Genetic Disorders and Biochemical Pathology Practice Questions

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

Maternal nondisjunction. **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).

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

All the above. **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.

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

Macrocephaly. ### 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.**

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

Glycogen storage disorders. **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.

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

Mental retardation. **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).

Q: Homocystinuria is due to abnormal metabolism of

Methionine. **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).

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

McArdle's disease. **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.

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

Tryptophan. **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.

Question 1

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

Accepted Answer

Maternal nondisjunction

Answer

Translocation

Answer

Point mutations

Answer

Paternal nondisjunction

Question 2

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

Accepted Answer

All the above

Answer

Chlorpropamide

Answer

Phenytoin

Answer

Griseofulvin

Question 3

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

Accepted Answer

Macrocephaly

Answer

Mental retardation

Answer

Growth retardation

Answer

Congenital heart disease

Question 4

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

Accepted Answer

Glycogen storage disorders

Answer

Galactosemia

Answer

Hereditary fructose intolerance

Answer

Hyperinsulinism

Question 5

What is the treatment for mitochondrial disorders?

Accepted Answer

All of the above

Answer

Ascorbic acid

Answer

Folic acid

Answer

Coenzyme Q-10

Question 6

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

Accepted Answer

Mental retardation

Answer

Cerebellar ataxia

Answer

Cutaneous photosensitivity

Answer

Psychological disturbance

Question 7

Homocystinuria is due to abnormal metabolism of

Accepted Answer

Methionine

Answer

Valine

Answer

Tryptophan

Answer

Leucine

Question 8

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

Accepted Answer

Glycogen storage disorder

Answer

Carbohydrate metabolic disorder

Answer

Lipoprotein deficiency disorder

Answer

Protein folding disorder

Question 9

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

Accepted Answer

McArdle's disease

Answer

Von Gierke's disease

Answer

Her's disease

Answer

Pompe's disease

Question 10

Which amino acid is excreted in Hartnup's disease?

Accepted Answer

Tryptophan

Answer

Arginine

Answer

Hydroxyproline

Answer

Proline

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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