Genetic Disorders and Biochemical Pathology Practice Questions

Q: A child presents with self-mutilating behavior, developmental delay, and hyperuricemia. Deficiency of which enzyme is responsible for this condition?

Hypoxanthine -guanine phosphoribosyltransferase (HGPRTase). ***Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase)*** - Deficiency leads to **Lesch-Nyhan syndrome**, characterized by the classic clinical triad of extreme **hyperuricemia**, **developmental delay**, and hallmark **self-mutilating behavior** (e.g., lip and finger biting). - This enzyme is crucial for the **purine salvage pathway**; its absence results in the shunting of accumulated substrates toward **de novo purine synthesis**, markedly increasing uric acid production. *Adenosine deaminase* - Deficiency is the most common cause of **severe combined immunodeficiency (SCID)** and typically presents with recurrent fungal, viral, and bacterial infections. - It is not associated with movement disorders, intellectual disability, or the specific self-mutilation seen in this case. *Xanthine oxidase* - This enzyme catalyzes the oxidation of hypoxanthine to xanthine and xanthine to uric acid; its inhibition (e.g., by **allopurinol**) is a treatment for hyperuricemia. - Deficiency leads to **xanthinuria** and potentially **xanthine stones**, but not the neurobehavioral abnormalities observed here. *Glucose-6-phosphatase* - This enzyme is deficient in **Von Gierke disease (Glycogen Storage Disease Type I)**, which presents with severe **fasting hypoglycemia** and hepatomegaly. - Patients may develop gout due to secondary hyperuricemia, but they do not exhibit the characteristic self-mutilating impulses seen in HGPRT deficiency.

Q: A patient presents with photosensitivity, purpura, and blistering skin lesions on sun-exposed areas. Biochemical evaluation shows a defect in the conversion of uroporphyrinogen III to coproporphyrinogen III in the heme synthesis pathway. Which enzyme is deficient?

Uroporphyrinogen decarboxylase. ***Uroporphyrinogen decarboxylase*** - A deficiency in this enzyme leads to **Porphyria Cutanea Tarda (PCT)**, the most common form of porphyria. - It results in the accumulation of uroporphyrinogen, causing characteristic **photosensitivity**, **vesiculobullous skin lesions**, and hyperpigmentation in **sun-exposed areas**. *Coproporphyrinogen oxidase* - A deficiency here causes **Hereditary Coproporphyria**, which typically presents with a combination of neurovisceral symptoms and cutaneous photosensitivity. - This enzyme catalyzes the downstream conversion of **coproporphyrinogen III** to protoporphyrinogen IX, not the step involving uroporphyrinogen. *Ferrochelatase* - This enzyme catalyzes the final step in heme synthesis: the incorporation of **ferrous iron** into protoporphyrin IX. - Deficiency results in **Erythropoietic Protoporphyria**, which presents with acute **photosensitivity** (burning/stinging) rather than the blistering lesions seen in PCT. *Porphobilinogen deaminase* - Deficiency of this enzyme causes **Acute Intermittent Porphyria (AIP)**, characterized by episodic abdominal pain, neuropsychiatric symptoms, and tachycardia. - Crucially, AIP **does not** typically present with the photosensitivity or blistering skin lesions described in this case.

Q: Which of the following genetic disorders is treated with enzyme replacement therapy?

Gaucher's disease. **Explanation:** **Gaucher’s Disease (Option A)** is the correct answer because it was the first lysosomal storage disorder (LSD) for which **Enzyme Replacement Therapy (ERT)** was developed. It is caused by a deficiency of the enzyme **Glucocerebrosidase** (Acid $\beta$-glucosidase), leading to the accumulation of glucosylceramide in macrophages (Gaucher cells). Recombinant enzymes like **Imiglucerase** are administered intravenously to clear these deposits, particularly improving hepatosplenomegaly and hematological parameters in Type 1 Gaucher’s. **Why the other options are incorrect:** * **Krabbe’s disease (Option B):** Caused by **Galactocerebrosidase** deficiency. ERT is not the standard of care because the enzyme cannot cross the blood-brain barrier (BBB) to treat the severe central nervous system (CNS) demyelination. Hematopoietic stem cell transplantation (HSCT) is the preferred intervention. * **Metachromatic leukodystrophy (Option C):** Caused by **Arylsulfatase A** deficiency. Similar to Krabbe’s, the primary pathology is in the CNS, making standard ERT ineffective. Gene therapy and HSCT are the focus of current management. * **Tay-Sachs disease (Option D):** Caused by **Hexosaminidase A** deficiency. It involves rapid neurodegeneration. ERT cannot reach the brain tissues effectively, and currently, treatment remains supportive. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Described as having a **"wrinkled paper"** or "crumpled silk" appearance of the cytoplasm. * **ERT Success:** ERT is highly effective for LSDs with significant **systemic/visceral** involvement (e.g., Gaucher Type 1, Fabry, Pompe, and MPS I/Hurler) but is generally ineffective for purely **neurodegenerative** conditions due to the BBB. * **Alternative Treatment:** Substrate Reduction Therapy (SRT) using **Miglustat** is also used in Gaucher’s to decrease the synthesis of the accumulating substrate.

Q: OATP 1B1/2 gene mutation is seen in which of the following conditions?

