Genetic Disorders and Biochemical Pathology Practice Questions

Q: An infant is unable to feed properly, is weak, and not gaining weight. The mother reports multiple episodes of urination with crying each time the baby passes urine. She also notes that the baby often smells of rotten fish in his urine and sweat. Which of the following would you test for in the infant's urine?

Trimethylamine. ### Explanation **Correct Option: C. Trimethylamine** The clinical presentation of a "rotten fish" odor in the urine and sweat is the pathognomonic sign of **Trimethylaminuria**, also known as **Fish Odor Syndrome**. This condition is caused by a deficiency of the enzyme **Flavin-containing monooxygenase 3 (FMO3)**. Normally, FMO3 converts trimethylamine (TMA)—a malodorous byproduct of bacterial degradation of choline, lecithin, and carnitine in the gut—into the odorless **trimethylamine N-oxide (TMAO)**. When FMO3 is deficient, TMA accumulates and is excreted through sweat, urine, and breath, resulting in the characteristic fishy smell. The infant’s failure to thrive and distress during urination (likely due to skin irritation or systemic malaise) are consistent with severe metabolic presentations. **Analysis of Incorrect Options:** * **A. Vinylmandelic acid (VMA):** This is a urinary metabolite of catecholamines (epinephrine/norepinephrine). It is used to diagnose **Neuroblastoma** in children or Pheochromocytoma in adults, but it does not cause a fishy odor. * **B. Isovaleric acid:** This accumulates in **Isovaleric Acidemia**. While it causes a distinct odor, it is described as **"sweaty feet"** or "cheesy" rather than rotten fish. * **D. Oxoisocaproic acid:** This is a branched-chain keto acid that accumulates in **Maple Syrup Urine Disease (MSUD)**. The characteristic odor associated with this is **burnt sugar** or maple syrup. **High-Yield Clinical Pearls for NEET-PG:** * **Trimethylaminuria (FMO3 deficiency):** Autosomal recessive; management involves dietary restriction of precursors (fish, eggs, legumes). * **Odor Associations:** * **Mousy/Musty:** Phenylketonuria (PKU). * **Cabbage-like/Rancid butter:** Tyrosinemia Type I. * **Sweaty feet:** Isovaleric acidemia or Glutaric aciduria type II. * **Swimming pool odor:** Hawkinsinuria.

Q: A 9-month-old infant presented with recurrent infections. Investigations revealed a near absence of B and T cells and a significantly diminished thymic shadow on chest X-ray. Which of the following metabolites would be elevated in this patient?

Deoxyadenosine. ### Explanation The clinical presentation of recurrent infections, near absence of both B and T cells (lymphopenia), and a diminished thymic shadow in a 9-month-old infant is characteristic of **Severe Combined Immunodeficiency (SCID)**. Approximately 15% of SCID cases are caused by a deficiency in the enzyme **Adenosine Deaminase (ADA)**. **Why Deoxyadenosine is Correct:** Adenosine Deaminase is a crucial enzyme in the purine salvage pathway that converts adenosine to inosine and deoxyadenosine to deoxyinosine. In ADA deficiency, **deoxyadenosine** accumulates and is phosphorylated into **dATP (deoxyadenosine triphosphate)**. High levels of dATP are toxic to lymphocytes because they inhibit **Ribonucleotide Reductase**, the enzyme responsible for DNA synthesis. This leads to a failure of T and B cell proliferation, resulting in the SCID phenotype. **Analysis of Incorrect Options:** * **A. Uric Acid:** This is the end product of purine catabolism. In ADA deficiency, the pathway is blocked upstream, typically leading to *decreased* uric acid production. * **B. Orotic Acid:** Elevated in Urea Cycle disorders (like OTC deficiency) or Orotic Aciduria. It is not directly linked to the purine salvage pathway or SCID. * **D. NADPH:** This is a cofactor produced in the HMP shunt. While essential for respiratory burst in neutrophils (deficient in Chronic Granulomatous Disease), it does not accumulate in ADA deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** ADA deficiency is the most common **Autosomal Recessive** cause of SCID (X-linked SCID is the overall most common type). * **First-line Treatment:** Bone marrow transplantation or Enzyme Replacement Therapy (PEG-ADA). * **Gene Therapy:** ADA deficiency was the first genetic disease for which gene therapy was attempted. * **Radiology:** A "missing thymic shadow" in a pediatric chest X-ray should immediately raise suspicion for SCID or DiGeorge Syndrome.

Q: For which of the following diseases is enzyme replacement therapy available?

