Genetic Disorders and Biochemical Pathology Practice Questions

Q: Deficiency of which of the following enzymes leads to toxicity of 5-fluorouracil?

Dihydropyrimidine dehydrogenase. **Explanation:** The correct answer is **Dihydropyrimidine dehydrogenase (DPD)**. **Why DPD is the correct answer:** 5-Fluorouracil (5-FU) is a pyrimidine analog used in cancer chemotherapy. Under normal physiological conditions, more than 80% of administered 5-FU is catabolized and inactivated in the liver. **Dihydropyrimidine dehydrogenase (DPD)** is the rate-limiting enzyme in this catabolic pathway, responsible for converting 5-FU into its inactive metabolite, dihydrofluorouracil (DHFU). Patients with **DPD deficiency** cannot efficiently clear the drug, leading to an accumulation of toxic levels of 5-FU. This results in severe, potentially fatal toxicities, including profound myelosuppression, mucositis, and neurotoxicity. **Why the other options are incorrect:** * **A. Purine nucleoside phosphorylase (PNP):** This enzyme is involved in the purine salvage pathway. Its deficiency leads to T-cell immunodeficiency but has no role in 5-FU metabolism. * **C. PRPP synthase:** This enzyme catalyzes the formation of phosphoribosyl pyrophosphate (PRPP), a key precursor for both purine and pyrimidine synthesis. Overactivity leads to gout; it is not involved in 5-FU catabolism. * **D. Adenosine deaminase (ADA):** ADA deficiency leads to Severe Combined Immunodeficiency (SCID) due to the accumulation of dATP, which inhibits ribonucleotide reductase. It is specific to purine metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** 5-FU acts primarily by inhibiting **Thymidylate Synthase**, leading to "thymineless death" of cells. * **Antidote:** **Uridine triacetate** is the FDA-approved antidote for 5-FU overdose or severe toxicity. * **Pharmacogenomics:** Screening for *DPYD* gene mutations is increasingly recommended before starting 5-FU or Capecitabine to prevent severe adverse reactions.

Q: What is characteristically seen in Hartnup disorder?

Pellagra-like rash. **Explanation:** **Hartnup disorder** is an autosomal recessive metabolic disorder caused by a mutation in the **SLC6A19 gene**, which encodes a sodium-dependent neutral amino acid transporter. This defect leads to the malabsorption of neutral amino acids (most importantly **Tryptophan**) from the small intestine and a failure of their reabsorption in the proximal renal tubules. **Why Option A is correct:** Tryptophan is an essential precursor for the endogenous synthesis of **Niacin (Vitamin B3)**. In Hartnup disease, the profound deficiency of Tryptophan results in secondary Niacin deficiency. This manifests clinically as **Pellagra**, characterized by the "3 Ds": Dermatitis (a photosensitive, scaly rash), Diarrhea, and Dementia. The rash is typically seen on sun-exposed areas (e.g., Casal’s necklace). **Why other options are incorrect:** * **B. Burned feet rash:** This is associated with **Pantothenic acid (Vitamin B5)** deficiency, characterized by paresthesia and burning sensations in the lower extremities. * **C. Alopecia:** While seen in Zinc deficiency or Biotin deficiency, it is not a primary feature of Hartnup disorder. * **D. Vitiligo:** This is an autoimmune destruction of melanocytes and is unrelated to amino acid transport defects. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Confirmed by detecting **neutral aminoaciduria** (specifically Alanine, Serine, Threonine, Valine, Leucine, Isoleucine, Phenylalanine, Tyrosine, and Tryptophan) in a 24-hour urine sample. * **Treatment:** High-protein diet and **Nicotinic acid/Nicotinamide** supplementation. * **Differential:** Unlike dietary Pellagra, Hartnup disorder will show high levels of neutral amino acids in the urine.

Q: A 19-year-old male presents for a routine physical examination for sports activities. His fasting blood glucose is 7.5 mM. C-peptide and insulin levels are near normal under fasting conditions. Postprandially, his blood glucose levels are only slightly elevated before stabilizing at fasting levels. He denies excessive thirst or urination but recalls his mother had gestational diabetes. What type of inheritance best characterizes this alteration in glucose homeostasis?

