Genetic Disorders and Biochemical Pathology — MCQs

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

Question 581: Which of the following is a persistent biochemical marker of rickets?

A. Serum calcium
B. Serum alkaline phosphatase (Correct Answer)
C. Serum acid phosphatase
D. Serum phosphate

Explanation: **Explanation:** In rickets, the primary pathology is a failure of osteoid mineralization. To compensate for the weakened bone structure and defective mineralization, **osteoblastic activity** increases significantly. **Serum Alkaline Phosphatase (ALP)** is a byproduct of osteoblast activity; therefore, its levels rise sharply. It is considered the most sensitive and **persistent biochemical marker** because it remains elevated throughout the active phase of the disease and is often the last parameter to normalize during the healing process. **Analysis of Incorrect Options:** * **Serum Calcium (A):** In early rickets, calcium may be low, but it often returns to the **low-normal range** due to compensatory secondary hyperparathyroidism (which mobilizes calcium from bones). Thus, it is not a persistent indicator of the disease state. * **Serum Acid Phosphatase (C):** This is a marker of **osteoclast** activity (bone resorption) and is typically associated with conditions like prostate cancer or Gaucher disease, not primarily with rickets. * **Serum Phosphate (D):** While hypophosphatemia is a hallmark of most forms of rickets (due to PTH-induced renal wasting), it can fluctuate based on dietary intake and is not as reliable a marker of bone turnover as ALP. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Biochemical Change:** Decreased 25-hydroxyvitamin D [25(OH)D] levels. * **Radiological Hallmark:** Fraying, cupping, and splaying of the metaphysis (best seen at the wrist or knee). * **Healing Marker:** The appearance of a "line of calcification" at the junction of the epiphysis and metaphysis on X-ray, accompanied by a gradual fall in ALP. * **Rule of Thumb:** In Vitamin D deficiency rickets: **Calcium (↓/Normal), Phosphate (↓), ALP (↑↑), PTH (↑).**

Question 582: Tomcat urine odor is characteristic of which of the following conditions?

A. Multiple carboxylase deficiency (Correct Answer)
B. Phenylketonuria
C. Hawkinuria
D. Maple syrup urine disease

Explanation: **Explanation:** **Multiple Carboxylase Deficiency (MCD)** is the correct answer. This condition results from a deficiency in **Holocarboxylase synthetase** or **Biotinidase**, leading to the dysfunction of four biotin-dependent enzymes: Pyruvate carboxylase, Acetyl-CoA carboxylase, Propionyl-CoA carboxylase, and 3-Methylcrotonyl-CoA carboxylase. The "tomcat urine" odor is specifically attributed to the accumulation of **3-hydroxyisovaleric acid** and **3-methylcrotonylglycine**, which are metabolites of leucine that cannot be properly processed due to the deficiency of 3-Methylcrotonyl-CoA carboxylase. **Analysis of Incorrect Options:** * **Phenylketonuria (PKU):** Characterized by a **mousy or musty odor** due to the accumulation of phenylacetic acid. * **Hawkinuria:** A rare defect in tyrosine metabolism (4-hydroxyphenylpyruvate dioxygenase deficiency) characterized by a **swimming pool or chlorine-like odor**. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex, leading to a **burnt sugar or maple syrup odor**. **High-Yield Clinical Pearls for NEET-PG:** * **Isovaleric Acidemia:** Characterized by a **"sweaty feet"** or "cheese-like" odor. * **Trimethylaminuria:** Known as **"Fish odor syndrome."** * **Tyrosinemia Type I:** Characterized by a **boiled cabbage or rancid butter** odor. * **Oasthouse Urine Disease:** Characterized by a **dried malt or hops** odor. * **MCD Clinical Presentation:** Often presents with metabolic acidosis, skin rash (alopecia/dermatitis), and neurological symptoms; it is highly responsive to **Biotin supplementation**.

Question 583: All of the following are autosomal dominant disorders except?

A. Von Hippel-Lindau disease
B. Achondroplasia
C. Duchenne muscular dystrophy (Correct Answer)
D. Multiple endocrine neoplasia

Explanation: **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).

Question 584: Which of the following statements is FALSE about Wolman disease?

