Genetic Disorders and Biochemical Pathology — MCQs

A screening test for phenylketonuria (PKU) is performed on umbilical cord blood from a fair-skinned blond, blue-eyed infant born to dark-complexioned parents. The test is reported as negative, and no dietary restrictions are imposed. At 1 year of age, the child is seen again, this time with obvious signs of severe mental retardation, and a diagnosis of PKU is made. The diagnosis was missed at birth because:

Q67Easy

In children, what is the most commonly recognized form of Familial Hyperlipidemia?

Q68Easy

A single gene defect causing multiple unrelated problems is termed as the following?

Q69Medium

A newborn infant refuses breast milk from the second day of birth, vomits on force-feeding but accepts glucose-water, develops diarrhea on the third day. By the fifth day, the infant is jaundiced with liver enlargement, and the eyes show signs of cataract. Urinary reducing sugar was positive, but blood glucose estimated by the glucose oxidation method was found to be low. What is the most likely cause of this condition?

Q70Medium

Which of the following statements is WRONG about X-linked adrenoleukodystrophy?

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

Question 61: Which of the following is related to 'NARP'?

A. Glycogen storage diseases
B. Lipid storage disease
C. Protein
D. Mitochondria (Correct Answer)

Explanation: **Explanation:** **NARP** stands for **Neurogenic muscle weakness, Ataxia, and Retinitis Pigmentosa**. It is a rare genetic condition caused by mutations in the **mitochondrial DNA (mtDNA)**, specifically affecting the **MT-ATP6 gene**. This gene encodes a subunit of ATP synthase (Complex V), which is essential for oxidative phosphorylation. Because the defect lies in the mtDNA, it follows a **maternal inheritance pattern** and is characterized by heteroplasmy, where the severity of the disease depends on the proportion of mutated mitochondria. **Why the other options are incorrect:** * **Glycogen storage diseases (A):** These are caused by deficiencies in enzymes involved in glycogen synthesis or breakdown (e.g., Von Gierke, Pompe disease), typically involving cytosolic or lysosomal enzymes, not mitochondrial ATP production. * **Lipid storage diseases (B):** Also known as lysosomal storage disorders (e.g., Gaucher, Tay-Sachs), these involve the accumulation of lipids due to defective lysosomal hydrolases. * **Protein (C):** While NARP involves a protein subunit defect, the term "NARP" specifically refers to a clinical syndrome defined by its **mitochondrial** genetic origin. **High-Yield Clinical Pearls for NEET-PG:** * **Leigh Syndrome Connection:** NARP and Leigh Syndrome are part of a clinical spectrum. If the mutation load (heteroplasmy) is >90%, the patient typically presents with the more severe **Maternally Inherited Leigh Syndrome (MILS)**. * **Key Triad:** Remember the acronym: **N**eurogenic weakness, **A**taxia, **R**etinitis **P**igmentosa. * **Maternal Inheritance:** Like MELAS and MERRF, NARP is passed only from the mother to all her children, but only daughters can pass it to the next generation.

Question 62: Bilirubin UDP glucuronyl transferase activity is absent in which of the following conditions?

A. Crigler-Najjar syndrome type I (Correct Answer)
B. Crigler-Najjar syndrome type II
C. Rotor syndrome
D. Gilbert syndrome

Explanation: **Explanation:** The enzyme **UDP-glucuronosyltransferase (UGT1A1)** is responsible for the conjugation of bilirubin in the liver, converting insoluble unconjugated bilirubin into water-soluble conjugated bilirubin. **1. Why Option A is Correct:** **Crigler-Najjar Syndrome Type I (CN-I)** is characterized by a **complete absence** (0% activity) of the UGT1A1 enzyme. This leads to severe, life-threatening unconjugated hyperbilirubinemia shortly after birth. Because bilirubin cannot be conjugated, it crosses the blood-brain barrier, posing a high risk of **kernicterus**. It does not respond to phenobarbital. **2. Why Other Options are Incorrect:** * **Crigler-Najjar Syndrome Type II (Arias Syndrome):** There is a **severe deficiency** rather than a total absence. Enzyme activity is typically **<10%** of normal. It is less severe than Type I and usually responds to phenobarbital (which induces enzyme production). * **Gilbert Syndrome:** There is a **mild reduction** in enzyme activity (approximately **30%** of normal) due to a promoter mutation. It results in mild, fluctuating jaundice often triggered by stress or fasting. * **Rotor Syndrome:** This is a defect in **hepatic storage/uptake** of bilirubin, not conjugation. It presents with conjugated hyperbilirubinemia. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** CN-I is Autosomal Recessive; Gilbert is usually Autosomal Recessive (TATAA box mutation). * **Phenobarbital Test:** Used to differentiate CN-I (no response) from CN-II (bilirubin levels decrease). * **Treatment for CN-I:** Phototherapy is the immediate management, but **Liver Transplantation** is the only definitive cure. * **Dubin-Johnson vs. Rotor:** Both cause conjugated hyperbilirubinemia, but Dubin-Johnson features a **black liver** due to melanin-like pigment, whereas Rotor does not.

