Genetic Disorders and Biochemical Pathology — MCQs

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

Question 481: In Down syndrome, the chromosomal abnormality is most commonly due to which of the following?

A. Translocation
B. Point mutations
C. Paternal nondisjunction
D. Maternal nondisjunction (Correct Answer)

Explanation: **Explanation:** **1. Why Maternal Nondisjunction is Correct:** Down syndrome (Trisomy 21) is primarily caused by **meiotic nondisjunction**, where chromosomes fail to separate during gametogenesis. In approximately **95% of cases**, the extra chromosome 21 is derived from the mother. This occurs most frequently during **Meiosis I**. The risk of maternal nondisjunction increases significantly with **advanced maternal age** (typically >35 years) due to the prolonged "dictyotene" stage (prophase I) in which oocytes are arrested from fetal life until ovulation. **2. Analysis of Incorrect Options:** * **Translocation (Option A):** Accounts for only **3–4%** of cases (Robertsonian translocation, usually between chromosomes 14 and 21). Unlike nondisjunction, this is independent of maternal age and can be inherited from a carrier parent. * **Point Mutations (Option B):** Down syndrome is a **numerical chromosomal aberration** (aneuploidy), not a single-gene disorder or a change in the DNA sequence at a specific locus. * **Paternal Nondisjunction (Option C):** While nondisjunction can occur during spermatogenesis, it accounts for only about **5%** of Down syndrome cases. **3. High-Yield Clinical Pearls for NEET-PG:** * **Most common cause:** Maternal Meiosis I nondisjunction (95%). * **Mosaicism:** Accounts for ~1–2% of cases; caused by **mitotic** nondisjunction during early embryogenesis. * **Biochemical Screening (Quadruple Test):** Characterized by **decreased** AFP and Estriol, and **increased** hCG and Inhibin-A (Mnemonic: **HI** is **High** – **H**CG and **I**nhibin). * **First Trimester Screening:** Increased Nuchal Translucency (NT) and decreased PAPP-A. * **Associated Pathology:** Early-onset Alzheimer’s (APP gene on Ch 21), Endocardial cushion defects (ASD/VSD), and increased risk of ALL/AML (M7).

Question 482: Acute intermittent porphyria is precipitated by which of the following drugs?

A. Chlorpropamide
B. Phenytoin
C. Griseofulvin
D. All the above (Correct Answer)

Explanation: **Explanation:** **Acute Intermittent Porphyria (AIP)** is an autosomal dominant metabolic disorder caused by a deficiency of the enzyme **Porphobilinogen (PBG) deaminase**. The clinical manifestations are triggered when there is an induction of the rate-limiting enzyme of heme synthesis, **ALA Synthase 1 (ALAS1)**, leading to the toxic accumulation of delta-aminolevulinic acid (ALA) and porphobilinogen. **Mechanism of Precipitation:** The drugs listed (Chlorpropamide, Phenytoin, and Griseofulvin) are potent inducers of the **Cytochrome P450 (CYP450)** system in the liver. 1. These drugs increase the demand for heme (a prosthetic group for CYP450 enzymes). 2. The resulting depletion of the free "regulatory heme pool" removes the feedback inhibition on **ALAS1**. 3. This causes a massive overproduction of heme precursors, precipitating an acute neurovisceral attack. **Analysis of Options:** * **A. Chlorpropamide:** A first-generation sulfonylurea known to trigger porphyric attacks. * **B. Phenytoin:** An antiepileptic that strongly induces hepatic enzymes. * **C. Griseofulvin:** An antifungal agent notorious for being one of the most potent precipitants of AIP. * **D. All the above:** Since all three drugs utilize the CYP450 pathway and induce ALAS1, they are all contraindicated. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Abdominal pain (most common), Neuropsychiatric symptoms, and Peripheral neuropathy. * **Diagnostic Sign:** Urine turns **"Port-wine" colored** on standing due to the oxidation of PBG to porphobilin. * **Management:** Treatment involves **IV Hemin** (suppresses ALAS1) and **Glucose loading** (high carbohydrate diet inhibits ALAS1). * **Safe Drugs:** Propranolol, Morphine, and Gabapentin are generally considered safe in AIP patients.

