Genetic Disorders and Biochemical Pathology Practice Questions

Q: Which of the following is a classic example of a missense mutation?

Sickle cell disease. **Explanation:** **Sickle Cell Disease (SCD)** is the classic prototype of a **missense mutation**. In SCD, a single nucleotide substitution occurs in the $\beta$-globin gene on chromosome 11. Specifically, adenine is replaced by thymine (**GAG $\rightarrow$ GTG**). This results in the substitution of the amino acid **Glutamic acid** (polar/hydrophilic) with **Valine** (non-polar/hydrophobic) at the **6th position** of the $\beta$-globin chain. This single amino acid change causes the hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of red blood cells. **Analysis of Incorrect Options:** * **Thalassemia:** These are typically caused by **quantitative** defects. $\beta$-thalassemia is most commonly due to **splicing mutations** or point mutations leading to premature stop codons (nonsense mutations), while $\alpha$-thalassemia is usually due to **gene deletions**. * **Sideroblastic Anemia:** This is a group of disorders characterized by impaired heme synthesis. The most common hereditary form is X-linked, caused by a mutation in the **ALAS2 gene**, but it is not defined as a classic missense model in the same way SCD is. * **Hemochromatosis:** While the most common mutation (C282Y) in the HFE gene is a missense mutation, it is not the "classic" textbook example used to teach the concept of point mutations in medical biochemistry. **High-Yield Clinical Pearls for NEET-PG:** * **Point Mutation Types:** * **Silent:** Same amino acid. * **Missense:** Different amino acid (e.g., Sickle Cell). * **Nonsense:** Results in a premature STOP codon (UAG, UAA, UGA). * **HbS Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because valine is less negatively charged than glutamic acid. * **Glutamic acid $\rightarrow$ Lysine** at the 6th position results in **HbC disease**.

Q: The FeCl3 test for urine in maple syrup urine disease gives which color?

Blue. **Explanation:** The **Ferric Chloride (FeCl₃) test** is a classic biochemical screening tool used to detect specific metabolites (usually phenols, enols, or keto-acids) in the urine. **1. Why Blue is Correct:** In **Maple Syrup Urine Disease (MSUD)**, there is a deficiency of the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD)** complex. This leads to the accumulation of branched-chain amino acids (Leucine, Isoleucine, Valine) and their corresponding **alpha-keto acids**. Specifically, the presence of **alpha-ketoisovaleric acid** reacts with ferric chloride to produce a characteristic **navy blue** or bluish-gray color. **2. Analysis of Incorrect Options:** * **Green (Option A):** This is the classic result for **Phenylketonuria (PKU)** due to the presence of phenylpyruvic acid. It is a very high-yield distinction for NEET-PG. * **Black (Option C):** A "transient blue-green" turning into a **persistent black** color is seen in **Alkaptonuria** (due to homogentisic acid). * **Red/Purple (Option D):** A purple or Bordeaux-red color is seen in **Salicylate poisoning** or the presence of **Acetoacetate** (Ketone bodies). **3. High-Yield Clinical Pearls for MSUD:** * **Defective Enzyme:** BCKAD (requires Thiamine/B1 as a cofactor). * **Odor:** Urine smells like burnt sugar or maple syrup due to **S-isoleucine**. * **Diagnosis:** Elevated levels of Leucine, Isoleucine, and Valine in plasma; presence of **Alloisoleucine** (pathognomonic). * **Management:** Dietary restriction of BCAA and, in some cases, high-dose Thiamine supplementation.

Q: Which type of mutation occurs in sickle cell anemia?

Point mutation. **Explanation:** **Sickle Cell Anemia** is a classic example of a **Point Mutation**, specifically a **missense mutation**. It involves a single nucleotide substitution in the $\beta$-globin gene on chromosome 11. 1. **Why Point Mutation is Correct:** The molecular basis involves the substitution of **Adenine (A) with Thymine (T)** in the GAG codon (GAG $\rightarrow$ GTG). This results in the replacement of **Glutamic acid** (a polar, negatively charged amino acid) with **Valine** (a non-polar, hydrophobic amino acid) at the **6th position** of the $\beta$-globin chain. This single change causes hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of RBCs. 2. **Why Other Options are Incorrect:** * **Frameshift Mutation:** Occurs when nucleotides are inserted or deleted in numbers not divisible by three, altering the entire reading frame (e.g., Tay-Sachs disease). Sickle cell does not change the reading frame. * **Deletion:** Involves the loss of genetic material. While $\alpha$-thalassemia often results from gene deletions, sickle cell is a substitution, not a loss. * **Crossover Mutation:** Refers to unequal crossing over during meiosis (e.g., Hb Lepore). This is a large-scale structural rearrangement, not a single base change. **High-Yield NEET-PG Pearls:** * **Inheritance:** Autosomal Recessive. * **Electrophoresis:** HbS moves **slower** than HbA toward the anode (+) because valine is neutral, whereas glutamic acid is negatively charged. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Metabolic Trigger:** Acidosis, dehydration, and hypoxia precipitate sickling crises.