Gaucher's disease. **Explanation:** **Gaucher’s Disease (Correct Answer):** Gaucher’s disease is a lysosomal storage disorder caused by a deficiency of the enzyme **β-glucocerebrosidase**, leading to the accumulation of glucocerebroside in macrophages. It was the first lysosomal storage disorder for which **Enzyme Replacement Therapy (ERT)** was successfully developed. Recombinant enzymes like **Imiglucerase**, Velaglucerase alfa, and Taliglucerase are the standard of care, significantly reducing hepatosplenomegaly and improving hematological parameters. **Why the other options are incorrect:** * **Albinism:** This is a defect in melanin synthesis (typically due to **Tyrosinase** deficiency). It is not a lysosomal storage disorder and cannot be treated with ERT because the enzyme cannot be targeted effectively to melanocytes via the bloodstream. * **Niemann-Pick Disease:** While ERT (Olipudase alfa) has recently been approved for Niemann-Pick Type B (Acid Sphingomyelinase deficiency), it is not yet the "classic" or widely established answer in standard medical examinations compared to Gaucher’s. Type A involves severe neurodegeneration where ERT cannot cross the blood-brain barrier. * **Metachromatic Leukodystrophy (MLD):** Caused by **Arylsulfatase A** deficiency. While gene therapy (Atidarsagene autotemcel) has shown promise, ERT is not the standard established treatment for MLD in the context of NEET-PG questions. **NEET-PG High-Yield Pearls:** * **Gaucher Cells:** Characterized by a "wrinkled paper" or "crumpled silk" appearance of the cytoplasm. * **Other ERT-treatable diseases:** Fabry’s disease (α-galactosidase A), Pompe disease (α-glucosidase), and Hurler syndrome (Iduronidase). * **Targeting Mechanism:** ERT for lysosomal disorders relies on the **Mannose-6-Phosphate (M6P)** receptor-mediated uptake of the enzyme into the cells.

Q: Barth syndrome is due to a defect in which of the following?

Cardiolipin. **Explanation:** **Barth Syndrome** (also known as 3-Methylglutaconic aciduria type II) is an X-linked recessive disorder caused by a mutation in the **TAZ gene**, which encodes the protein **Tafazzin**. Tafazzin is an acyltransferase responsible for the remodeling and maturation of **Cardiolipin**, a unique phospholipid found exclusively in the inner mitochondrial membrane. 1. **Why Cardiolipin is Correct:** Cardiolipin is essential for stabilizing the mitochondrial respiratory chain complexes and maintaining the structural integrity of the cristae. In Barth syndrome, defective remodeling leads to a deficiency of mature cardiolipin, resulting in mitochondrial dysfunction. This manifests clinically as the triad of **dilated cardiomyopathy, skeletal myopathy, and neutropenia.** 2. **Why Other Options are Incorrect:** * **Thermogenin (UCP1):** This is an uncoupling protein found in brown adipose tissue that generates heat. It is not associated with Barth syndrome. * **Ubiquinone (Coenzyme Q10):** This is a mobile electron carrier in the electron transport chain. While CoQ10 deficiency causes mitochondrial diseases, it is not the primary defect in Barth syndrome. * **Cytochrome:** Cytochromes (like Cytochrome c) are heme-containing proteins in the ETC. Mutations in cytochrome oxidase can cause Leigh syndrome, but not Barth syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked Recessive (affects males). * **Biochemical Marker:** Elevated **3-methylglutaconic acid** in urine. * **Key Clinical Features:** "Cardio-Skeletal-Neutro" (Cardiomyopathy, Myopathy, Neutropenia, and Growth retardation). * **Mnemonic:** Think of **"Barth"** as **"Heart"** (Cardiomyopathy) and **"Cardio"**-lipin.

Q: Different mutations in the same genetic locus causing similar or identical phenotypes is termed as?

Allelic heterogeneity. ### Explanation **1. Why Allelic Heterogeneity is Correct:** Allelic heterogeneity refers to a situation where **different mutations at the same genetic locus** (within the same gene) result in the same or a very similar clinical phenotype. In medical genetics, a single gene can be damaged in various ways—such as missense, nonsense, or frameshift mutations—yet all lead to the loss of function of the same protein, causing the same disease. * **Classic Example:** **Cystic Fibrosis.** Over 2,000 different mutations in the *CFTR* gene have been identified; while the specific molecular defect varies, they all manifest as Cystic Fibrosis. **2. Analysis of Incorrect Options:** * **Locus Heterogeneity:** This occurs when mutations at **different genetic loci** (different genes) produce the same phenotype. For example, Osteogenesis Imperfecta can be caused by mutations in either the *COL1A1* or *COL1A2* genes. * **Phenotypic Heterogeneity:** This is the opposite concept, where **different mutations in the same gene** result in **strikingly different clinical phenotypes**. For example, mutations in the *RET* proto-oncogene can cause either Hirschsprung disease (loss of function) or Multiple Endocrine Neoplasia type 2 (gain of function). * **Non-allelic Heterogeneity:** This is essentially a synonym for Locus Heterogeneity. **3. NEET-PG High-Yield Pearls:** * **Beta-Thalassemia** is a prime example of allelic heterogeneity (hundreds of different mutations in the $\beta$-globin gene). * **Compound Heterozygote:** A patient with two different mutant alleles at the same locus (common in allelic heterogeneity) rather than being a true homozygote. * **Pleiotropy:** One single mutation causing multiple, seemingly unrelated phenotypic effects (e.g., Marfan Syndrome affecting the eyes, heart, and skeleton).