Question 483: A child born to a mother with phenylketonuria will have all of the following conditions except?

A. Mental retardation
B. Macrocephaly (Correct Answer)
C. Growth retardation
D. Congenital heart disease

Explanation: ### Explanation This question refers to **Maternal Phenylketonuria (PKU) Syndrome**, a condition where a mother with PKU has poorly controlled phenylalanine (Phe) levels during pregnancy. High maternal Phe levels act as a **teratogen**, crossing the placenta and causing irreversible damage to the developing fetus, even if the fetus itself does not have the PKU genotype. #### Why Macrocephaly is the Correct Answer: The hallmark of Maternal PKU Syndrome is **Microcephaly** (small head size), not macrocephaly. Phenylalanine and its metabolites are neurotoxic and interfere with normal fetal brain development and myelination, leading to a significantly smaller brain and skull. #### Analysis of Incorrect Options: * **Mental Retardation (A):** This is the most common and severe outcome. High Phe levels disrupt amino acid transport across the blood-brain barrier and inhibit neurotransmitter synthesis in the fetus, leading to profound intellectual disability. * **Growth Retardation (C):** Intrauterine growth restriction (IUGR) is a classic feature. Elevated Phe interferes with general protein synthesis and cellular metabolism, resulting in low birth weight and postnatal growth failure. * **Congenital Heart Disease (D):** Maternal PKU is associated with a high incidence of structural cardiac defects, most commonly **Ventricular Septal Defects (VSD)** and Tetralogy of Fallot. #### NEET-PG High-Yield Pearls: * **Prevention:** To prevent Maternal PKU Syndrome, the mother must maintain a **strict low-phenylalanine diet** (Phe levels between 2–6 mg/dL) starting *before* conception and continuing throughout pregnancy. * **Teratogenic vs. Genetic:** The damage is caused by the **maternal environment**, not the fetal genotype. Even a heterozygous (carrier) fetus will be affected by the high maternal Phe levels. * **Classic Triad:** Think of Maternal PKU as causing **Microcephaly, Mental Retardation, and Congenital Heart Defects.**

Question 484: Non-ketotic hypoglycemia is seen in all of the following conditions except?

A. Galactosemia
B. Hereditary fructose intolerance
C. Hyperinsulinism
D. Glycogen storage disorders (Correct Answer)

Explanation: **Explanation:** The hallmark of **non-ketotic hypoglycemia** is the absence of ketone bodies despite low blood glucose levels. This occurs when there is either an inhibition of fatty acid oxidation or a suppression of lipolysis. **1. Why Glycogen Storage Disorders (GSDs) are the exception:** In most GSDs (specifically Type I - von Gierke disease), hypoglycemia occurs due to the inability to release glucose from glycogen. However, the counter-regulatory hormones (glucagon and epinephrine) are activated. This triggers **intact fatty acid oxidation**, leading to the production of acetyl-CoA and subsequently **ketone bodies**. Therefore, GSDs typically present with **ketotic hypoglycemia**. **2. Analysis of Incorrect Options:** * **Hyperinsulinism:** Insulin is a potent inhibitor of lipolysis and fatty acid oxidation. High insulin levels prevent the formation of ketones, leading to profound non-ketotic hypoglycemia. * **Galactosemia & Hereditary Fructose Intolerance (HFI):** In these conditions, the accumulation of phosphorylated sugars (Galactose-1-P or Fructose-1-P) depletes intracellular inorganic phosphate. This inhibits both glycogenolysis and gluconeogenesis. The acute metabolic derangement often interferes with the transition to ketosis, typically presenting as non-ketotic hypoglycemia in the acute phase. **3. NEET-PG High-Yield Pearls:** * **Ketotic Hypoglycemia:** Seen in GSD Type I, Ketotic hypoglycemia of childhood (most common cause in toddlers), and Maple Syrup Urine Disease (MSUD). * **Non-Ketotic Hypoglycemia:** Think of **MCAD deficiency** (impaired beta-oxidation), **Hyperinsulinism**, and **Systemic Carnitine deficiency**. * **Key Diagnostic:** If a question mentions "hypoglycemia with absent ketones," always prioritize Fatty Acid Oxidation Disorders or Insulin excess.