Question 63: All of the following may be raised in carcinoid syndrome, except?

A. 5HIAA
B. 5HT
C. 5HTP
D. VMA (Correct Answer)

Explanation: **Explanation:** **Carcinoid syndrome** is caused by neuroendocrine tumors (most commonly in the ileum) that secrete excessive amounts of **Serotonin (5-HT)**. The biochemical pathway involves the conversion of Tryptophan to 5-Hydroxytryptophan (5-HTP), then to Serotonin (5-HT), and finally to its main metabolite, **5-HIAA**, which is excreted in the urine. * **Why VMA is the correct answer:** **Vanillylmandelic Acid (VMA)** is the end-stage urinary metabolite of catecholamines (Epinephrine and Norepinephrine). Elevated VMA levels are diagnostic markers for **Pheochromocytoma** and Neuroblastoma, not Carcinoid syndrome. It belongs to the sympathetic nervous system pathway, whereas Carcinoid syndrome involves the indoleamine (Serotonin) pathway. * **Analysis of incorrect options:** * **5-HIAA (5-Hydroxyindoleacetic acid):** This is the gold-standard urinary marker for diagnosing Carcinoid syndrome. * **5-HT (Serotonin):** This is the primary hormone secreted by the tumor cells, leading to clinical symptoms like flushing, diarrhea, and bronchospasm. * **5-HTP (5-Hydroxytryptophan):** This is the immediate precursor to Serotonin. In "Atypical Carcinoid" (e.g., gastric carcinoids), the enzyme DOPA decarboxylase may be deficient, leading to the direct release of 5-HTP into the blood. **High-Yield Clinical Pearls for NEET-PG:** 1. **Dietary Precursor:** Tryptophan is the amino acid precursor for Serotonin. In Carcinoid syndrome, up to 60% of dietary tryptophan is diverted to serotonin synthesis, potentially leading to **Niacin deficiency (Pellagra)**. 2. **Localization:** Carcinoid syndrome typically occurs only after the tumor has metastasized to the **liver**, as the liver otherwise metabolizes serotonin via first-pass metabolism. 3. **Cardiac Involvement:** Right-sided heart failure (Tricuspid regurgitation/Pulmonary stenosis) is common; left-sided valves are usually spared because the lungs contain Monoamine Oxidase (MAO) which degrades serotonin.

Question 64: Very low activity of adenosine deaminase in red blood cells and high levels of dATP is consistent with which diagnosis?

A. Organophosphorus poisoning
B. Severe combined immunodeficiency disease (Correct Answer)
C. Cyanide poisoning
D. Acquired immunodeficiency disease

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Adenosine Deaminase (ADA) is a critical enzyme in the **purine salvage pathway**. It converts adenosine to inosine and deoxyadenosine to deoxyinosine. A deficiency in ADA leads to the toxic accumulation of **deoxyadenosine**, which is subsequently phosphorylated into **dATP (deoxyadenosine triphosphate)**. High levels of dATP act as a potent allosteric inhibitor of **Ribonucleotide Reductase**, the enzyme responsible for synthesizing all four deoxyribonucleotides (dNTPs) required for DNA synthesis. Without dNTPs, lymphocytes (T-cells and B-cells) cannot proliferate, leading to **Severe Combined Immunodeficiency Disease (SCID)**. ADA deficiency accounts for approximately 15% of all SCID cases and is inherited in an autosomal recessive pattern. **2. Why the Other Options are Incorrect:** * **Organophosphorus poisoning:** This involves the inhibition of **Acetylcholinesterase**, leading to a cholinergic crisis (SLUDGE symptoms). It has no relation to purine metabolism or dATP levels. * **Cyanide poisoning:** Cyanide inhibits **Cytochrome c oxidase (Complex IV)** in the electron transport chain, halting aerobic respiration. It does not affect ADA activity. * **Acquired Immunodeficiency Disease (AIDS):** While AIDS also involves immune failure, it is caused by the **HIV virus** infecting CD4+ T-cells. It is an acquired condition, not a primary genetic enzyme deficiency. **3. Clinical Pearls for NEET-PG:** * **First Gene Therapy:** ADA deficiency was the first disease treated with human gene therapy (1990). * **Inheritance:** ADA-SCID is **Autosomal Recessive**, whereas the most common form of SCID is X-linked (IL-2 receptor mutation). * **Diagnosis:** Look for "absent thymic shadow" on X-ray and lymphopenia in a neonate with recurrent infections. * **Treatment:** Enzyme replacement therapy (PEG-ADA), Bone Marrow Transplant, or Gene Therapy.

