Genetic Disorders and Biochemical Pathology Practice Questions

Q: Which of the following gene mutations occurs in cleidocranial dysplasia?

CBFA1 gene. **Explanation:** **Cleidocranial Dysplasia (CCD)** is an autosomal dominant skeletal disorder primarily characterized by defective intramembranous and endochondral ossification. **1. Why CBFA1 is Correct:** The correct answer is **CBFA1** (Core-Binding Factor Alpha 1), now more commonly known as **RUNX2** (Runt-related transcription factor 2). This gene, located on chromosome 6p21, encodes a transcription factor essential for **osteoblast differentiation**. It acts as a "master switch" for bone formation. Mutations lead to a failure in the maturation of precursor cells into osteoblasts, resulting in the classic skeletal and dental abnormalities seen in CCD. **2. Why Other Options are Incorrect:** * **CDFA1:** This is a distractor and not a recognized gene associated with skeletal dysplasias. * **CBFA2:** Also known as **RUNX1**, this gene is primarily involved in hematopoiesis (blood cell formation). Mutations in CBFA2 are associated with Acute Myeloid Leukemia (AML) and familial platelet disorders, not skeletal morphogenesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** 1. **Aplasia/Hypoplasia of Clavicles:** Patients can often bring their shoulders together in the midline. 2. **Delayed Closure of Fontanelles:** Resulting in a large head with frontal bossing and open skull sutures (Wormian bones). 3. **Dental Abnormalities:** Presence of multiple **supernumerary teeth**, delayed eruption of permanent teeth, and a high arched palate. * **Inheritance:** Autosomal Dominant. * **Stature:** Patients usually have moderately short stature but normal intelligence.

Q: Which amino acids are excreted in the urine in cystinosis?

All of the above. **Explanation:** The correct answer is **D. All of the above**. To understand this, it is crucial to distinguish between **Cystinuria** and **Cystinosis**, as they are frequently confused in exams. 1. **Cystinosis:** This is a lysosomal storage disorder caused by a defect in the **CTNS gene**, which encodes **cystinosin**, a lysosomal cystine transporter. This leads to the accumulation of cystine crystals within lysosomes across various organs. In the kidneys, this accumulation causes generalized proximal tubular damage, known as **Renal Fanconi Syndrome**. Because the entire proximal tubule is dysfunctional, there is a failure to reabsorb *all* amino acids (generalized aminoaciduria), glucose, phosphates, and bicarbonates. Therefore, cystine, ornithine, arginine, and lysine (the COAL group) are all excreted, along with many others. 2. **Cystinuria (The "COAL" mnemonic):** This is a specific transport defect in the proximal tubule and small intestine involving only the dibasic amino acids: **C**ystine, **O**rnithine, **A**rginine, and **L**ysine. While "COAL" excretion is the hallmark of Cystinuria, these same amino acids are also excreted in Cystinosis due to the broader Fanconi Syndrome. **Clinical Pearls for NEET-PG:** * **Cystinosis:** Characterized by growth retardation, photophobia (due to corneal crystals), and early-onset renal failure. It is the most common cause of Fanconi Syndrome in children. * **Cystinuria:** Presents with recurrent **hexagonal cystine stones** (calculi). Diagnosis is via the **Cyanide-Nitroprusside test** (purple color). * **Treatment:** Cystinosis is treated with **Cysteamine** (depletes lysosomal cystine); Cystinuria is managed with hydration, urinary alkalinization, and Penicillamine.

Q: Prenatal determination of osteogenesis imperfecta is done by?

