Genetic Disorders and Biochemical Pathology Practice Questions

Q: All of the following are autosomal dominant disorders except?

Duchenne muscular dystrophy. **Explanation:** The correct answer is **C. Duchenne muscular dystrophy (DMD)**. **1. Why Duchenne Muscular Dystrophy is the correct answer:** DMD is an **X-linked recessive (XLR)** disorder, not autosomal dominant. It is caused by a mutation in the *DMD* gene located on the X chromosome (Xp21), which encodes the protein **dystrophin**. Since it is X-linked, it primarily affects males, while females are typically asymptomatic carriers. **2. Analysis of Incorrect Options (Autosomal Dominant Disorders):** * **A. Von Hippel-Lindau (VHL) disease:** An autosomal dominant (AD) condition caused by a mutation in the *VHL* tumor suppressor gene on chromosome 3. It is characterized by hemangioblastomas, renal cell carcinoma, and pheochromocytoma. * **B. Achondroplasia:** The most common form of dwarfism, inherited in an AD pattern. It results from a gain-of-function mutation in the **FGFR3** gene. Note: 80% of cases arise from *de novo* mutations. * **D. Multiple Endocrine Neoplasia (MEN):** Both MEN1 and MEN2 (2A and 2B) are inherited in an AD fashion. They involve a predisposition to tumors in various endocrine glands (e.g., parathyroid, pancreas, thyroid). **3. High-Yield Clinical Pearls for NEET-PG:** * **DMD Hallmark:** Gower’s sign (using hands to "climb up" the legs to stand) and pseudohypertrophy of the calves (fatty replacement of muscle). * **Biochemical Marker:** Significantly elevated **Creatine Kinase (CK-MM)** levels are seen in DMD even before clinical symptoms appear. * **Rule of Thumb:** Most structural protein defects (e.g., Achondroplasia, Marfan) are **Autosomal Dominant**, while most enzyme deficiencies are **Autosomal Recessive**. DMD is a notable X-linked exception involving a structural protein (dystrophin).

Q: The phenomenon of anticipation is observed in which type of genetic condition?

Trinucleotide repeat expansion disorders. **Explanation:** **1. Why the Correct Answer is Right:** **Anticipation** is a genetic phenomenon where a disease becomes more severe or presents at an earlier age in succeeding generations. This is the hallmark of **Trinucleotide Repeat Expansion Disorders**. During gametogenesis, these unstable repeats (e.g., CAG, CGG) can expand in number. A larger number of repeats often correlates with increased protein dysfunction or gene silencing, leading to an earlier and more aggressive clinical phenotype in offspring. **2. Why the Incorrect Options are Wrong:** * **Mitochondrial Inheritance:** Characterized by maternal transmission to all offspring, but with variable expression due to *heteroplasmy* (mixture of normal and mutated mtDNA), not repeat expansion. * **X-linked Dominant Disorders:** These follow standard Mendelian inheritance patterns where a single mutant allele on the X chromosome causes the disease; they do not typically show progressive worsening across generations. * **Genomic Imprinting:** This involves the differential expression of a gene depending on whether it is inherited from the mother or father (e.g., Prader-Willi vs. Angelman syndrome). While it involves epigenetic modification, it does not explain the "anticipation" of symptoms. **3. Clinical Pearls & High-Yield Facts:** * **Fragile X Syndrome:** Most common cause of inherited intellectual disability; shows anticipation via **CGG** repeats (expansion occurs during **oogenesis**). * **Huntington Disease:** Autosomal dominant; shows **CAG** repeats (expansion occurs during **spermatogenesis**). * **Myotonic Dystrophy:** Shows **CTG** repeats; often presents with the most dramatic examples of anticipation. * **Friedreich Ataxia:** The only common trinucleotide disorder that is **Autosomal Recessive** (**GAA** repeats).

Q: Enzyme alglucerase is used in the treatment of?