Q: All of the following are examples of uniparental disomy except?

Bloom syndrome. **Explanation:** The correct answer is **Bloom syndrome** because it is an **autosomal recessive** disorder characterized by chromosomal instability, not uniparental disomy (UPD). It is caused by a mutation in the *BLM* gene (RECQL3), which encodes a DNA helicase. This leads to excessive sister chromatid exchanges, resulting in short stature, photosensitive rashes, and a high predisposition to various cancers. **Uniparental Disomy (UPD)** occurs when an individual inherits two copies of a chromosome (or part of a chromosome) from one parent and no copy from the other. The incorrect options are classic examples of UPD: * **Prader-Willi Syndrome (PWS):** Approximately 25–30% of cases are caused by **maternal UPD** of chromosome 15 (inheriting two maternal copies, lacking the paternal 15q11-q13 region). * **Angelman Syndrome (AS):** Approximately 3–7% of cases are caused by **paternal UPD** of chromosome 15 (inheriting two paternal copies, lacking the maternal 15q11-q13 region). * **Russell-Silver Syndrome:** About 10% of cases are due to **maternal UPD of chromosome 7**. It presents with intrauterine growth restriction (IUGR), triangular facies, and limb asymmetry. **High-Yield Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** PWS and AS are the "poster children" for imprinting. Remember: **P**ader-Willi = **P**aternal deletion; **A**ngelman = **M**aternal deletion (or vice versa via UPD). * **Bloom Syndrome Hallmark:** Look for "quadriradial figures" on cytogenetic analysis and "sister chromatid exchange" (SCE). * **Beckwith-Wiedemann Syndrome:** Another high-yield UPD example (paternal UPD of chromosome 11).

Q: The provided diagram indicates what type of inheritance?

Mitochondrial. ***Mitochondrial*** - Shows **strict maternal transmission** where all children of affected mothers inherit the condition, regardless of sex - **No paternal transmission** occurs since mitochondria are inherited exclusively from the mother through the egg cytoplasm *Autosomal recessive* - Requires **two affected alleles** for expression, typically showing affected individuals with unaffected parents - Demonstrates **horizontal pattern** with affected siblings but unaffected parents, unlike the vertical maternal pattern seen here *Autosomal dominant* - Shows **vertical transmission** through multiple generations but affects **both maternal and paternal lineages** - Typically has **50% risk** for each child of an affected parent, not the 100% maternal transmission pattern observed *X-linked recessive* - Primarily affects **males** due to hemizygous state, with affected males connected through **carrier females** - Shows **criss-cross inheritance** pattern where affected grandfathers pass the trait through carrier daughters to affected grandsons

Q: Menke's disease is due to a defect in the metabolism of which element?

Copper. **Explanation:** **Menkes Disease (Kinky Hair Syndrome)** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This gene encodes a copper-transporting ATPase responsible for the absorption of copper from the gastrointestinal tract and its distribution to various tissues. In Menkes disease, copper is trapped within intestinal mucosal cells, leading to severe systemic **copper deficiency**. Copper is a vital cofactor for several enzymes. Its deficiency leads to the characteristic clinical features: * **Lysyl Oxidase deficiency:** Causes defective collagen cross-linking, leading to arterial tortuosity and skeletal abnormalities. * **Tyrosinase deficiency:** Results in hypopigmentation. * **Cytochrome c oxidase deficiency:** Leads to neurodegeneration and hypothermia. * **Clinical Hallmark:** "Steely" or "Kinky" brittle hair due to defective keratin disulfide bond formation. **Why other options are incorrect:** * **Zinc:** Deficiency causes *Acrodermatitis enteropathica*, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Selenium:** Deficiency is associated with *Keshan disease* (dilated cardiomyopathy). * **Iron:** Metabolism defects lead to *Iron Deficiency Anemia* or *Hemochromatosis* (iron overload), not Menkes disease. **High-Yield NEET-PG Pearls:** 1. **ATP7A vs. ATP7B:** Remember **A** for **A**bsorption (Menkes/ATP7A - deficiency) and **B** for **B**iliary excretion (Wilson’s/ATP7B - overload). 2. **Diagnosis:** Low serum copper and low serum ceruloplasmin levels. 3. **Inheritance:** Menkes is X-linked Recessive (mostly affects males), whereas Wilson’s is Autosomal Recessive.