Abnormal Pro- alpha chain. **Explanation:** **Osteogenesis Imperfecta (OI)**, also known as "Brittle Bone Disease," is a genetic disorder primarily caused by mutations in the **COL1A1** and **COL1A2** genes. These genes are responsible for the synthesis of **Type 1 Collagen**, the most abundant protein in bone matrix. 1. **Why "Abnormal Pro-alpha chain" is correct:** Type 1 collagen is composed of a triple helix formed by two pro-alpha 1 chains and one pro-alpha 2 chain. In OI, mutations lead to either a quantitative deficiency or a qualitative defect (structural abnormality) in these **pro-alpha chains**. Prenatal diagnosis is achieved by analyzing collagen synthesized by cultured **chorionic villus cells** or amniocytes. Biochemical analysis (electrophoresis) detects the presence of these abnormal pro-alpha chains, confirming the diagnosis before birth. 2. **Why other options are incorrect:** * **Alkaline Phosphatase (ALP):** While ALP is a marker of osteoblastic activity and is often elevated in bone remodeling disorders (like Paget’s disease or Rickets), it is not a specific or reliable prenatal marker for OI. * **Acid Phosphatase:** This is a marker of osteoclastic activity (bone resorption) and is not used for the prenatal diagnosis of collagen defects. * **Neutral Phosphatase:** This has no clinical relevance in the diagnosis of metabolic or genetic bone diseases. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Most common types (Type I and IV) are Autosomal Dominant. * **Clinical Triad:** Fragile bones (multiple fractures), **Blue Sclera** (due to thinning of collagen allowing uveal pigment to show), and **Early-onset Hearing loss** (Otosclerosis). * **Radiology:** Look for "Zebra line" sign (post-bisphosphonate treatment) and "Wormian bones" in the skull. * **Treatment:** Bisphosphonates (e.g., Pamidronate) are the mainstay to increase bone mineral density and reduce fractures.

Q: Which of the following conditions is due to a point mutation?

Sickle cell anemia. **Explanation:** **Sickle Cell Anemia (Correct Answer):** Sickle cell anemia is the classic example of a **missense point mutation**. It occurs due to a single nucleotide substitution (A to T) in the 6th codon of the **β-globin gene** on chromosome 11. This results in the replacement of **Glutamic acid** (polar/negative charge) with **Valine** (non-polar/hydrophobic). This single amino acid change causes hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of RBCs, hemolysis, and vaso-occlusive crises. **Analysis of Incorrect Options:** * **Color Blindness:** This is typically an **X-linked recessive** disorder caused by **unequal crossing over** or deletions/rearrangements in the opsin genes, rather than a single point mutation. * **Diabetes Mellitus:** This is a **multifactorial/polygenic** disorder involving a complex interplay of multiple genetic loci and environmental factors (Type 2) or autoimmune destruction (Type 1). * **Porphyria:** While some forms can involve point mutations, the group as a whole is heterogeneous. Most porphyrias (like Acute Intermittent Porphyria) are **Autosomal Dominant** and involve various types of mutations (insertions, deletions, or splice-site mutations) leading to enzyme deficiencies in the heme synthesis pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Transition vs. Transversion:** The mutation in Sickle Cell (GAG → GTG) is a **transversion** (purine to pyrimidine). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because Valine is less negative than Glutamic acid. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria.

Q: Proximal tubular dysfunction is seen in all of the following disorders EXCEPT?

Fabry disease. The question tests the ability to distinguish between various lysosomal storage disorders and their specific renal manifestations. Proximal tubular dysfunction, often manifesting as **Fanconi Syndrome** (characterized by glucosuria, phosphaturia, aminoaciduria, and bicarbonate loss), is a hallmark of certain metabolic diseases but is notably absent in others [1]. **1. Why Fabry Disease is the Correct Answer:** Fabry disease is an X-linked recessive deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of globotriaosylceramide (Gb3). While it causes significant renal pathology, the primary site of damage is the **glomerulus** (podocytes) and the **distal nephron/collecting ducts**. It presents with proteinuria and progressive renal failure, but it does **not** typically cause proximal tubular dysfunction or Fanconi syndrome. **2. Analysis of Incorrect Options:** * **Cystinosis:** This is the **most common cause** of inherited Fanconi syndrome in children. Cystine crystals accumulate within the lysosomes of proximal tubular cells, leading to severe dysfunction. * **Lowe Syndrome (Oculocerebrorenal syndrome):** An X-linked disorder affecting the *OCRL1* gene. It presents with a classic triad of congenital cataracts, intellectual disability, and **proximal renal tubular acidosis**. * **Pompe Disease (GSD Type II):** While primarily a neuromuscular disorder (acid maltase deficiency), glycogen accumulation can occur in the proximal tubules, leading to tubular dysfunction in some clinical presentations. **Clinical Pearls for NEET-PG:** * **Fanconi Syndrome Mnemonic:** "**L**ow **C**ystine **W**ilson **G**lycogen" (**L**owe, **C**ystinosis, **W**ilson disease, **G**alactosemia/Glycogen storage diseases) [2]. * **Fabry Disease Key Finding:** Look for "Maltese cross" appearance of lipid droplets in urine and angiokeratomas on physical exam. * **Cystinosis Key Finding:** Corneal cystine crystals on slit-lamp exam.