Gaucher's disease. **Explanation:** **Alglucerase** is a modified form of the human enzyme **β-glucocerebrosidase**. It is used as Enzyme Replacement Therapy (ERT) for **Gaucher’s disease**, the most common lysosomal storage disorder. In Gaucher’s disease, a deficiency of β-glucocerebrosidase leads to the accumulation of glucosylceramide in macrophages (Gaucher cells), primarily affecting the bone marrow, liver, and spleen. Alglucerase (derived from human placental tissue) or its recombinant counterpart, **Imiglucerase**, works by breaking down these accumulated lipids, thereby reducing organomegaly and improving hematological parameters. **Analysis of Incorrect Options:** * **B. Galactosemia:** This is a carbohydrate metabolism disorder caused by deficiencies in enzymes like GALT. Treatment involves dietary restriction of galactose/lactose, not ERT. * **C. Niemann-Pick Disease:** Caused by a deficiency of **Sphingomyelinase**. While ERT (Olipudase alfa) exists for Type A/B, Alglucerase is specific only to the glucocerebrosidase enzyme. * **D. Pompe’s Disease:** A glycogen storage disorder (Type II) caused by **Acid α-glucosidase** deficiency. The specific ERT for Pompe’s is **Alglucosidase alfa**. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Described as having a **"wrinkled paper"** or "crumpled silk" appearance of the cytoplasm. * **Markers:** Elevated levels of **Serum ACE** and **TRAP** (Tartrate-resistant acid phosphatase) are often seen in Gaucher’s. * **Bone Findings:** Look for the **"Erlenmeyer flask deformity"** of the distal femur on X-ray. * **Drug Suffix Tip:** Most ERT drugs end in **"-ase"**. Always match the prefix to the deficient enzyme (e.g., *Gluc*-erase for *Gluco*-cerebrosidase).

Q: What enzyme deficiency is characteristic of Hurler syndrome?

alpha-L-iduronidase. **Explanation:** **Hurler Syndrome (MPS IH)** is the most severe form of Mucopolysaccharidosis. It is an **autosomal recessive** lysosomal storage disorder caused by a deficiency of the enzyme **$\alpha$-L-iduronidase**. This enzyme is essential for the degradation of glycosaminoglycans (GAGs), specifically **Dermatan sulfate and Heparan sulfate**. When deficient, these GAGs accumulate in lysosomes, leading to progressive multi-organ dysfunction. **Analysis of Options:** * **Option B (Correct): $\alpha$-L-iduronidase** is the specific enzyme deficient in Hurler syndrome. Its absence leads to the classic triad of coarse facial features, hepatosplenomegaly, and corneal clouding. * **Option A: Iduronate sulfatase** deficiency causes **Hunter Syndrome (MPS II)**. Key differentiator: Hunter syndrome is X-linked recessive and lacks corneal clouding ("The Hunter needs clear eyes to see the target"). * **Option C: $\beta$-Galactosidase** deficiency is associated with **Morquio Syndrome Type B** or GM1 gangliosidosis. * **Option D: Galactosamine 6-sulfatase** deficiency results in **Morquio Syndrome Type A (MPS IV)**, characterized primarily by severe skeletal dysplasia (dysostosis multiplex) without intellectual disability. **High-Yield NEET-PG Pearls:** 1. **Corneal Clouding:** Present in Hurler (MPS I); **Absent** in Hunter (MPS II). 2. **Urinary Findings:** Both Hurler and Hunter syndromes show elevated levels of **Dermatan sulfate and Heparan sulfate** in the urine. 3. **Clinical Features:** "Gargoylism" (coarse facies), macroglossia, umbilical hernia, and developmental delay. 4. **Diagnosis:** Confirmed by enzyme activity assays in leukocytes or fibroblasts. 5. **Treatment:** Enzyme Replacement Therapy (Laronidase) or Hematopoietic Stem Cell Transplant (HSCT).

Q: DNA analysis of chorionic villus sampling or amniocentesis is not likely to detect which of the following conditions?

Tay-Sach's disease. **Explanation:** The core concept tested here is the distinction between **genetic mutations** (detectable via DNA analysis) and **metabolic/enzymatic deficiencies** (traditionally detected via biochemical assays). **Why Tay-Sachs Disease is the correct answer:** Tay-Sachs disease is a lysosomal storage disorder caused by a deficiency of the enzyme **Hexosaminidase A**. While the underlying genetic mutation can technically be identified, the standard and most reliable prenatal diagnostic method for Tay-Sachs is the **biochemical assay of enzyme activity** rather than direct DNA analysis. This is because the disease exhibits significant genetic heterogeneity (many different mutations can lead to the same enzyme deficiency), making enzymatic levels a more definitive marker for diagnosis in a clinical setting. **Analysis of Incorrect Options:** * **Hemophilia A:** This is an X-linked recessive disorder caused by mutations in the Factor VIII gene. It is routinely diagnosed prenatally using DNA analysis (linkage analysis or direct mutation detection) from CVS or amniocentesis samples. * **Sickle Cell Disease:** This is a classic example of a single-point mutation (substitution of valine for glutamic acid). It is easily and definitively detected via DNA analysis (PCR or RFLP). * **Duchenne Muscular Dystrophy (DMD):** Caused by deletions or duplications in the Dystrophin gene, DMD is primarily diagnosed prenatally through DNA-based techniques like MLPA or PCR. **High-Yield Clinical Pearls for NEET-PG:** * **CVS Timing:** Performed at **10–13 weeks**. It cannot detect Neural Tube Defects (NTDs) as it doesn't measure AFP. * **Amniocentesis Timing:** Performed at **15–20 weeks**. * **Enzyme vs. DNA:** For metabolic disorders like Tay-Sachs or Gaucher’s, enzyme activity in cultured amniocytes or villi is the traditional "gold standard," whereas structural gene defects (Hemoglobinopathies, Muscular dystrophies) rely on DNA analysis.

