Genetic Disorders and Biochemical Pathology Practice Questions

Q: Which of the following is related to 'NARP'?

Mitochondria. **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.

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

Crigler-Najjar syndrome type I. **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.

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

Severe combined immunodeficiency disease. **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.

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

More formation of pentose. **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.

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

Combined hyperlipidemia. **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.

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

It is a mitochondrial disorder.. **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.

Question 1

Which of the following is related to 'NARP'?

Accepted Answer

Mitochondria

Answer

Glycogen storage diseases

Answer

Lipid storage disease

Answer

Protein

Question 2

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

Accepted Answer

Crigler-Najjar syndrome type I

Answer

Crigler-Najjar syndrome type II

Answer

Rotor syndrome

Answer

Gilbert syndrome

Question 3

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

Accepted Answer

VMA

Answer

5HIAA

Answer

5HT

Answer

5HTP

Question 4

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

Accepted Answer

Severe combined immunodeficiency disease

Answer

Organophosphorus poisoning

Answer

Cyanide poisoning

Answer

Acquired immunodeficiency disease

Question 5

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

Accepted Answer

More formation of pentose

Answer

Decreased availability of glucose to tissues

Answer

Increased accumulation of sorbitol

Answer

Impaired degradation of free radicals

Question 6

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:

Accepted Answer

the test was performed too early, before the enzyme deficiency manifested significantly.

Answer

cord blood is not a good source of fetal blood.

Answer

the screening (Guthrie) test has low sensitivity.

Answer

the test should have been performed on maternal blood.

Question 7

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

Accepted Answer

Combined hyperlipidemia

Answer

Hypertriglyceridemia

Answer

Hypercholesterolemia

Answer

Hyperchylomicronemia

Question 8

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

Accepted Answer

Pleiotropism

Answer

Pseudodominance

Answer

Penetrance

Answer

Anticipation

Question 9

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?

Accepted Answer

Galactose-1-phosphate uridyl transferase deficiency

Answer

Beta galactosidase deficiency

Answer

Glucose-6-phosphatase deficiency

Answer

Galactokinase deficiency

Question 10

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

Accepted Answer

It is a mitochondrial disorder.

Answer

It may present as Addison's disease.

Answer

It involves the accumulation of very long chain fatty acids.

Answer

It involves progressive neuronal demyelination.

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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