Question 485: What is the treatment for mitochondrial disorders?

A. Ascorbic acid
B. Folic acid
C. Coenzyme Q-10
D. All of the above (Correct Answer)

Explanation: ### Explanation Mitochondrial disorders (e.g., MELAS, MERFF, Leigh Syndrome) result from defects in the **Mitochondrial Respiratory Chain (Electron Transport Chain)**, leading to impaired ATP production and increased oxidative stress. Since there is no definitive cure, management focuses on a **"Mitochondrial Cocktail"** designed to bypass enzymatic blocks and scavenge free radicals. **Why "All of the Above" is correct:** The treatment strategy involves three main components: 1. **Coenzyme Q-10 (Ubiquinone):** This is the most critical component. It acts as an essential electron carrier between Complexes I/II and Complex III. Supplementation improves electron flow and acts as a potent lipid-soluble antioxidant. 2. **Ascorbic Acid (Vitamin C):** It serves as a powerful water-soluble antioxidant that neutralizes reactive oxygen species (ROS) generated by a dysfunctional ETC. It can also act as an electron donor to Cytochrome C. 3. **Folic Acid:** Many mitochondrial diseases (like Kearns-Sayre syndrome) are associated with secondary cerebral folate deficiency. Supplementation helps maintain normal neurological function and supports one-carbon metabolism. **Other Components of the "Mitochondrial Cocktail":** * **Riboflavin (B2):** A precursor for FAD; specifically helpful in Complex I and II defects. * **L-Carnitine:** Aids in the transport of fatty acids into the mitochondria and helps remove toxic acyl-CoA metabolites. * **Thiamine (B1):** A cofactor for Pyruvate Dehydrogenase; helps reduce lactic acidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Mitochondrial DNA (mtDNA) disorders show **maternal inheritance** (all children of an affected mother are affected; fathers do not pass it on). * **Heteroplasmy:** The presence of a mixture of wild-type and mutant mtDNA within a single cell, explaining the clinical variability. * **Biochemical Marker:** Elevated **Lactic acid** in blood and CSF is a hallmark of mitochondrial dysfunction. * **Muscle Biopsy:** Characterized by **"Ragged Red Fibers"** on Gomori trichrome stain.

Question 486: Which of the following is NOT a feature of Hartnup's disease?

A. Cerebellar ataxia
B. Cutaneous photosensitivity
C. Mental retardation (Correct Answer)
D. Psychological disturbance

Explanation: **Explanation:** **Hartnup’s disease** is an autosomal recessive disorder characterized by a defect in the **SLC6A19 transporter**, which is responsible for the absorption of neutral amino acids (especially **Tryptophan**) in the renal proximal tubules and the small intestine. **Why "Mental Retardation" is the correct answer:** While patients may experience transient neurological symptoms during flares, **permanent mental retardation is NOT a characteristic feature** of Hartnup’s disease. This distinguishes it from other metabolic disorders like Phenylketonuria (PKU). Most patients lead a normal life with proper nutrition. **Analysis of other options:** * **Cutaneous photosensitivity (B):** Tryptophan is a precursor for **Niacin (Vitamin B3)**. Deficiency leads to "Pellagra-like" skin rashes, specifically a scaly, erythematous eruption on sun-exposed areas. * **Cerebellar ataxia (A):** Intermittent neurological symptoms, such as unsteady gait and incoordination, occur during periods of severe tryptophan depletion or metabolic stress. * **Psychological disturbance (D):** Patients often present with emotional lability, irritability, or even delirium during acute exacerbations. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmark:** Neutral aminoaciduria (specifically excluding proline, hydroxyproline, and arginine). * **The 3 D's:** Symptoms mimic Pellagra (Dermatitis, Diarrhea, Dementia/Depression), but the cause is amino acid transport failure, not primary dietary niacin deficiency. * **Diagnosis:** High levels of neutral amino acids in urine (Chromatography). * **Treatment:** High-protein diet and **Nicotinamide** supplementation. * **Indicanuria:** Unabsorbed tryptophan in the gut is converted by bacteria into indoles, which are excreted in urine (blue diaper syndrome variant).

