Genetic Disorders and Biochemical Pathology — MCQs

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?

Q556Easy

Wilson disease is caused by a defect in which gene?

Q557Medium

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?

Q558Easy

Wilson's disease is inherited in which pattern?

Q559Easy

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

Q560Medium

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?

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

Question 551: What is the enzyme defect in acute intermittent porphyria?

A. Hydroxymethylbilane synthase (Correct Answer)
B. Aminolevulinate dehydratase
C. Uroporphyrinogen I synthase
D. Uroporphyrinogen III synthase

Explanation: **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).

Question 552: 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?

A. Maple syrup urine disease
B. Isovaleric aciduria
C. Phenylketonuria (Correct Answer)
D. Canavan disease

Explanation: **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).

Question 553: Which protein is implicated in Alzheimer's disease?

A. Presenilin II
B. Apolipoprotein E gene
C. Amyloid precursor protein
D. All of the above (Correct Answer)

Explanation: **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.

Question 554: Alpha fetoprotein is genetically and structurally related to which of the following?

A. Albumin (Correct Answer)
B. Transferrin
C. Fibrinogen
D. Growth hormone

Explanation: **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.

Question 555: 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?

A. Altering the tertiary structure of the protein
B. Inhibiting DNA replication
C. Introducing a premature stop codon into the protein
D. Creating an alternative splice site in the gene (Correct Answer)

Explanation: ### **Explanation** The clinical presentation described—alopecia, "beak" nose, scleroderma-like skin, and premature aging—is classic for **Hutchinson-Gilford Progeria Syndrome (HGPS)**. **1. Why the Correct Answer is Right:** HGPS is caused by a specific mutation in the **LMNA gene**, which encodes **Lamin A**, a structural protein of the nuclear scaffold. The most common mutation is a **silent mutation (C1824T)**. Although this mutation does not change the amino acid (Glycine remains Glycine), it creates a **cryptic (alternative) 5' splice site** within exon 11. * During mRNA processing, the spliceosome recognizes this new site, leading to the deletion of 150 nucleotides. * This results in an abnormal protein called **Progerin**, which lacks a critical cleavage site. Progerin remains permanently farnesylated and anchored to the nuclear envelope, causing nuclear instability and the characteristic phenotype of accelerated aging. **2. Why Incorrect Options are Wrong:** * **Option A:** A silent mutation, by definition, does not change the primary amino acid sequence; therefore, it cannot directly alter the protein's tertiary structure through R-group interactions. * **Option B:** While nuclear instability eventually affects the cell cycle, the primary molecular defect is a splicing error, not the direct inhibition of DNA polymerase or replication machinery. * **Option C:** A mutation that introduces a premature stop codon is a **nonsense mutation**, not a silent mutation. **3. Clinical Pearls for NEET-PG:** * **Gene Involved:** *LMNA* (Lamin A/C). * **Molecular Mechanism:** Activation of a cryptic splice site (Synonymous mutation). * **Key Feature:** "Progerin" accumulation leads to nuclear blebbing. * **Differential Diagnosis:** **Werner Syndrome** (Adult-onset progeria) caused by a defect in the *WRN* gene (DNA Helicase). * **High-Yield Fact:** HGPS is an example of how "silent" mutations can be pathogenic by affecting **pre-mRNA splicing** rather than protein translation.

Question 556: Wilson disease is caused by a defect in which gene?

A. ATP7A mutation
B. ATP6A mutation
C. ATP6B mutation
D. ATP7B mutation (Correct Answer)

Explanation: **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).

Question 557: 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?

A. Alcohol dehydrogenase
B. Aldehyde dehydrogenase (Correct Answer)
C. Isocitrate dehydrogenase
D. Alpha-keto glutarate dehydrogenase

Explanation: **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.

Question 558: Wilson's disease is inherited in which pattern?

A. Autosomal Dominant (AD)
B. Autosomal Recessive (AR) (Correct Answer)
C. X-linked Dominant
D. X-linked Recessive

Explanation: **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).

Question 559: Lesch-Nyhan syndrome is associated with a deficiency of which enzyme?

A. Hypoxanthine-guanine phosphoribosyltransferase (partial)
B. Hypoxanthine-guanine phosphoribosyltransferase (total) (Correct Answer)
C. Phosphoribosyl pyrophosphate amidotransferase (partial)
D. Phosphoribosyl pyrophosphate amidotransferase (total)

Explanation: **Explanation:** **Lesch-Nyhan Syndrome (LNS)** is an X-linked recessive disorder characterized by a **total (near-complete) deficiency** of the enzyme **Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. 1. **Why Option B is Correct:** HGPRT is a key enzyme in the **Purine Salvage Pathway**. It converts hypoxanthine to IMP and guanine to GMP. In LNS, the total absence of HGPRT leads to: * **Failure of Salvage:** Purine bases cannot be recycled, leading to their degradation into **Uric Acid**. * **Increased De Novo Synthesis:** Low levels of IMP/GMP and high levels of PRPP (the substrate) stimulate the *de novo* purine synthesis pathway, further exacerbating hyperuricemia. * **Clinical Manifestations:** The "total" deficiency results in the classic triad of hyperuricemia (gout, stones), neurological impairment (chorea, spasticity), and hallmark **self-mutilating behavior**. 2. **Why Other Options are Incorrect:** * **Option A:** A **partial** deficiency of HGPRT results in **Kelley-Seegmiller Syndrome**. While these patients have severe gout and kidney stones, they typically lack the severe neurological deficits and self-mutilation seen in LNS. * **Options C & D:** PRPP amidotransferase is the rate-limiting enzyme of *de novo* purine synthesis. It is inhibited by IMP/GMP (feedback inhibition) but is not the primary enzyme deficient in LNS. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked recessive (mostly affects males). * **Biochemical marker:** Elevated serum uric acid and **Orange sand-like crystals** (sodium urate) in the diaper of infants. * **Mnemonic (HGPRT):** **H**yperuricemia, **G**out, **P**issed off (self-mutilation), **R**etardation (intellectual disability), **T**one (dystonia). * **Treatment:** Allopurinol or Febuxostat (inhibits xanthine oxidase) to manage uric acid levels, though it does not reverse neurological symptoms.

Question 560: 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?

A. 0% and 100% (Correct Answer)
B. 25% and 25%
C. 50% and 50%
D. 10% and 50%

Explanation: ### 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.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

Practice Questions

Lysosomal Storage Diseases

Practice Questions

Glycogen Storage Diseases

Practice Questions

Disorders of Lipoprotein Metabolism

Practice Questions

Disorders of Purine and Pyrimidine Metabolism

Practice Questions

Hemoglobinopathies

Practice Questions

Porphyrias

Practice Questions

Biochemical Markers for Disease Diagnosis

Practice Questions

Newborn Screening for Genetic Disorders

Practice Questions

Enzyme Replacement Therapy

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free