Genetic Disorders and Biochemical Pathology Practice Questions

Q: What is the enzyme defect in acute intermittent porphyria?

Hydroxymethylbilane synthase. **Explanation:** **Acute Intermittent Porphyria (AIP)** is an autosomal dominant metabolic disorder caused by a deficiency in the enzyme **Hydroxymethylbilane synthase (HMBS)**. This enzyme is responsible for the third step of heme biosynthesis, converting Porphobilinogen (PBG) into Hydroxymethylbilane. 1. **Why Option A is correct:** HMBS is the modern nomenclature for the enzyme deficient in AIP. When this enzyme is defective, there is a backup of precursors, specifically **Delta-aminolevulinic acid (ALA)** and **Porphobilinogen (PBG)**. These accumulated precursors are neurotoxic, leading to the classic clinical triad of abdominal pain, neuropsychiatric symptoms, and peripheral neuropathy. 2. **Why Option C is also technically correct (but less preferred):** **Uroporphyrinogen I synthase** is the older name for Hydroxymethylbilane synthase. In many exams, both terms are used interchangeably; however, HMBS is the current biochemical standard. 3. **Why other options are incorrect:** * **Option B (ALA Dehydratase):** Deficiency leads to ALA Dehydratase Deficiency Porphyria (Doss Porphyria), which is extremely rare. * **Option D (Uroporphyrinogen III synthase):** Deficiency causes **Congenital Erythropoietic Porphyria (Gunther’s disease)**, characterized by severe photosensitivity and erythrodontia (red teeth). **High-Yield Clinical Pearls for NEET-PG:** * **The "5 Ps" of AIP:** **P**ainful abdomen, **P**ort-wine colored urine (on standing), **P**olyneuropathy, **P**sychological disturbances, and **P**recipitated by drugs (e.g., Barbiturates, Sulfonamides). * **Key Diagnostic Finding:** Elevated urinary PBG and ALA. * **Crucial Fact:** AIP is a **non-photosensitive** porphyria because the metabolic block occurs before the formation of porphyrin rings. * **Management:** Treatment includes IV Hemin and Glucose (which inhibits ALA synthase, the rate-limiting enzyme).

Q: A 2-year-old child with intellectual disability presents with blue eyes, blonde hair, fair skin, and a peculiar body odor. What is the most likely diagnosis?

Phenylketonuria. **Explanation:** **Phenylketonuria (PKU)** is the correct diagnosis based on the classic clinical triad of intellectual disability, pigmentary dilution, and a characteristic odor. 1. **Why PKU is correct:** PKU is most commonly caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts phenylalanine to tyrosine. * **Intellectual Disability:** Accumulation of phenylalanine and its metabolites (phenylpyruvate, phenyllactate) is neurotoxic. * **Fair Skin, Blue Eyes, Blonde Hair:** Tyrosine is a precursor for **melanin**. Reduced tyrosine levels lead to hypopigmentation. * **Mousy/Musty Odor:** This is due to the excretion of **phenylacetic acid** in sweat and urine. 2. **Why other options are incorrect:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in Branched-Chain Alpha-Keto Acid Dehydrogenase. It presents with a **burnt sugar/maple syrup odor**, not fair skin or mousy odor. * **Isovaleric Aciduria:** A disorder of leucine metabolism characterized by a distinct **"sweaty feet" odor**. * **Canavan Disease:** A leukodystrophy presenting with macrocephaly, hypotonia, and developmental delay, but it lacks the specific pigmentary changes and mousy odor of PKU. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** Diet low in phenylalanine; **Tyrosine becomes an essential amino acid** for these patients. * **Maternal PKU:** If a mother with PKU doesn't maintain a strict diet during pregnancy, the fetus may develop microcephaly and congenital heart defects (even if the fetus doesn't have the genotype).

Q: Which protein is implicated in Alzheimer's disease?