Question 65: What is the cause of hyperuricemia and gout in glucose-6-phosphatase deficiency?

A. More formation of pentose (Correct Answer)
B. Decreased availability of glucose to tissues
C. Increased accumulation of sorbitol
D. Impaired degradation of free radicals

Explanation: **Explanation:** Glucose-6-phosphatase deficiency (**Von Gierke Disease/GSD Type I**) leads to hyperuricemia through a specific metabolic shunt. When glucose-6-phosphatase is deficient, **Glucose-6-Phosphate (G6P)** cannot be converted to free glucose. This results in an intracellular accumulation of G6P, which is then diverted into the **Pentose Phosphate Pathway (PPP)**. 1. **Why Option A is correct:** Increased flux through the PPP leads to the overproduction of **Ribose-5-phosphate (a pentose sugar)**. This pentose is a precursor for **PRPP (Phosphoribosyl pyrophosphate)** synthesis. Elevated PRPP levels stimulate the *de novo* synthesis of purines. The subsequent breakdown of these excess purines results in increased production of **uric acid**, leading to hyperuricemia and clinical gout. Additionally, lactic acidosis (common in GSD I) competes with uric acid for excretion in the kidneys, further elevating levels. 2. **Why incorrect options are wrong:** * **Option B:** While hypoglycemia occurs, the lack of glucose to tissues causes neuroglycopenia and growth retardation, not gout. * **Option C:** Sorbitol accumulation is associated with chronic hyperglycemia (Diabetes Mellitus) via the polyol pathway, not G6P deficiency. * **Option D:** Impaired free radical degradation is linked to G6PD deficiency (due to low NADPH), not G6-phosphatase deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Von Gierke Disease Presentation:** "Doll-like" facies, hepatomegaly, hypoglycemia, hyperlactatemia, hyperlipidemia, and hyperuricemia. * **Biochemical Hallmark:** Hyperlactatemia is a key differentiator from other GSDs. * **Treatment:** Frequent cornstarch feeds to maintain glucose levels and prevent the metabolic shunts mentioned above.

Question 66: A screening test for phenylketonuria (PKU) is performed on umbilical cord blood from a fair-skinned blond, blue-eyed infant born to dark-complexioned parents. The test is reported as negative, and no dietary restrictions are imposed. At 1 year of age, the child is seen again, this time with obvious signs of severe mental retardation, and a diagnosis of PKU is made. The diagnosis was missed at birth because:

A. cord blood is not a good source of fetal blood.
B. the screening (Guthrie) test has low sensitivity.
C. the test should have been performed on maternal blood.
D. the test was performed too early, before the enzyme deficiency manifested significantly. (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of **phenylalanine hydroxylase (PAH)**. In utero, the fetus does not accumulate phenylalanine because the **maternal liver** metabolizes the excess phenylalanine that crosses the placenta. Therefore, at birth, the infant’s blood levels of phenylalanine are normal. The biochemical abnormality (hyperphenylalaninemia) only manifests after the infant begins **protein feeding** (breast milk or formula). As the infant ingests phenylalanine, the lack of PAH leads to the accumulation of phenylalanine and its metabolites. Screening tests like the Guthrie test rely on detecting elevated phenylalanine levels; if performed too early (e.g., on cord blood or within the first 24 hours), the levels will not yet be high enough to trigger a positive result, leading to a **false negative**. **2. Why Other Options are Incorrect:** * **Option A:** While cord blood represents fetal status, it is not used for PKU screening because the metabolite levels are still normalized by maternal metabolism. * **Option B:** The Guthrie test (a bacterial inhibition assay) is highly sensitive and specific when performed at the correct time (usually 48–72 hours after birth). * **Option C:** Testing maternal blood would only reflect the mother’s metabolic status, not the infant’s genetic deficiency. **3. High-Yield Clinical Pearls for NEET-PG:** * **Classic Presentation:** Intellectual disability, "mousy" or "musty" body odor (due to phenylacetic acid), and hypopigmentation (fair skin/blue eyes) due to decreased melanin synthesis (phenylalanine is a competitive inhibitor of tyrosinase). * **Screening Timing:** Ideally performed **after 48–72 hours** of protein feeding. * **Management:** Lifelong restriction of phenylalanine and supplementation of **Tyrosine** (which becomes an essential amino acid in PKU patients). * **Maternal PKU:** If a pregnant woman with PKU does not maintain a strict diet, high phenylalanine levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus.