Question 487: Homocystinuria is due to abnormal metabolism of

A. Methionine (Correct Answer)
B. Valine
C. Tryptophan
D. Leucine

Explanation: **Explanation:** **Homocystinuria** is a group of inherited metabolic disorders characterized by the accumulation of homocysteine in the blood and urine. The most common form (Classical Homocystinuria) is caused by a deficiency of the enzyme **Cystathionine β-synthase (CBS)**. **Why Methionine is the correct answer:** Homocysteine is an intermediate in the metabolism of **Methionine**, a sulfur-containing amino acid. In the normal pathway, Methionine is converted to S-adenosylmethionine (SAM), then to S-adenosylhomocysteine (SAH), and finally to Homocysteine. Under normal conditions, Homocysteine is then converted to Cystathionine by CBS (requiring Vitamin B6). A defect in this pathway leads to a "backlog," causing elevated levels of both Homocysteine and its precursor, Methionine. **Why the other options are incorrect:** * **Valine and Leucine:** These are **Branched-Chain Amino Acids (BCAAs)**. Abnormal metabolism of these leads to **Maple Syrup Urine Disease (MSUD)**, not Homocystinuria. * **Tryptophan:** This is an aromatic amino acid. Defects in its metabolism or transport are associated with **Hartnup disease** or Pellagra-like symptoms, but not Homocystinuria. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** 1. Ectopia lentis (downward subluxation of lens), 2. Intellectual disability, 3. Thromboembolic episodes (major cause of early death). * **Skeletal features:** Marfanoid habitus (tall stature, long limbs). * **Treatment:** High doses of **Vitamin B6 (Pyridoxine)** are effective in ~50% of cases (B6-responsive). Other treatments include a low-methionine diet and Cysteine supplementation. * **Diagnosis:** Positive **Cyanide-nitroprusside test** (detects sulfhydryl groups in urine).

Question 488: Severe hypoglycemia, increased uric acid, and renal failure are characteristic findings in which of the following types of disorders?

A. Carbohydrate metabolic disorder
B. Glycogen storage disorder (Correct Answer)
C. Lipoprotein deficiency disorder
D. Protein folding disorder

Explanation: **Explanation:** The clinical triad of **severe hypoglycemia, hyperuricemia (increased uric acid), and renal involvement** is the hallmark of **Glycogen Storage Disease (GSD) Type I (Von Gierke Disease)**. 1. **Why the correct answer is right:** GSD Type I is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is essential for both glycogenolysis and gluconeogenesis. Its absence leads to: * **Severe Hypoglycemia:** The liver cannot release free glucose into the blood during fasting. * **Hyperuricemia:** Accumulated Glucose-6-Phosphate enters the Pentose Phosphate Pathway, increasing ribose-5-phosphate production, which drives purine synthesis and subsequent degradation to uric acid. Additionally, lactic acidosis (from diverted glycolysis) competes with uric acid for excretion in the kidneys. * **Renal Failure:** Glucose-6-Phosphatase is also expressed in the kidneys; its deficiency leads to glycogen deposition in renal tubules, causing nephromegaly and progressive renal dysfunction. 2. **Why incorrect options are wrong:** * **Carbohydrate metabolic disorders:** While GSD is a subset, this category is too broad (includes Galactosemia or Hereditary Fructose Intolerance). While they cause hypoglycemia, they do not typically present with the specific renal failure/hyperuricemia profile of GSD Type I. * **Lipoprotein deficiency:** These (e.g., Abetalipoproteinemia) present with malabsorption and neurological issues, not acute hypoglycemia. * **Protein folding disorders:** These (e.g., Amyloidosis, Prion diseases) involve organ-specific deposition but do not cause acute metabolic hypoglycemia. **High-Yield NEET-PG Pearls:** * **GSD Type I (Von Gierke):** Look for "Doll-like facies," massive hepatomegaly, and hyperlipidemia (xanthomas). * **Key Biochemical Markers:** Hyper-**L**actatemia, Hyper-**U**ricemia, Hyper-**L**ipidemia, and **H**ypoglycemia (Mnemonic: **LULH**). * **Treatment:** Frequent feeds with uncooked cornstarch to maintain glucose levels.