A. It is a lysosomal storage disorder.
B. It is characterized by deficient acid lysosomal lipase.
C. It shows an autosomal recessive inheritance pattern.
D. It is characterized by adrenal calcification and corneal clouding. (Correct Answer)

Explanation: **Explanation** Wolman disease is a severe, early-onset **lysosomal storage disorder (LSD)**. The question asks for the false statement, and Option D is incorrect because while **adrenal calcification** is a hallmark feature of Wolman disease, **corneal clouding is not**. Corneal clouding is typically associated with certain Mucopolysaccharidoses (like Hurler syndrome) or Mucolipidosis, but not with acid lipase deficiencies. **Analysis of Options:** * **Option A & B (True):** Wolman disease is caused by a mutation in the *LIPA* gene, leading to a functional deficiency of **Lysosomal Acid Lipase (LAL)**. This enzyme is essential for hydrolyzing cholesteryl esters and triglycerides. Its deficiency leads to the massive accumulation of these lipids within lysosomes across various tissues. * **Option C (True):** Like most lysosomal storage disorders (with the exception of Fabry and Hunter syndromes), Wolman disease follows an **autosomal recessive** inheritance pattern. * **Option D (False):** Adrenal calcification occurs in about 50% of patients due to the accumulation of lipids in the adrenal cortex, which subsequently undergoes necrosis and calcification. However, the absence of corneal clouding makes this statement false. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Hepatosplenomegaly (leading to abdominal distension), steatorrhea (malabsorption), and bilateral adrenal calcification. * **Radiology:** Bilateral enlarged, "stippled" adrenal calcification on X-ray/CT is a pathognomonic sign. * **Prognosis:** Usually fatal within the first year of life if untreated. * **Related Condition:** **Cholesteryl Ester Storage Disease (CESD)** is the milder, adult-onset form of LAL deficiency. * **Treatment:** Enzyme replacement therapy with **Sebelipase alfa**.

Question 585: The phenomenon of anticipation is observed in which type of genetic condition?

A. Mitochondrial inheritance
B. X-linked dominant disorders
C. Trinucleotide repeat expansion disorders (Correct Answer)
D. Genomic imprinting

Explanation: **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).

Question 586: Von Gierke's disease is due to a deficiency of which enzyme?

A. Branching enzyme
B. Debranching enzyme
C. Phosphorylase
D. Glucose 6 phosphatase (Correct Answer)

Explanation: **Explanation:** **Von Gierke’s Disease (Glycogen Storage Disease Type I)** is caused by a deficiency of the enzyme **Glucose-6-Phosphatase (G6Pase)**. This enzyme is responsible for the final step in both glycogenolysis and gluconeogenesis: converting Glucose-6-Phosphate into free glucose. Because this enzyme is primarily located in the liver and kidneys, its deficiency prevents the release of glucose into the bloodstream, leading to severe fasting hypoglycemia and massive hepatomegaly due to glycogen accumulation. **Analysis of Options:** * **Option A (Branching enzyme):** Deficiency causes **Andersen’s disease (GSD Type IV)**. It results in the accumulation of abnormal glycogen with long outer chains (amylopectin-like), leading to early liver cirrhosis. * **Option B (Debranching enzyme):** Deficiency causes **Cori’s disease (GSD Type III)**. Clinical features are similar to Von Gierke’s but milder, as gluconeogenesis remains intact. * **Option C (Phosphorylase):** Deficiency in the liver causes **Hers disease (GSD Type VI)**, while deficiency in the muscle causes **McArdle disease (GSD Type V)**. These enzymes are responsible for the initial breakdown of glycogen into Glucose-1-Phosphate. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmarks:** Severe fasting hypoglycemia, **Hyperuricemia** (due to increased PPP shunt activity), **Hyperlactatemia**, and **Hyperlipidemia** (doll-like facies). * **Diagnostic Clue:** Administration of glucagon or epinephrine does *not* raise blood glucose levels in these patients. * **Management:** Frequent oral cornstarch (to maintain glucose levels) and avoidance of fructose/galactose.

Question 587: Abnormalities of copper metabolism are implicated in the pathogenesis of all the following conditions except?

A. Wilson's disease
B. Menkes syndrome
C. Indian childhood cirrhosis
D. Keshan disease (Correct Answer)

Explanation: **Explanation:** The correct answer is **Keshan disease** because it is a cardiomyopathy caused by a deficiency of **Selenium**, not copper. It is often exacerbated by the presence of the Coxsackie B virus and was historically prevalent in regions of China with selenium-poor soil. **Analysis of Options:** * **Wilson’s Disease (Hepatolenticular Degeneration):** This is an autosomal recessive disorder caused by mutations in the **ATP7B gene**. It leads to impaired biliary copper excretion and failure to incorporate copper into ceruloplasmin, resulting in toxic copper accumulation in the liver, brain (basal ganglia), and cornea (Kayser-Fleischer rings). * **Menkes Syndrome (Kinky Hair Disease):** This is an X-linked recessive disorder caused by mutations in the **ATP7A gene**. It results in impaired intestinal copper absorption and transport, leading to systemic **copper deficiency**. Clinical features include "steely" or "kinky" hair, growth failure, and neurological degeneration. * **Indian Childhood Cirrhosis (ICC):** This is a progressive liver disorder associated with excessive **intake of copper**, traditionally from milk stored or boiled in brass or copper vessels. It is characterized by massive copper deposition in hepatocytes and rapid progression to cirrhosis. **High-Yield Clinical Pearls for NEET-PG:** * **ATP7A vs. ATP7B:** Remember "**A**bsorption" for ATP7**A** (Menkes) and "**B**iliary excretion" for ATP7**B** (Wilson). * **Ceruloplasmin:** Decreased in both Wilson’s and Menkes, but for different reasons (transport failure vs. systemic deficiency). * **Selenium Enzymes:** Selenium is a vital component of **Glutathione Peroxidase**, which protects cells from oxidative damage. Deficiency leads to Keshan disease (cardiomyopathy) and Kashin-Beck disease (osteoarthritis).