All of the above. **Explanation:** Alzheimer’s Disease (AD) is a neurodegenerative disorder characterized by the accumulation of extracellular **amyloid-beta (Aβ) plaques** and intracellular **tau protein neurofibrillary tangles**. The pathogenesis involves several key proteins and genetic factors: 1. **Amyloid Precursor Protein (APP):** Located on **Chromosome 21**, APP is the parent molecule from which Aβ peptides are cleaved by β and γ-secretases. Mutations in APP lead to increased production of amyloidogenic peptides, explaining why Down Syndrome patients (Trisomy 21) develop early-onset AD. 2. **Presenilins (PSEN1 & PSEN2):** These are essential components of the **γ-secretase complex**. Mutations in **Presenilin I (Chr 14)** and **Presenilin II (Chr 1)** are the most common causes of Familial Early-Onset Alzheimer’s Disease, as they cause abnormal cleavage of APP. 3. **Apolipoprotein E (ApoE):** Located on **Chromosome 19**, the **ε4 allele** of ApoE is the strongest genetic risk factor for late-onset (sporadic) AD. It is involved in the impaired clearance and increased deposition of Aβ plaques. **Why "All of the above" is correct:** All three proteins—APP, Presenilin II, and ApoE—play definitive roles in the biochemical pathway leading to the neurotoxicity seen in Alzheimer's. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of Early-Onset AD:** Mutations in **Presenilin 1** (Chromosome 14). * **Protective Factor:** The **ApoE ε2** allele is associated with a *decreased* risk of Alzheimer's. * **Histopathology:** Silver stains (like Bielschowsky) are used to visualize plaques and tangles. * **Biochemical Marker:** Decreased levels of **Acetylcholine** in the nucleus basalis of Meynert.

Q: Alpha fetoprotein is genetically and structurally related to which of the following?

Albumin. **Explanation:** **Alpha-fetoprotein (AFP)** is a major plasma protein produced by the fetal yolk sac and liver. It is genetically and structurally related to **Albumin** because both belong to the same multigene family (the albuminoid family), which also includes vitamin D-binding protein and afamin. 1. **Why Albumin is Correct:** * **Genetic Link:** The genes for AFP and Albumin are located in tandem on the long arm of **chromosome 4 (4q11-q13)**. * **Structural Similarity:** They share approximately 40% sequence homology. Both are single-chain polypeptides with similar tertiary structures characterized by three repeating domains and a high content of disulfide bonds. * **Functional Analogy:** AFP acts as the fetal counterpart of albumin, maintaining oncotic pressure and serving as a carrier protein for steroids, fatty acids, and bilirubin in the fetus. 2. **Why Other Options are Incorrect:** * **Transferrin:** This is an iron-transport protein synthesized in the liver. While it is a globulin, it belongs to a different genetic family and lacks the structural domain homology of the albuminoid group. * **Fibrinogen:** A large, complex glycoprotein involved in clotting. It is structurally distinct (hexameric) and unrelated to the albumin family. * **Growth Hormone:** A peptide hormone produced by the anterior pituitary. Its structure and genetic locus (chromosome 17) are entirely different. **High-Yield Clinical Pearls for NEET-PG:** * **AFP Levels in Pregnancy:** Elevated in **Neural Tube Defects (NTDs)** (e.g., spina bifida, anencephaly) and abdominal wall defects (omphalocele). Decreased in **Down Syndrome (Trisomy 21)**. * **Tumor Marker:** AFP is a highly specific marker for **Hepatocellular Carcinoma (HCC)** and **Non-seminomatous Germ Cell Tumors (Yolk sac tumors)**. * **Switch:** After birth, AFP synthesis is rapidly repressed, and albumin synthesis becomes dominant.

Q: Wilson disease is caused by a defect in which gene?

ATP7B mutation. **Explanation:** Wilson disease (Hepatolenticular degeneration) is an autosomal recessive disorder of copper metabolism. The correct answer is **ATP7B mutation** because this gene, located on **chromosome 13**, encodes a copper-transporting P-type ATPase primarily expressed in the liver. This protein is essential for two processes: transporting copper into the Golgi apparatus for incorporation into **ceruloplasmin** and facilitating the excretion of excess copper into **bile**. A defect leads to toxic copper accumulation in the liver, brain (basal ganglia), and cornea. **Analysis of Incorrect Options:** * **ATP7A mutation (Option A):** This causes **Menkes disease** ("Kinky Hair Syndrome"). ATP7A is responsible for intestinal copper absorption and systemic distribution. Its deficiency leads to systemic copper deficiency, unlike the overload seen in Wilson disease. * **ATP6A/ATP6B (Options B & C):** These genes encode subunits of vacuolar ATPase (V-ATPase) involved in proton pumping and acidification of intracellular organelles. Mutations in ATP6B1, for example, are associated with distal renal tubular acidosis with sensorineural deafness, not copper metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Low serum ceruloplasmin, increased urinary copper excretion, and increased hepatic copper content on biopsy. * **Kayser-Fleischer (KF) rings:** Copper deposition in the **Descemet’s membrane** of the cornea (best seen on slit-lamp exam). * **Neurological sign:** "Giant Panda face" appearance on MRI midbrain. * **Treatment:** Chelating agents like **D-Penicillamine** (first-line) or Trientine, and Zinc (inhibits intestinal absorption).