Question 489: After strenuous exercise, an alkaline pH of skeletal muscle is characteristic of which glycogen storage disease?

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

Explanation: **Explanation:** **McArdle’s Disease (GSD Type V)** is caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**. Under normal physiological conditions, strenuous exercise triggers glycogenolysis, converting muscle glycogen into glucose-1-phosphate, which eventually enters glycolysis to produce **lactic acid**. The accumulation of lactic acid typically causes the intramuscular pH to drop (become acidic). In McArdle’s disease, the inability to break down glycogen means no lactic acid is produced during exercise. Instead, there is a compensatory increase in blood flow and the breakdown of creatine phosphate and proteins, leading to an accumulation of **ammonia**. This lack of lactate combined with increased ammonia results in an **alkaline pH** (or a failure of the pH to fall) during an ischemic exercise test—a classic diagnostic hallmark. **Why other options are incorrect:** * **Von Gierke’s Disease (Type I):** Caused by Glucose-6-Phosphatase deficiency. It primarily affects the liver, leading to severe fasting hypoglycemia and **lactic acidosis**, not alkalinity. * **Hers’ Disease (Type VI):** Caused by liver phosphorylase deficiency. It presents with hepatomegaly and mild hypoglycemia; it does not affect muscle metabolism or muscle pH. * **Pompe’s Disease (Type II):** Caused by lysosomal α-1,4-glucosidase deficiency. It affects the heart and muscles via lysosomal accumulation but does not specifically cause an alkaline pH during exercise. **High-Yield Clinical Pearls for NEET-PG:** * **Ischemic Exercise Test:** Failure of blood lactate to rise with a concomitant rise in ammonia is diagnostic for McArdle’s. * **Second Wind Phenomenon:** Patients often experience relief after 10–15 minutes of exercise as the body switches to using fatty acids and blood glucose. * **Clinical Triad:** Exercise intolerance, muscle cramps, and **myoglobinuria** (burgundy-colored urine) after exertion.

Question 490: Which amino acid is excreted in Hartnup's disease?

A. Arginine
B. Hydroxyproline
C. Tryptophan (Correct Answer)
D. Proline

Explanation: **Explanation:** **Hartnup’s disease** is an autosomal recessive disorder caused by a mutation in the **SLC6A19 gene**, which encodes a sodium-dependent neutral amino acid transporter. This defect occurs in both the proximal renal tubules and the intestinal mucosa. **1. Why Tryptophan is the Correct Answer:** The primary biochemical defect is the impaired transport of **neutral amino acids** (monoamino-monocarboxylic acids). While several neutral amino acids are affected (including alanine, serine, and valine), the clinical manifestations are primarily due to the loss of **Tryptophan**. Tryptophan is a precursor for **Niacin (Vitamin B3)** via the kynurenine pathway. Deficiency leads to "Pellagra-like" symptoms, including dermatitis, diarrhea, and dementia. **2. Why the Incorrect Options are Wrong:** * **Arginine (Option A):** This is a basic amino acid. Defective transport of basic amino acids (Cystine, Lysine, Arginine, Ornithine) is characteristic of **Cystinuria**, not Hartnup’s. * **Proline & Hydroxyproline (Options B & D):** These are imino acids. Their excretion in urine is seen in **Familial Iminoglycinuria**, a benign condition involving a defect in the transport of proline, hydroxyproline, and glycine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Photosensitive skin rash (Pellagra-like), cerebellar ataxia, and emotional lability. * **Diagnosis:** Characterized by **neutral aminoaciduria**. A key diagnostic finding is the presence of **Indican** in the urine (formed by bacterial degradation of unabsorbed tryptophan in the gut). * **Treatment:** High-protein diet and **Nicotinamide (Niacin)** supplementation. * **Mnemonic:** Hartnup = **N**eutral amino acids = **N**iacin deficiency.

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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