Q: Unconjugated hyperbilirubinemia is seen in which of the following conditions?

Gilbe syndrome. **Explanation:** Hyperbilirubinemia is classified into unconjugated (pre-microsomal) and conjugated (post-microsomal) based on where the metabolic defect occurs in the liver. **Why Gilbert Syndrome is Correct:** Gilbert syndrome is a common, benign autosomal recessive condition characterized by reduced activity of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**. This enzyme is responsible for conjugating bilirubin with glucuronic acid in the hepatocytes. A deficiency leads to impaired conjugation, resulting in isolated **unconjugated hyperbilirubinemia**, typically triggered by stress, fasting, or illness. **Analysis of Incorrect Options:** * **Rotor Syndrome:** An autosomal recessive condition caused by a defect in hepatic storage and re-uptake of bilirubin. It presents with **conjugated hyperbilirubinemia**. * **Dubin-Johnson Syndrome:** Caused by a mutation in the **MRP2 gene**, leading to defective excretion of conjugated bilirubin into the bile canaliculi. It presents with **conjugated hyperbilirubinemia** and a characteristic "black liver" due to melanin-like pigment deposition. * **Bile Duct Obstruction:** This is a post-hepatic (obstructive) cause of jaundice. Since the bilirubin has already been processed by the liver, it results in **conjugated hyperbilirubinemia** and features like pale stools and dark urine. **High-Yield Clinical Pearls for NEET-PG:** * **Crigler-Najjar Syndrome (Type I & II):** Also causes unconjugated hyperbilirubinemia due to UGT1A1 deficiency (Type I is total absence; Type II is severe deficiency). * **Distinguishing Dubin-Johnson vs. Rotor:** Dubin-Johnson shows a black liver and abnormal urinary coproporphyrin I levels (>80%), whereas Rotor syndrome has a normal-appearing liver. * **Gilbert Syndrome** is often a "spot diagnosis" in exams when a patient presents with mild jaundice after a period of fasting or infection with otherwise normal Liver Function Tests (LFTs).

Question 1

An infant is unable to feed properly, is weak, and not gaining weight. The mother reports multiple episodes of urination with crying each time the baby passes urine. She also notes that the baby often smells of rotten fish in his urine and sweat. Which of the following would you test for in the infant's urine?

Accepted Answer

Trimethylamine

Answer

Viny/mandelic acid

Answer

Isovaleric acid

Answer

Oxoisocaproic acid

Question 2

A 9-month-old infant presented with recurrent infections. Investigations revealed a near absence of B and T cells and a significantly diminished thymic shadow on chest X-ray. Which of the following metabolites would be elevated in this patient?

Accepted Answer

Deoxyadenosine

Answer

Uric acid

Answer

Orotic acid

Answer

NADPH

Question 3

For which of the following diseases is enzyme replacement therapy available?

Accepted Answer

Gaucher's disease

Answer

Albinism

Answer

Niemann-Pick disease

Answer

Metachromatic leukodystrophy

Question 4

Barth syndrome is due to a defect in which of the following?

Accepted Answer

Cardiolipin

Answer

Thermogenic

Answer

Ubiquinone

Answer

Cytochrome

Question 5

Different mutations in the same genetic locus causing similar or identical phenotypes is termed as?

Accepted Answer

Allelic heterogeneity

Answer

Phenotypic heterogeneity

Answer

Locus heterogeneity

Answer

Non-allelic heterogeneity

Question 6

All of the following are examples of uniparental disomy except?

Accepted Answer

Bloom syndrome

Answer

Russell-Silver syndrome

Answer

Prader-Willi syndrome

Answer

Angelman syndrome

Question 7

The provided diagram indicates what type of inheritance?

Accepted Answer

Mitochondrial

Answer

Autosomal recessive

Answer

Autosomal dominant

Answer

X-linked recessive

Question 8

Menke's disease is due to a defect in the metabolism of which element?

Accepted Answer

Copper

Answer

Zinc

Answer

Selenium

Answer

Iron

Question 9

Which of the following is not characteristic of vitamin-D resistant rickets?

Accepted Answer

Reduced calcium absorption from the gut

Answer

X-linked inheritance

Answer

Renal stones

Answer

Hypophosphatemia

Question 10

Unconjugated hyperbilirubinemia is seen in which of the following conditions?

Accepted Answer

Gilbe syndrome

Answer

Rotor syndrome

Answer

Dubin-Johnson syndrome

Answer

Bile duct obstruction

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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