Q: Which of the following is an autosomal dominant disorder?

Huntington's chorea. **Explanation:** **Huntington’s Chorea (Option D)** is the correct answer because it is a classic example of an **Autosomal Dominant (AD)** neurodegenerative disorder. It is caused by the expansion of **CAG trinucleotide repeats** in the *HTT* gene on chromosome 4. This leads to a "gain-of-function" mutation resulting in the accumulation of huntingtin protein, causing atrophy of the caudate nucleus and putamen. **Analysis of Incorrect Options:** * **Duchenne Muscular Dystrophy (Option A):** This is an **X-linked recessive** disorder caused by a mutation in the *DMD* gene, leading to a complete absence of the protein dystrophin. * **Fragile X Syndrome (Option B):** This is an **X-linked dominant** disorder (with variable expressivity) caused by **CGG repeat** expansion in the *FMR1* gene. It is the most common cause of inherited intellectual disability. * **Fanconi’s Syndrome (Option C):** This is a generalized dysfunction of the proximal renal tubules. While it can be acquired, the most common inherited form (Fanconi Anemia) is **Autosomal Recessive**. (Note: Do not confuse this with Fanconi Anemia, though both are typically recessive). **High-Yield Clinical Pearls for NEET-PG:** * **Anticipation:** Huntington’s chorea exhibits anticipation (earlier onset in successive generations), particularly when inherited from the **father** (spermatogenesis increases repeat instability). * **Trinucleotide Repeat Mnemonic:** **CAG** = **C**audate **A**trophy, **G**ABA decreased. * **Biochemical Change:** There is a marked **decrease in GABA and Acetylcholine**, and an **increase in Dopamine** in the basal ganglia. * **Imaging:** MRI typically shows "boxcar ventricles" due to atrophy of the caudate head.

Q: One of the following disorders is due to maternal disomy?

Prader Willi syndrome. This question tests your knowledge of **Genomic Imprinting**, a phenomenon where certain genes are expressed in a parent-of-origin-specific manner. [1] ### **Explanation of the Correct Answer** **Prader-Willi Syndrome (PWS)** occurs due to the loss of expression of genes on the **paternal** chromosome 15 (specifically the 15q11-q13 region). While the most common cause is a paternal deletion (70%), approximately 25-30% of cases are caused by **Maternal Uniparental Disomy (UPD)**. [1] In maternal UPD, the offspring inherits two copies of chromosome 15 from the mother and none from the father. Since the maternal copies are "silenced" (imprinted) in this region, the individual lacks the necessary active paternal genes, leading to the clinical phenotype. [1] ### **Analysis of Incorrect Options** * **B. Angelman Syndrome:** This is the "sister" condition. It results from the loss of the **maternal** 15q11-q13 region (specifically the *UBE3A* gene). It is most commonly due to maternal deletion or **Paternal UPD**. * **C. Hydatidiform Mole:** A complete mole is an example of **androgenesis**, where all 46 chromosomes are of paternal origin (usually due to a sperm fertilizing an empty egg). * **D. Klinefelter Syndrome:** This is a numerical chromosomal aberration (47, XXY) caused by non-disjunction during meiosis, not imprinting or disomy of a single homologous pair. ### **High-Yield NEET-PG Pearls** * **Prader-Willi Mnemonic:** **P**aternal deletion / **P**rader-Willi / **P**hyperphagia (Hyperphagia), **P**hypogonadism, and **P**loppy (hypotonia). * **Angelman Mnemonic:** **M**aternal deletion / **M**appy (Happy puppet), **M**ental retardation, **M**outh (large), and **M**ovement (ataxia). * **Chromosomal Region:** Both involve **15q11-q13**. * **Diagnosis:** The gold standard screening test for both is **DNA Methylation Analysis**.