Q: Asians and Native Americans may experience flushing and feel ill after consuming small amounts of ethanol. This reaction is due to a genetic variation in which enzyme?

Aldehyde dehydrogenase. **Explanation:** The "Alcohol Flush Reaction" (commonly known as the Asian Flush) is caused by a genetic deficiency in **Aldehyde Dehydrogenase (ALDH)**, specifically the mitochondrial **ALDH2** isoform. **1. Why Aldehyde Dehydrogenase is Correct:** Alcohol metabolism occurs in two steps: * **Step 1:** Ethanol is converted to **Acetaldehyde** by Alcohol Dehydrogenase (ADH). * **Step 2:** Acetaldehyde is converted to **Acetate** by Aldehyde Dehydrogenase (ALDH). In many individuals of East Asian descent, a point mutation leads to an inactive form of ALDH2. This causes a toxic accumulation of **Acetaldehyde**, a potent vasodilator that triggers flushing, tachycardia, nausea, and dizziness. **2. Why Other Options are Incorrect:** * **Alcohol Dehydrogenase (ADH):** While some populations have a "fast-acting" variant of ADH (ADH1B), which produces acetaldehyde quickly, the primary cause of the severe illness and flushing is the inability to *clear* the acetaldehyde via ALDH. * **Isocitrate Dehydrogenase:** This is a rate-limiting enzyme of the TCA cycle, converting isocitrate to alpha-ketoglutarate; it is not involved in ethanol metabolism. * **Alpha-keto glutarate Dehydrogenase:** This is a TCA cycle enzyme complex requiring Thiamine (B1) as a cofactor. While chronic alcoholism leads to B1 deficiency affecting this enzyme (Wernicke-Korsakoff), it does not cause acute flushing. **Clinical Pearls for NEET-PG:** * **Disulfiram (Antabuse):** This drug works by inhibiting ALDH, mimicking the genetic deficiency to discourage alcohol consumption. * **Metronidazole:** Known for causing a "Disulfiram-like reaction" when taken with alcohol. * **Cofactor:** Both ADH and ALDH require **NAD+** as a cofactor. The high NADH/NAD+ ratio in alcoholics leads to lactic acidosis and hepatic steatosis.

Q: Wilson's disease is inherited in which pattern?

Autosomal Recessive (AR). **Explanation:** Wilson’s disease (Hepatolenticular degeneration) is an **Autosomal Recessive (AR)** disorder caused by mutations in the **ATP7B gene** located on **Chromosome 13**. This gene encodes a copper-transporting P-type ATPase, which is essential for the excretion of excess copper into bile and its incorporation into ceruloplasmin. A defect in this protein leads to toxic copper accumulation in the liver, brain (basal ganglia), cornea, and kidneys. **Why the other options are incorrect:** * **Autosomal Dominant (AD):** AD disorders (e.g., Huntington’s, Marfan syndrome) typically involve structural proteins or receptors. Wilson’s disease involves an enzyme/transporter defect, which characteristically follows an AR pattern where both alleles must be mutated for the phenotype to manifest. * **X-linked Recessive (XLR):** These primarily affect males (e.g., Hemophilia, G6PD deficiency). Wilson’s disease affects males and females equally. Note: Menkes disease (kinky hair syndrome) is an X-linked copper disorder, but it involves copper *deficiency* due to ATP7A mutation. * **X-linked Dominant:** These are rare (e.g., Alport syndrome, Vitamin D resistant rickets) and do not match the inheritance or pathophysiology of copper metabolism disorders. **NEET-PG High-Yield Pearls:** * **Diagnosis:** Low serum ceruloplasmin, increased urinary copper excretion, and increased hepatic copper content (Gold Standard). * **Clinical Signs:** **Kayser-Fleischer (KF) rings** in Descemet’s membrane, Parkinsonian symptoms, and Coombs-negative hemolytic anemia. * **Treatment:** Copper chelators like **D-Penicillamine** (first-line) or Trientine, and Zinc (inhibits intestinal copper absorption).

Q: A mother has sickle cell disease and the father is normal. What are the chances of their children having sickle cell disease and sickle cell trait, respectively?