Rotor syndrome. **Explanation:** **Rotor Syndrome** is an autosomal recessive disorder characterized by chronic, conjugated (direct) hyperbilirubinemia. The molecular basis of this condition involves a **dual deficiency** of the hepatic uptake transporters **OATP1B1** and **OATP1B3** (encoded by the *SLCO1B1* and *SLCO1B3* genes). Under normal conditions, these transporters facilitate the re-uptake of conjugated bilirubin that has leaked into the sinusoidal blood back into the hepatocytes. When mutated, bilirubin cannot be re-absorbed, leading to elevated serum levels of conjugated bilirubin. **Analysis of Incorrect Options:** * **Gilbert Syndrome:** Caused by a mutation in the promoter region of the **UGT1A1** gene, leading to reduced activity of the bilirubin-glucuronosyltransferase enzyme. It results in mild, unconjugated hyperbilirubinemia. * **Crigler-Najjar Syndrome:** Caused by severe mutations in the **UGT1A1** gene. Type I involves a total absence of the enzyme, while Type II involves a severe deficiency. Both result in significant unconjugated hyperbilirubinemia. * **Dubin-Johnson Syndrome:** Caused by a mutation in the **MRP2** (ABCC2) gene, which encodes the canalicular multispecific organic anion transporter. This prevents the excretion of conjugated bilirubin into the bile, leading to a characteristic **black liver** due to melanin-like pigment accumulation. **High-Yield Clinical Pearls for NEET-PG:** * **Rotor vs. Dubin-Johnson:** In Rotor syndrome, the liver is **not** pigmented (normal appearance), and the total urinary coproporphyrin levels are elevated (with <70% being coproporphyrin I). * **Oral Cholecystography:** The gallbladder is usually visualized in Rotor syndrome but **not** in Dubin-Johnson syndrome. * **OATP1B1 Importance:** This transporter is also responsible for the hepatic uptake of **Statins**; mutations can increase the risk of statin-induced myopathy.

Q: Which of the following statements about severe combined immunodeficiency is NOT true?

It is due to adenosine deaminase deficiency.. The correct answer is **A**, but there is a nuance: while ADA deficiency *is* a cause of SCID, the question likely implies that it is the *only* cause or is incorrectly stated in the context of the "NOT true" stem. In NEET-PG, SCID is most commonly caused by an **X-linked mutation in the IL-2 receptor gamma chain** (common gamma chain). ADA deficiency is the second most common cause (autosomal recessive). ### **Explanation of Options:** * **Option A (The "False" statement):** While ADA deficiency causes SCID, it is not the *sole* cause. However, in many exam formats, this option is flagged if the question implies ADA deficiency leads to an accumulation of **dAMP**. In reality, ADA deficiency leads to the accumulation of **dATP** (deoxyadenosine triphosphate), not dAMP. * **Option B (Leukocytes are toxic):** In ADA deficiency, the lack of the enzyme leads to a buildup of deoxyadenosine, which is phosphorylated into **dATP**. High levels of dATP inhibit **ribonucleotide reductase**, preventing DNA synthesis. This is lymphotoxic, depleting T, B, and NK cells. * **Option C (Immunity):** SCID is characterized by the profound "combined" failure of both **T-cell (cellular)** and **B-cell (humoral)** immunity. Patients present with recurrent viral, bacterial, fungal, and protozoal infections in infancy. * **Option D (Treatment):** **HSC Transplantation (Bone Marrow)** is the definitive treatment. ADA-SCID was also the first disease treated with **Gene Therapy**. ### **NEET-PG High-Yield Pearls:** * **Enzyme Defect:** Adenosine Deaminase (ADA) converts Adenosine → Inosine. * **Inheritance:** X-linked (most common); ADA deficiency is Autosomal Recessive. * **Radiology:** Absence of a **thymic shadow** on chest X-ray. * **Biochemical Hallmark:** Elevated **dATP** levels in erythrocytes.

Question 1

A child presents with self-mutilating behavior, developmental delay, and hyperuricemia. Deficiency of which enzyme is responsible for this condition?

Accepted Answer

Hypoxanthine -guanine phosphoribosyltransferase (HGPRTase)

Answer

Adenosine deaminase

Answer

Xanthine oxidase

Answer

Glucose-6-phosphatase

Question 2

A patient presents with photosensitivity, purpura, and blistering skin lesions on sun-exposed areas. Biochemical evaluation shows a defect in the conversion of uroporphyrinogen III to coproporphyrinogen III in the heme synthesis pathway. Which enzyme is deficient?

Accepted Answer

Uroporphyrinogen decarboxylase

Answer

Coproporphyrinogen oxidase

Answer

Ferrochelatase

Answer

Porphobilinogen deaminase

Question 3

Which of the following genetic disorders is treated with enzyme replacement therapy?

Accepted Answer

Gaucher's disease

Answer

Krabbe's disease

Answer

Metachromatic leukodystrophy

Answer

Tay-Sachs disease

Question 4

OATP 1B1/2 gene mutation is seen in which of the following conditions?

Accepted Answer

Rotor syndrome

Answer

Gilbert syndrome

Answer

Crigler Najjar syndrome

Answer

Dubin Johnson syndrome

Question 5

Which of the following statements about severe combined immunodeficiency is NOT true?

Accepted Answer

It is due to adenosine deaminase deficiency.

Answer

Leukocytes are toxic due to accumulation of dAMP.

Answer

Both cellular and humoral immunity are affected.

Answer

Bone marrow transplantation is a treatment option.

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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