Q: A 10-year-old child presents with aggressive, self-mutilating behavior, poor concentration, joint pain, and reduced urinary output. Deficiency of which enzyme is most likely responsible for this presentation?

Hypoxanthine-guanine phosphoribosyltransferase (HGPT). ### Explanation The clinical presentation of **self-mutilation** (biting of lips and fingers), **aggressive behavior**, and signs of **hyperuricemia** (joint pain/gout and renal failure/reduced urinary output) is a classic triad for **Lesch-Nyhan Syndrome**. **1. Why Option A is Correct:** Lesch-Nyhan Syndrome is an X-linked recessive disorder caused by a deficiency of **Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. This enzyme is crucial for the **Purine Salvage Pathway**, where it converts hypoxanthine to IMP and guanine to GMP. Its deficiency leads to: * **Hyperuricemia:** Failure to salvage purines causes them to be degraded into excessive Uric Acid. * **Neurological Symptoms:** The brain relies heavily on the salvage pathway; low HGPRT levels lead to dopamine dysfunction, causing the characteristic self-mutilation and cognitive impairment. **2. Why Incorrect Options are Wrong:** * **B. Adenosine Deaminase (ADA):** Deficiency leads to **Severe Combined Immunodeficiency (SCID)** due to the accumulation of dATP, which is toxic to lymphocytes. It does not cause self-mutilation. * **C. Alkaline Phosphatase (APase):** Low levels are seen in Hypophosphatasia (bone mineralization defects), while high levels indicate liver or bone disease. * **D. Acid Maltase (α-1,4-glucosidase):** Deficiency causes **Pompe Disease** (Glycogen Storage Disease Type II), characterized by cardiomegaly and muscle weakness, not hyperuricemia. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** X-linked Recessive (mostly affects males). * **Biochemical Hallmark:** Increased levels of **PRPP** (Phosphoribosyl pyrophosphate) and decreased IMP/GMP. * **Diagnosis:** "Orange sand" crystals (sodium urate) often found in the diapers of affected infants. * **Treatment:** Allopurinol or Febuxostat (manages uric acid but does not fix neurological symptoms).

Question 1

All of the following are autosomal dominant disorders except?

Accepted Answer

Duchenne muscular dystrophy

Answer

Von Hippel-Lindau disease

Answer

Achondroplasia

Answer

Multiple endocrine neoplasia

Question 2

Which of the following statements is FALSE about Wolman disease?

Accepted Answer

It is characterized by adrenal calcification and corneal clouding.

Answer

It is a lysosomal storage disorder.

Answer

It is characterized by deficient acid lysosomal lipase.

Answer

It shows an autosomal recessive inheritance pattern.

Question 3

The phenomenon of anticipation is observed in which type of genetic condition?

Accepted Answer

Trinucleotide repeat expansion disorders

Answer

Mitochondrial inheritance

Answer

X-linked dominant disorders

Answer

Genomic imprinting

Question 4

Enzyme alglucerase is used in the treatment of?

Accepted Answer

Gaucher's disease

Answer

Galactosemia

Answer

Niemann Pick disease

Answer

Pompe's disease

Question 5

What enzyme deficiency is characteristic of Hurler syndrome?

Accepted Answer

alpha-L-iduronidase

Answer

Iduronate sulfatase

Answer

beta-Galactosidase

Answer

Galactosamine 6-sulfatase

Question 6

DNA analysis of chorionic villus sampling or amniocentesis is not likely to detect which of the following conditions?

Accepted Answer

Tay-Sach's disease

Answer

Hemophilia A

Answer

Sickle cell disease

Answer

Duchenne muscular dystrophy

Question 7

Which of the following is an autosomal dominant disorder?

Accepted Answer

Huntington's chorea

Answer

Duchenes muscular dystrophy

Answer

Fragile X syndrome

Answer

Fanconi's syndrome

Question 8

A female whose father had vitamin D resistant rickets marries a normal male. What are the chances that her children will be color blind?

Accepted Answer

50%

Answer

0%

Answer

75%

Answer

100%

Question 9

One of the following disorders is due to maternal disomy?

Accepted Answer

Prader Willi syndrome

Answer

Angelman syndrome

Answer

Hydatidiform mole

Answer

Klinefelter syndrome

Question 10

A 10-year-old child presents with aggressive, self-mutilating behavior, poor concentration, joint pain, and reduced urinary output. Deficiency of which enzyme is most likely responsible for this presentation?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPT)

Answer

Adenosine Deaminase

Answer

Alkaline Phosphatase (APase)

Answer

Acid Maltase

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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