Q: What syndrome is associated with the deficiency of Dermatan sulfate, heparan sulfate, chondroitin 4-sulfate, and chondroitin 6-sulfate?

Sly syndrome. ### Explanation **Correct Answer: C. Sly syndrome** **Underlying Medical Concept:** Sly syndrome (Mucopolysaccharidosis type VII) is caused by a deficiency of the enzyme **β-glucuronidase**. This specific enzyme is required for the degradation of glucuronic acid residues found in multiple glycosaminoglycans (GAGs). Because glucuronic acid is a common constituent of **Dermatan sulfate, Heparan sulfate, and Chondroitin 4/6-sulfates**, a deficiency in β-glucuronidase leads to the systemic accumulation of all four substances. This distinguishes it from other MPS types which typically involve only one or two GAGs. **Analysis of Incorrect Options:** * **A. Hunter syndrome (MPS II):** Caused by a deficiency of **Iduronate sulfatase**. It leads to the accumulation of **Dermatan sulfate and Heparan sulfate** only. (Key NEET-PG fact: It is X-linked recessive and lacks corneal clouding). * **B. Morquio syndrome B (MPS IVB):** Caused by a deficiency of **β-galactosidase**. It primarily results in the accumulation of **Keratan sulfate**. * **D. Hurler syndrome (MPS IH):** Caused by a deficiency of **α-L-iduronidase**. Like Hunter syndrome, it leads to the accumulation of **Dermatan sulfate and Heparan sulfate**, but presents with more severe features and corneal clouding. **High-Yield Clinical Pearls for NEET-PG:** * **Sly Syndrome Unique Feature:** It is the only MPS that can present as **Hydrops fetalis** in utero. * **GAG Triad:** If a question mentions the accumulation of **Chondroitin sulfates** alongside Dermatan and Heparan, always think of **Sly Syndrome**. * **Enzyme Mnemonic:** "Sly (7) Glucuronidase" — MPS **VII** is **β-glucuronidase** deficiency.

Q: In Phenylketonuria, what is the main aim of first-line therapy?

Limiting the substrate for the deficient enzyme. ### Explanation **1. Why Option C is Correct:** Phenylketonuria (PKU) is most commonly caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts the essential amino acid **Phenylalanine** into **Tyrosine**. In its absence, Phenylalanine accumulates to toxic levels in the blood and brain, leading to severe intellectual disability and seizures. The primary therapeutic strategy is **dietary restriction of Phenylalanine**. By limiting the substrate (Phenylalanine intake), we prevent its toxic accumulation, thereby bypassing the metabolic block. **2. Why the Other Options are Incorrect:** * **Option A:** Enzyme Replacement Therapy (ERT) exists (e.g., Pegvaliase), but it is not the "first-line" therapy. It is generally reserved for adults who cannot maintain metabolic control through diet alone. * **Option B:** While Tyrosine becomes a "conditionally essential" amino acid in PKU, simply replacing the product (Tyrosine) does not solve the primary problem of Phenylalanine neurotoxicity. * **Option C vs D:** Option D is a distractor. Phenylalanine is an essential amino acid; we do not "give" it because it is missing—we strictly "limit" it because it is in excess. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Cofactor:** A subset of PKU is caused by a deficiency of **Tetrahydrobiopterin (BH4)**. * **Clinical Features:** "Mousy" or "Musty" body odor (due to phenylacetic acid), hypopigmentation (low melanin), and intellectual disability. * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry (TMS) for newborn screening. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high phenylalanine levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus.

Q: What is the most appropriate treatment for Homocystinuria?

Vitamin B6. **Explanation:** **Homocystinuria** is most commonly caused by a deficiency of the enzyme **Cystathionine β-synthase (CBS)**. This enzyme facilitates the conversion of homocysteine to cystathionine. Crucially, CBS requires **Pyridoxine (Vitamin B6)** as a mandatory cofactor. 1. **Why Vitamin B6 is correct:** Approximately 50% of patients with Homocystinuria are "B6-responsive." High doses of Vitamin B6 can stabilize the mutated CBS enzyme, enhancing its residual activity and effectively lowering toxic homocysteine levels. It is considered the first-line pharmacological intervention. 2. **Why other options are incorrect:** * **Betaine (A):** While used as an adjunct treatment, it acts by providing methyl groups to convert homocysteine back to methionine (remethylation pathway). It is typically reserved for B6-non-responsive patients. * **Folic acid (B):** Folate and Vitamin B12 are essential cofactors for the remethylation of homocysteine, but they are supportive therapies rather than the primary corrective treatment for the classic CBS deficiency. * **Vitamin C (C):** Has no direct role in the metabolic pathway of sulfur-containing amino acids or the treatment of Homocystinuria. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Intellectual disability, Ectopia lentis (downward dislocation of lens), and Thromboembolic events (major cause of mortality). * **Marfanoid Habitus:** Patients often present with long limbs and chest deformities, but unlike Marfan syndrome (upward lens dislocation), Homocystinuria features downward lens dislocation and autosomal recessive inheritance. * **Diagnosis:** Elevated homocysteine in urine (Cyanide-nitroprusside test) and blood. * **Dietary Management:** Low methionine diet and Cysteine supplementation (as Cysteine becomes an essential amino acid).