Q: What is the chromosomal complement in individuals with Klinefelter's syndrome?

47, XXY. **Explanation:** **1. Why Option D is Correct:** Klinefelter’s syndrome is the most common sex chromosome disorder affecting males, occurring in approximately 1 in 600 live births. It is characterized by the presence of an **extra X chromosome** in a phenotypic male, resulting in the **47, XXY** karyotype. This occurs primarily due to **meiotic non-disjunction** of the sex chromosomes during gametogenesis (more commonly maternal in origin). The presence of the Y chromosome ensures male differentiation, but the extra X chromosome leads to testicular dysgenesis and subsequent androgen deficiency. **2. Analysis of Incorrect Options:** * **Option A (45, XO):** This represents **Turner Syndrome**, characterized by a phenotypic female with short stature and streak ovaries. * **Option B (47, XXX):** This is **Triple X Syndrome** (Superfemale), which typically presents in females who may be asymptomatic or have mild learning disabilities. * **Option C (46, XY):** This is the normal male chromosomal complement. **3. NEET-PG High-Yield Clinical Pearls:** * **Clinical Features:** Tall stature, eunuchoid body proportions (increased arm span), gynecomastia, small firm testes, and infertility (azoospermia). * **Biochemical Profile:** Due to testicular atrophy (specifically Leydig cell dysfunction and seminiferous tubule hyalinization), there is **decreased Testosterone**, **increased LH**, and **increased FSH** (Hypergonadotropic Hypogonadism). * **Cytogenetics:** A **Barr body** (inactivated X chromosome) is visible on a buccal smear, which is normally absent in males. * **Associated Risks:** Increased risk of breast cancer, extragonadal germ cell tumors, and autoimmune diseases (like SLE).

Q: All of the following are lysosomal storage disorders, except?

Von Gierke's disease. **Explanation:** The correct answer is **Von Gierke’s disease** because it is a **Glycogen Storage Disease (GSD Type I)**, not a Lysosomal Storage Disorder (LSD). It is caused by a deficiency of the enzyme **Glucose-6-Phosphatase** in the liver and kidneys. Unlike LSDs, where undigested substrates accumulate within lysosomes, Von Gierke’s involves the accumulation of glycogen in the **cytosol** and endoplasmic reticulum, leading to severe fasting hypoglycemia, hepatomegaly, and hyperuricemia. **Analysis of Incorrect Options:** * **Gaucher’s Disease:** The most common LSD. It is caused by a deficiency of **$\beta$-Glucocerebrosidase**, leading to the accumulation of glucocerebroside in macrophages (Gaucher cells). * **Tay-Sachs Disease:** An LSD caused by a deficiency of **Hexosaminidase A**, resulting in the accumulation of GM2 gangliosides. It is characterized by a cherry-red spot on the macula and neurodegeneration. * **Niemann-Pick Disease:** An LSD caused by a deficiency of **Sphingomyelinase**, leading to sphingomyelin accumulation. It presents with hepatosplenomegaly and a cherry-red spot. **NEET-PG High-Yield Pearls:** 1. **Pompe’s Disease (GSD Type II)** is the only Glycogen Storage Disease that is *also* classified as a Lysosomal Storage Disorder (deficiency of lysosomal $\alpha$-1,4-glucosidase). 2. **Enzyme Replacement Therapy (ERT)** is available for Gaucher’s but not for Von Gierke’s (managed by frequent cornstarch feeds). 3. **I-Cell Disease** is a unique LSD where multiple lysosomal enzymes are missing due to a failure in Golgi trafficking (Mannose-6-Phosphate tagging).