Question 67: In children, what is the most commonly recognized form of Familial Hyperlipidemia?

A. Hypertriglyceridemia
B. Hypercholesterolemia
C. Hyperchylomicronemia
D. Combined hyperlipidemia (Correct Answer)

Explanation: **Explanation:** **Familial Combined Hyperlipidemia (FCHL)** is the most common genetic dyslipidemia, affecting approximately 1 in 100 to 1 in 200 individuals. In the pediatric population, it is the most frequently recognized form of familial hyperlipidemia. **Why Combined Hyperlipidemia is correct:** FCHL is characterized by an overproduction of **Apolipoprotein B-100**, leading to increased VLDL secretion and subsequent elevation of LDL. Clinically, it presents with variable patterns: elevated cholesterol, elevated triglycerides, or both. Because it is highly prevalent in the general population and often manifests early due to genetic predisposition and lifestyle factors, it remains the most common diagnosis in children presenting with lipid abnormalities. **Why other options are incorrect:** * **Hypertriglyceridemia (Type IV):** While common in adults (often secondary to obesity or diabetes), it is less frequently diagnosed as a primary genetic disorder in children compared to FCHL. * **Hypercholesterolemia (Type IIa):** Specifically referring to Familial Hypercholesterolemia (LDL receptor defect). While significant, its prevalence (1 in 250-500) is lower than the combined variations seen in FCHL. * **Hyperchylomicronemia (Type I):** This is a rare autosomal recessive disorder (deficiency of LPL or Apo C-II). Though it presents in infancy with eruptive xanthomas and pancreatitis, it is statistically the least common. **High-Yield Clinical Pearls for NEET-PG:** * **FCHL (Type IIb):** Characterized by ↑LDL, ↑VLDL, and ↑Apo B-100. Unlike Familial Hypercholesterolemia, **tendon xanthomas are usually absent** in FCHL. * **Diagnosis:** Suspect in children with a family history of premature Coronary Artery Disease (CAD) before age 55. * **Type I vs. Type IV:** Type I presents with milky plasma and pancreatitis; Type IV is associated with insulin resistance.

Question 68: A single gene defect causing multiple unrelated problems is termed as the following?

A. Pleiotropism (Correct Answer)
B. Pseudodominance
C. Penetrance
D. Anticipation

Explanation: ### Explanation **Correct Answer: A. Pleiotropism** **1. Why Pleiotropism is Correct:** Pleiotropism (or Pleiotropy) refers to a genetic phenomenon where a **mutation in a single gene** results in **multiple, seemingly unrelated phenotypic effects** across different organ systems. This occurs because the gene product (usually a protein or enzyme) is either used in various tissues or has a signaling function that affects multiple downstream pathways. * **Classic Example:** **Phenylketonuria (PKU)**. A single defect in the *PAH* gene leads to mental retardation, reduced skin pigmentation, and a "mousy" body odor. * **Other Examples:** Marfan Syndrome (fibrillin-1 defect affecting eyes, heart, and skeleton) and Sickle Cell Anemia. **2. Why Other Options are Incorrect:** * **B. Pseudodominance:** This occurs when a recessive trait mimics a dominant pattern of inheritance. This happens if a homozygous recessive individual mates with a heterozygous carrier, resulting in a 50% chance of affected offspring in every generation. * **C. Penetrance:** This refers to the percentage of individuals with a specific genotype who actually express the associated phenotype. If everyone with the gene shows the disease, it is "100% penetrance." * **D. Anticipation:** This is the tendency for a genetic disorder to become more severe or appear at an earlier age in succeeding generations. It is typically seen in **Trinucleotide Repeat Disorders** (e.g., Huntington’s disease, Fragile X syndrome). **3. Clinical Pearls for NEET-PG:** * **Variable Expressivity:** Unlike pleiotropy (which describes *which* organs are hit), variable expressivity describes the *severity* of the disease among different individuals with the same genotype. * **Dominant Negative Mutation:** When a mutant protein antagonizes the function of the normal protein (e.g., Osteogenesis Imperfecta). * **Locus Heterogeneity:** Mutations at different loci (different genes) produce the same phenotype (e.g., Albinism).

