF (Lactic acidosis) due to the inability of mitochondria to process pyruvate efficiently. **2. Why Other Options are Incorrect:** * **A. Adrenoleukodystrophy:** This is a **peroxisomal** disorder (not mitochondrial) caused by a defect in the transport of Very Long Chain Fatty Acids (VLCFA), leading to their accumulation in the adrenal glands and white matter of the brain. * **B. Central Pontine Myelinolysis (CPM):** This is an **iatrogenic** condition caused by the rapid correction of chronic hyponatremia. It is not a genetic mitochondrial enzyme deficiency. * **C. Krabbe’s Disease:** This is a **lysosomal** storage disorder (sphingolipidosis) caused by a deficiency of the enzyme **galactocerebrosidase**. It leads to the accumulation of galactocerebroside and psychosine, destroying the myelin sheath. **3. NEET-PG High-Yield Pearls:** * **Mitochondrial Inheritance:** Leigh’s disease can be inherited via mitochondrial DNA (maternal inheritance) or nuclear DNA (autosomal recessive). * **Imaging Gold Standard:** MRI typically shows characteristic **bilateral, symmetrical T2-hyperintense lesions** in the basal ganglia, thalamus, and brainstem. * **Key Enzyme Association:** If a question mentions "mitochondrial enzyme" and "lactic acidosis" in a pediatric/young adult neurological context, prioritize **Leigh’s Disease** or **PDH deficiency**.

Question 120: Gyrate atrophy of the retina is caused by the accumulation of which substance?

A. Ornithine (Correct Answer)
B. Arginine
C. Citrulline
D. None of the above

Explanation: **Explanation:** **Gyrate atrophy of the choroid and retina** is a rare, autosomal recessive metabolic disorder caused by a deficiency of the mitochondrial enzyme **Ornithine Aminotransferase (OAT)**. 1. **Why Ornithine is correct:** The OAT enzyme normally catalyzes the conversion of **ornithine** into glutamate-gamma-semialdehyde (a precursor for proline and glutamate). When this enzyme is deficient, ornithine cannot be metabolized, leading to severe **hyperornithinemia** (10–20 times normal levels). This accumulation is toxic to the retinal pigment epithelium and the choroid, leading to progressive vision loss and characteristic circular (gyrate) chorioretinal lesions. 2. **Why other options are incorrect:** * **Arginine:** While ornithine is derived from arginine via the urea cycle (arginase enzyme), arginine itself does not accumulate to toxic levels in this specific condition. * **Citrulline:** Accumulation of citrulline is seen in **Citrullinemia Type I** (deficiency of Argininosuccinate synthase), which presents with hyperammonemia and neurological distress, not retinal atrophy. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Clinical Triad:** Progressive night blindness, constricted visual fields, and "gyrate" (scalloped) chorioretinal atrophy. * **Treatment:** A subset of patients responds to **Vitamin B6 (Pyridoxine)**, which is a cofactor for the OAT enzyme. Arginine-restricted diets are also used to lower ornithine levels. * **Biochemical Link:** It is one of the few urea cycle-related disorders that does **not** typically present with hyperammonemia.

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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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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