Q: What type of inheritance is seen in MELAS syndrome?

Mitochondrial. **Explanation:** **MELAS syndrome** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) is a classic example of **Mitochondrial Inheritance** (maternal inheritance) [2]. It is most commonly caused by a point mutation in the mitochondrial DNA (mtDNA) at position A3243G in the *MT-TL1* gene, which encodes tRNA leucine. 1. **Why Mitochondrial is correct:** Unlike nuclear DNA, mitochondrial DNA is inherited exclusively from the mother (as the sperm contributes negligible cytoplasm to the zygote). A hallmark of this inheritance is **Heteroplasmy**, where a cell contains a mixture of both mutant and normal mtDNA. The clinical severity depends on the proportion of mutant DNA in specific tissues [1]. 2. **Why other options are incorrect:** * **Autosomal Dominant/Recessive:** These involve mutations in the nuclear genome (chromosomes 1–22). While some mitochondrial proteins are encoded by nuclear DNA, the specific MELAS phenotype is linked to the mitochondrial genome [1]. * **X-linked:** This involves genes on the X chromosome. MELAS affects both males and females equally, but only females can transmit the disease to their offspring. **Clinical Pearls for NEET-PG:** * **Classic Triad:** Stroke-like episodes (before age 40), Encephalopathy (seizures/dementia), and Lactic Acidosis [2]. * **Muscle Biopsy:** Shows **"Ragged Red Fibers"** (Gomori trichrome stain) due to subsarcolemmal accumulation of abnormal mitochondria. * **Other Mitochondrial Diseases:** MERRF (Myoclonic Epilepsy with Ragged Red Fibers), LHON (Leber’s Hereditary Optic Neuropathy), and Kearns-Sayre Syndrome [2]. * **High-Yield Fact:** All children of an affected mother are at risk, but children of an affected father will not inherit the disease.

Q: What is the chance of a child having cystic fibrosis if one parent is a carrier and the other is unaffected and not a carrier?

25%. ### Explanation **1. Understanding the Correct Answer (B):** Cystic Fibrosis (CF) is an **Autosomal Recessive (AR)** disorder caused by mutations in the *CFTR* gene on chromosome 7. For a child to manifest the disease, they must inherit two defective alleles (homozygous recessive). In this scenario: * **Parent 1 (Carrier):** Genotype **Aa** (one normal allele, one mutant allele). * **Parent 2 (Unaffected/Non-carrier):** Genotype **AA** (two normal alleles). Using a Punnett Square (Aa x AA), the possible offspring genotypes are: * 50% **AA** (Unaffected, non-carrier) * 50% **Aa** (Unaffected carrier) * **0% aa** (Affected with Cystic Fibrosis) **Wait!** There is a discrepancy in the provided key. Mathematically, the chance of having the disease (aa) is **0%**. However, in medical entrance exams, if "25%" is marked as correct, it typically refers to the standard AR inheritance pattern where *both* parents are carriers (Aa x Aa). If the question strictly defines one parent as a non-carrier (AA), the biological risk is 0%. **2. Analysis of Incorrect Options:** * **Option A (0%):** This is the genetically accurate answer for the specific scenario described (Aa x AA). * **Option B (25%):** This is the risk when **both** parents are carriers (Aa x Aa). This is the most "high-yield" number for AR disorders in exams. * **Option C (50%):** This is the risk if one parent is affected (aa) and the other is a carrier (Aa)—a scenario known as "Pseudodominance." * **Option D (100%):** This only occurs if both parents are affected (aa x aa). **3. NEET-PG Clinical Pearls:** * **Most common mutation:** ΔF508 (deletion of phenylalanine at position 508), leading to protein misfolding in the ER. * **Diagnosis:** Sweat Chloride Test (>60 mEq/L) is the gold standard. * **Key Association:** Congenital Bilateral Absence of Vas Deferens (CBAVD) leading to male infertility. * **Screening:** Immunoreactive Trypsinogen (IRT) in newborns.

Q: An infant presents with a history of seizures and skin rashes. Investigations show metabolic acidosis, increased blood ketone levels, and normal ammonia (NH3). What is the likely diagnosis?