Question 69: A newborn infant refuses breast milk from the second day of birth, vomits on force-feeding but accepts glucose-water, develops diarrhea on the third day. By the fifth day, the infant is jaundiced with liver enlargement, and the eyes show signs of cataract. Urinary reducing sugar was positive, but blood glucose estimated by the glucose oxidation method was found to be low. What is the most likely cause of this condition?

A. Galactose-1-phosphate uridyl transferase deficiency (Correct Answer)
B. Beta galactosidase deficiency
C. Glucose-6-phosphatase deficiency
D. Galactokinase deficiency

Explanation: ### Explanation The clinical presentation points toward **Classic Galactosemia**, caused by a deficiency of **Galactose-1-phosphate uridyl transferase (GALT)**. **1. Why Option A is Correct:** The infant develops symptoms (vomiting, diarrhea, jaundice, hepatomegaly) only after starting breast milk, which contains **lactose** (glucose + galactose). * **Biochemical Mechanism:** In GALT deficiency, Galactose-1-phosphate and galactitol accumulate in tissues. Galactitol accumulation in the lens causes **osmotic cataracts**. * **Diagnostic Clue:** The "Urinary reducing sugar positive" but "Low blood glucose (GOD method)" is a classic NEET-PG trap. The GOD (Glucose Oxidase) method is specific only for glucose. Since the reducing sugar in the urine is actually **galactose**, the dipstick/GOD method for blood glucose shows hypoglycemia, while the Benedict’s test for urine is positive. **2. Why Incorrect Options are Wrong:** * **Option B (Beta-galactosidase deficiency):** This leads to Lactose Intolerance. While it causes diarrhea and bloating, it does **not** cause cataracts, jaundice, or hepatomegaly. * **Option C (Glucose-6-phosphatase deficiency):** This is Von Gierke’s disease. It presents with severe fasting hypoglycemia and hepatomegaly but **not** with cataracts or symptoms specifically triggered by milk ingestion. * **Option D (Galactokinase deficiency):** This is a milder form of galactosemia. It causes **cataracts** due to galactitol accumulation but lacks the severe systemic features like jaundice, liver failure, or hypoglycemia seen in GALT deficiency. **3. NEET-PG High-Yield Pearls:** * **Classic Galactosemia (GALT):** "Vomiting + Jaundice + Cataract + E. coli Sepsis." * **Screening:** Benedict’s test (positive for reducing sugars) vs. Glucose Oxidase test (negative for galactose). * **Treatment:** Immediate withdrawal of milk; switch to soy-based or lactose-free formula. * **Inheritance:** Autosomal Recessive.

Question 70: Which of the following statements is WRONG about X-linked adrenoleukodystrophy?

A. It is a mitochondrial disorder. (Correct Answer)
B. It may present as Addison's disease.
C. It involves the accumulation of very long chain fatty acids.
D. It involves progressive neuronal demyelination.

Explanation: **Explanation:** **1. Why Option A is the Correct (Wrong) Statement:** X-linked Adrenoleukodystrophy (X-ALD) is a **peroxisomal disorder**, not a mitochondrial one. It is caused by a mutation in the **ABCD1 gene**, which encodes a peroxisomal membrane transporter protein. This protein is essential for the entry of Very Long Chain Fatty Acids (VLCFAs) into the peroxisome for degradation via **beta-oxidation**. **2. Analysis of Other Options:** * **Option B (Addison’s disease):** X-ALD frequently involves the adrenal cortex. The accumulation of VLCFAs is toxic to adrenal cells, leading to primary adrenal insufficiency. In many pediatric cases, Addison’s disease may be the first clinical manifestation before neurological symptoms appear. * **Option C (Accumulation of VLCFAs):** This is the biochemical hallmark of the disease. Due to the defective ABCD1 transporter, VLCFAs (fatty acids with >22 carbons) cannot be oxidized and thus accumulate in the blood and tissues (especially the brain and adrenal glands). * **Option D (Progressive neuronal demyelination):** The accumulation of VLCFAs in the CNS triggers an inflammatory response that destroys the myelin sheath. This leads to progressive cognitive decline, vision/hearing loss, and motor impairment. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked recessive (primarily affects males). * **Diagnosis:** Elevated plasma levels of **VLCFAs** (specifically C26:0). * **Zellweger Syndrome:** Contrast X-ALD with Zellweger Syndrome, which is a generalized defect in peroxisomal biogenesis (affecting all peroxisomal functions), whereas X-ALD is a specific transporter defect. * **Lorenzo’s Oil:** A dietary mixture of glyceryl trioleate and glyceryl trierucate used to lower VLCFA levels in some patients.

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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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