Q: Which of the following findings is associated with alkaptonuria?

Ochronosis. **Explanation:** **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**. This enzyme is essential in the catabolic pathway of phenylalanine and tyrosine. Its deficiency leads to the accumulation of **homogentisic acid (HGA)** in the body. 1. **Why Ochronosis is Correct:** Excess homogentisic acid is oxidized and polymerized into a melanin-like pigment. This pigment deposits in connective tissues, cartilages (like the pinna of the ear and nose), and joints. This bluish-black discoloration of connective tissue is termed **ochronosis**. Long-term deposition leads to ochronotic arthritis, typically affecting large weight-bearing joints and the spine. 2. **Why Other Options are Incorrect:** * **Red urine:** This is characteristic of Porphyrias or Hematuria. In Alkaptonuria, the urine turns **black** upon standing or alkalinization due to the oxidation of HGA. * **Anuria:** This refers to the failure of kidneys to produce urine, seen in acute renal failure, not in amino acid metabolism disorders. * **Myositis:** This is an inflammatory muscle condition (e.g., Dermatomyositis) and is not a feature of Alkaptonuria. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** Homogentisic aciduria (urine turns black on standing), Ochronosis, and Arthritis. * **Diagnostic Test:** Ferric chloride test (gives a transient deep blue color) and Silver nitrate test. * **Radiological Sign:** "Bamboo spine" appearance due to calcification of intervertebral discs (mimicking Ankylosing Spondylitis). * **Management:** Dietary restriction of Tyrosine and Phenylalanine; **Nitisinone** is the drug of choice as it inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase, preventing HGA formation.

Question 1

A child presents with seizures and developmental delay, and is diagnosed with phenylketonuria. What is the initial line of treatment for this child?

Accepted Answer

Restriction of the dietary substrate of the deficient enzyme

Answer

Dietary supply of deficient protein

Answer

Supplementation of enzyme products

Answer

Dietary supply of the deficient proenzyme

Question 2

Which of the following is a classic example of a missense mutation?

Accepted Answer

Sickle cell disease

Answer

Thalassemia

Answer

Sideroblastic anemia

Answer

Hemochromatosis

Question 3

The FeCl3 test for urine in maple syrup urine disease gives which color?

Accepted Answer

Blue

Answer

Green

Answer

Black

Answer

Red

Question 4

Which type of mutation occurs in sickle cell anemia?

Accepted Answer

Point mutation

Answer

Crossover mutation

Answer

Frameshift

Answer

Deletion

Question 5

What syndrome is associated with the deficiency of Dermatan sulfate, heparan sulfate, chondroitin 4-sulfate, and chondroitin 6-sulfate?

Accepted Answer

Sly syndrome

Answer

Hunter syndrome

Answer

Morquio syndrome B

Answer

Hurler syndrome

Question 6

In Phenylketonuria, what is the main aim of first-line therapy?

Accepted Answer

Limiting the substrate for the deficient enzyme

Answer

Replacement of the defective enzyme

Answer

Replacement of the deficient product

Answer

Giving the missing amino acid by diet

Question 7

Which glycogen storage disease does not affect muscles?

Accepted Answer

Type I

Answer

Type II

Answer

Type III

Answer

Type V

Question 8

An 8-day-old child presents with yellow sclera, whitish stool, and turmeric-colored urine on the 3rd day of septicemia treated with broad-spectrum antibiotics. What is the likely diagnosis?

Accepted Answer

Galactose-1-phosphate uridyltransferase deficiency

Answer

Galactosidase deficiency

Answer

Ammonia toxicity

Answer

All of the above

Question 9

What is the most appropriate treatment for Homocystinuria?

Accepted Answer

Vitamin B6

Answer

Betaine

Answer

Folic acid

Answer

Vitamin C

Question 10

Which of the following findings is associated with alkaptonuria?

Accepted Answer

Ochronosis

Answer

Red urine

Answer

Anuria

Answer

Myositis

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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