Question 588: All are features of Pompe's disease except?

A. Hyperglycemia (Correct Answer)
B. Cardiomyopathy
C. Skeletal myopathy
D. Hepatomegaly

Explanation: **Explanation:** **Pompe’s Disease (Glycogen Storage Disease Type II)** is unique among GSDs because it is a **lysosomal storage disorder**, not a disorder of glycogenolysis or gluconeogenesis. It is caused by a deficiency of **Acid α-1,4-glucosidase (Acid Maltase)**, which breaks down glycogen within lysosomes. 1. **Why Hyperglycemia is the correct answer (the "Except"):** In Pompe’s disease, the metabolic pathways in the cytosol (glycogenolysis and gluconeogenesis) remain **intact**. Therefore, the liver can still maintain blood glucose levels normally. Patients do **not** present with hypoglycemia; conversely, they definitely do not present with **hyperglycemia**. Blood glucose levels are typically normal. 2. **Why the other options are incorrect (Features of Pompe’s):** * **Cardiomyopathy (Option B):** This is the hallmark of the infantile-onset form. Massive glycogen accumulation in cardiac muscle leads to hypertrophic cardiomyopathy and "globular heart" on X-ray. * **Skeletal myopathy (Option C):** Glycogen accumulates in skeletal muscle lysosomes, causing profound hypotonia (Floppy Baby Syndrome) and respiratory muscle weakness. * **Hepatomegaly (Option D):** While the liver is not the primary organ of clinical failure (unlike GSD Type I), mild to moderate hepatomegaly occurs due to lysosomal glycogen buildup in hepatocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart), **Liver**, and **Muscle**." * **Enzyme Deficiency:** Acid Maltase (Acid α-1,4-glucosidase). * **Diagnosis:** PAS-positive material in lysosomes; "EKG with massive QRS complexes." * **Treatment:** Enzyme Replacement Therapy (Alglucosidase alfa). * **Key Distinction:** Unlike von Gierke’s (Type I), Pompe’s has **no** lactic acidosis, **no** hyperuricemia, and **normal** blood glucose.

Question 589: Enzyme alglucerase is used in the treatment of?

A. Gaucher's disease (Correct Answer)
B. Galactosemia
C. Niemann Pick disease
D. Pompe's disease

Explanation: **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).

Question 590: Which of the following is not inherited as X-linked recessive?

A. G-6-PD deficiency
B. Duchenne muscular dystrophy
C. Cystic fibrosis (Correct Answer)
D. Hemophilia

Explanation: **Explanation:** The question tests the knowledge of inheritance patterns of common genetic disorders. **1. Why Cystic Fibrosis is the correct answer:** Cystic Fibrosis (CF) is inherited as an **Autosomal Recessive** disorder. It is caused by a mutation in the *CFTR* (Cystic Fibrosis Transmembrane Conductance Regulator) gene located on **Chromosome 7**. For the disease to manifest, an individual must inherit two defective alleles (one from each parent). It is characterized by thick, tenacious secretions affecting the lungs, pancreas, and sweat glands. **2. Analysis of Incorrect Options (X-linked Recessive Disorders):** * **G-6-PD Deficiency:** This is a classic X-linked recessive (XLR) enzymopathy. It leads to episodic hemolytic anemia triggered by oxidative stress (e.g., fava beans, primaquine). * **Duchenne Muscular Dystrophy (DMD):** This is an XLR disorder caused by a mutation in the *Dystrophin* gene (the largest known human gene), leading to progressive muscle weakness and pseudohypertrophy. * **Hemophilia:** Both Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency) are inherited in an XLR fashion, primarily affecting males. **Clinical Pearls for NEET-PG:** * **Mnemonic for XLR disorders:** "**G**iants **D**o **H**ave **C**olorful **L**ittle **F**ragile **M**inds" (**G**6PD, **D**MD/DI, **H**emophilia, **C**olor blindness, **L**esch-Nyhan, **F**ragile X, **M**enkes). * **Cystic Fibrosis High-Yields:** Most common mutation is **ΔF508** (phenylalanine deletion). Screening is done via the **Sweat Chloride Test** (Pilocarpine iontophoresis). * **X-linked Recessive Rule:** These disorders never show male-to-male transmission; daughters of affected males are always obligate carriers.

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

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