Multiple carboxylase deficiency. ### Explanation **1. Why Multiple Carboxylase Deficiency (MCD) is correct:** MCD is caused by a deficiency in **Biotinidase** or **Holocarboxylase synthetase**, enzymes responsible for the attachment and recycling of biotin (Vitamin B7). Biotin is a mandatory cofactor for four key carboxylases: * **Pyruvate carboxylase:** Its failure leads to lactic acidosis. * **Acetyl-CoA carboxylase:** Impairs fatty acid synthesis. * **Propionyl-CoA carboxylase:** Leads to the accumulation of organic acids (metabolic acidosis). * **3-methylcrotonyl-CoA carboxylase:** Accumulation of metabolites causes the characteristic **skin rash (alopecia and dermatitis)** and neurological symptoms like **seizures**. The presence of metabolic acidosis with ketosis and dermatological findings is the classic triad for MCD. **2. Why the other options are incorrect:** * **Propionic acidemia:** While it causes metabolic acidosis and ketosis, it typically does **not** present with the significant skin rashes (dermatitis) seen in biotin-related disorders. * **Urea cycle disorder:** These typically present with **hyperammonemia** and respiratory alkalosis, not metabolic acidosis or ketosis. * **Phenylketonuria (PKU):** Presents with intellectual disability and a "mousy" odor, but not with acute metabolic acidosis or ketosis. **3. NEET-PG High-Yield Pearls:** * **The "B" Enzymes:** Biotin is required for all **Carboxylases** (except Vitamin K-dependent ones). * **Clinical Triad:** Seizures + Organic Acidosis + Alopecia/Skin Rash = Think Biotinidase Deficiency. * **Treatment:** This is a highly treatable condition; patients respond dramatically to oral **Biotin** supplementation. * **Diagnosis:** Confirmed by measuring biotinidase activity in serum or organic acid analysis via GC-MS (showing 3-hydroxyisovaleric acid).

Question 1

Which of the following gene mutations occurs in cleidocranial dysplasia?

Accepted Answer

CBFA1 gene

Answer

CDFA1 gene

Answer

CBFA2 gene

Answer

None

Question 2

Which amino acids are excreted in the urine in cystinosis?

Accepted Answer

All of the above

Answer

Cystine

Answer

Ornithine

Answer

Arginine

Question 3

Prenatal determination of osteogenesis imperfecta is done by?

Accepted Answer

Abnormal Pro- alpha chain

Answer

Acid phosphatase

Answer

Alkaline phosphatase

Answer

Neutral phosphatase

Question 4

Which of the following conditions is due to a point mutation?

Accepted Answer

Sickle cell anemia

Answer

Color blindness

Answer

Diabetes

Answer

Porphyria

Question 5

Proximal tubular dysfunction is seen in all of the following disorders EXCEPT?

Accepted Answer

Fabry disease

Answer

Pompe disease

Answer

Lowe syndrome

Answer

Cystinosis

Question 6

What is the chromosomal complement in individuals with Klinefelter's syndrome?

Accepted Answer

47, XXY

Answer

45, XO

Answer

47, XXX

Answer

46, XY

Question 7

All of the following are lysosomal storage disorders, except?

Accepted Answer

Von Gierke's disease

Answer

Gaucher's disease

Answer

Tay-Sachs disease

Answer

Niemann-Pick disease

Question 8

What type of inheritance is seen in MELAS syndrome?

Accepted Answer

Mitochondrial

Answer

Autosomal Dominant

Answer

Autosomal Recessive

Answer

X-linked

Question 9

What is the chance of a child having cystic fibrosis if one parent is a carrier and the other is unaffected and not a carrier?

Accepted Answer

25%

Answer

0%

Answer

50%

Answer

100%

Question 10

An infant presents with a history of seizures and skin rashes. Investigations show metabolic acidosis, increased blood ketone levels, and normal ammonia (NH3). What is the likely diagnosis?

Accepted Answer

Multiple carboxylase deficiency

Answer

Propionic acidemia

Answer

Urea cycle disorder

Answer

Phenylketonuria

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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