0% and 100%. ### Explanation **1. Understanding the Genetics (Why A is correct):** Sickle cell disease (SCD) is an **autosomal recessive** disorder. To determine the inheritance pattern, we must identify the genotypes of the parents: * **Mother (Sickle Cell Disease):** Genotype is **HbSS** (homozygous recessive). * **Father (Normal):** Genotype is **HbAA** (homozygous dominant). Using a Punnett square, all possible offspring will receive one 'S' allele from the mother and one 'A' allele from the father. * **Offspring Genotype:** 100% **HbAS** (Heterozygous). * **Phenotype:** 100% will have **Sickle Cell Trait** (carriers), and **0%** will have **Sickle Cell Disease** (which requires HbSS). **2. Analysis of Incorrect Options:** * **Option B (25% and 25%):** This ratio does not fit any standard Mendelian cross for this disease. A 25% chance of disease occurs only when *both* parents are carriers (HbAS x HbAS). * **Option C (50% and 50%):** This occurs if one parent has the disease (HbSS) and the other is a carrier (HbAS). * **Option D (10% and 50%):** These percentages do not align with Mendelian inheritance patterns for single-gene autosomal disorders. **3. NEET-PG High-Yield Clinical Pearls:** * **Molecular Basis:** A point mutation (missense) in the β-globin gene on **Chromosome 11**, where **Glutamic acid** is replaced by **Valine** at the 6th position. * **Electrophoresis:** On alkaline electrophoresis, HbS moves slower than HbA toward the anode (mnemonic: **A**frican **F**at **S**low **C**hild – order of mobility A>F>S>C). * **Protective Effect:** Sickle cell trait (HbAS) provides a selective advantage against *Plasmodium falciparum* malaria. * **Precipitating Factors:** Sickling is induced by hypoxia, acidosis, dehydration, and cold.

Question 1

What is the enzyme defect in acute intermittent porphyria?

Accepted Answer

Hydroxymethylbilane synthase

Answer

Aminolevulinate dehydratase

Answer

Uroporphyrinogen I synthase

Answer

Uroporphyrinogen III synthase

Question 2

A 2-year-old child with intellectual disability presents with blue eyes, blonde hair, fair skin, and a peculiar body odor. What is the most likely diagnosis?

Accepted Answer

Phenylketonuria

Answer

Maple syrup urine disease

Answer

Isovaleric aciduria

Answer

Canavan disease

Question 3

Which protein is implicated in Alzheimer's disease?

Accepted Answer

All of the above

Answer

Presenilin II

Answer

Apolipoprotein E gene

Answer

Amyloid precursor protein

Question 4

Alpha fetoprotein is genetically and structurally related to which of the following?

Accepted Answer

Albumin

Answer

Transferrin

Answer

Fibrinogen

Answer

Growth hormone

Question 5

An 8-year-old boy presents with failure to thrive, alopecia totalis, localized scleroderma, a small face and jaw, a "beak" nose, wrinkled skin, and stiff joints. He has a single-point mutation in a nuclear protein that is silent in terms of the protein's primary structure. How could such a mutation lead to a disease?

Accepted Answer

Creating an alternative splice site in the gene

Answer

Altering the tertiary structure of the protein

Answer

Inhibiting DNA replication

Answer

Introducing a premature stop codon into the protein

Question 6

Wilson disease is caused by a defect in which gene?

Accepted Answer

ATP7B mutation

Answer

ATP7A mutation

Answer

ATP6A mutation

Answer

ATP6B mutation

Question 7

Asians and Native Americans may experience flushing and feel ill after consuming small amounts of ethanol. This reaction is due to a genetic variation in which enzyme?

Accepted Answer

Aldehyde dehydrogenase

Answer

Alcohol dehydrogenase

Answer

Isocitrate dehydrogenase

Answer

Alpha-keto glutarate dehydrogenase

Question 8

Wilson's disease is inherited in which pattern?

Accepted Answer

Autosomal Recessive (AR)

Answer

Autosomal Dominant (AD)

Answer

X-linked Dominant

Answer

X-linked Recessive

Question 9

Lesch-Nyhan syndrome is associated with a deficiency of which enzyme?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (total)

Answer

Hypoxanthine-guanine phosphoribosyltransferase (partial)

Answer

Phosphoribosyl pyrophosphate amidotransferase (partial)

Answer

Phosphoribosyl pyrophosphate amidotransferase (total)

Question 10

A mother has sickle cell disease and the father is normal. What are the chances of their children having sickle cell disease and sickle cell trait, respectively?

Accepted Answer

0% and 100%

Answer

25% and 25%

Answer

50% and 50%

Answer

10% and 50%

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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