Autosomal dominant. ### Explanation **1. Why Autosomal Dominant is Correct:** The clinical presentation is classic for **MODY (Maturity-Onset Diabetes of the Young)**, specifically **MODY-2**, which is caused by a mutation in the **Glucokinase (GCK) gene**. * **The Mechanism:** Glucokinase acts as the "glucose sensor" in pancreatic beta cells. A mutation increases the threshold for insulin release, leading to mild, stable, non-progressive fasting hyperglycemia (typically 5.5–8.0 mmol/L). * **The Inheritance:** All forms of MODY (including the most common types: HNF1A/MODY-3 and GCK/MODY-2) follow an **Autosomal Dominant** inheritance pattern. The mention of the mother having "gestational diabetes" (often misdiagnosed MODY-2) further supports a vertical transmission pattern. **2. Why Incorrect Options are Wrong:** * **Autosomal Recessive:** Rare forms of neonatal diabetes can be recessive, but MODY presents in adolescence/young adulthood and requires only one affected allele for the phenotype. * **Sex-linked:** There are no common primary disorders of glucose homeostasis linked to the X or Y chromosomes. * **Mitochondrial:** While Maternal Inherited Diabetes and Deafness (MIDD) exists, it is characterized by hearing loss and a different clinical progression, not the stable, asymptomatic hyperglycemia seen here. **3. Clinical Pearls for NEET-PG:** * **MODY-2 (Glucokinase):** Mildest form; usually asymptomatic; requires no treatment; no long-term microvascular complications. * **MODY-3 (HNF1A):** Most common type; progressive; high risk of complications; extremely sensitive to **Sulfonylureas**. * **Key Differentiator:** Unlike Type 1 DM, MODY patients have **detectable C-peptide** (insulin production is preserved) and lack autoantibodies. Unlike Type 2 DM, they are usually young, non-obese, and have no insulin resistance.

Q: In which of the following conditions is the formation of a calcium-containing kidney stone NOT likely?

Lesch-Nyhan syndrome. **Explanation:** The formation of kidney stones is primarily determined by the chemical composition of the solute in excess. To answer this question, one must distinguish between **Hypercalcemia** (leading to calcium stones) and **Hyperuricemia** (leading to urate stones). **Why Lesch-Nyhan Syndrome is the correct answer:** Lesch-Nyhan syndrome is an X-linked recessive disorder caused by a deficiency of the enzyme **HGPRT** (Hypoxanthine-Guanine Phosphoribosyltransferase). This deficiency leads to the failure of the purine salvage pathway, resulting in the massive overproduction of **Uric Acid**. Consequently, patients develop **Uric Acid stones** (radiolucent), not calcium stones. Clinical hallmarks include self-mutilation, choreoathetosis, and gouty arthritis. **Why the other options are incorrect:** * **Parathyroid Adenoma:** Causes Primary Hyperparathyroidism, leading to increased PTH, bone resorption, and hypercalcemia. This results in hypercalciuria and the formation of **Calcium Oxalate/Phosphate stones**. * **Multiple Myeloma:** This plasma cell dyscrasia causes extensive bone destruction via osteoclast activating factors, leading to significant **hypercalcemia** and subsequent calcium stone formation. * **Sarcoidosis:** Granulomatous macrophages in sarcoidosis contain 1-alpha-hydroxylase, which converts Vitamin D to its active form (1,25-dihydroxyvitamin D). This increases intestinal calcium absorption, causing **hypercalcemia** and calcium stones. **High-Yield Clinical Pearls for NEET-PG:** 1. **Most common kidney stone:** Calcium Oxalate (Radiopaque). 2. **Uric acid stones:** These are **Radiolucent** (not visible on X-ray) but visible on CT/Ultrasound. 3. **Lesch-Nyhan Enzyme:** HGPRT converts Hypoxanthine to IMP and Guanine to GMP. 4. **Treatment for Urate stones:** Alkalinization of urine (Uric acid is more soluble at higher pH) and Allopurinol.

Q: Which of the following is a Y-linked disease?

Leri-Weil dyschondrosteosis. **Explanation:** **Leri-Weil Dyschondrosteosis (LWD)** is the correct answer because it is inherited in a **pseudoautosomal dominant** manner. It is caused by mutations or deletions of the **SHOX gene**, located in the pseudoautosomal regions (PAR1) of the **X and Y chromosomes**. Because the gene is present on both sex chromosomes, it can be transmitted from father to son (Y-linked transmission), distinguishing it from classic X-linked disorders. Clinically, LWD is characterized by short stature, mesomelic shortening of limbs, and the **Madelung deformity** of the wrist. **Analysis of Incorrect Options:** * **Adrenoleukodystrophy (ALD):** This is an **X-linked recessive** peroxisomal disorder caused by a mutation in the *ABCD1* gene, leading to the accumulation of very-long-chain fatty acids (VLCFA). * **Fabry Disease:** This is an **X-linked recessive** lysosomal storage disorder caused by a deficiency of **alpha-galactosidase A**, leading to accumulation of globotriaosylceramide. * **Diabetes Insipidus (DI):** Congenital Nephrogenic DI is most commonly inherited as an **X-linked recessive** trait (mutation in the *AVPR2* gene). While autosomal forms exist, it is not Y-linked. [1] **NEET-PG High-Yield Pearls:** 1. **Pseudoautosomal Regions (PAR):** These are homologous sequences on the tips of X and Y chromosomes that allow them to pair and recombine during meiosis. Genes here (like *SHOX*) do not follow traditional sex-linked inheritance patterns. 2. **Pure Y-linked (Holandric) Traits:** These are rare. Classic examples include **Swyer syndrome** (SRY gene mutations) and **Hypertrichosis pinnae** (though the latter is debated). 3. **Madelung Deformity:** A hallmark of Leri-Weil syndrome, involving dorsal dislocation of the ulna and bowing of the radius.

Q: Which of the following lysosomal storage disorders is NOT inherited as an autosomal recessive trait?

Fabry disease. **Explanation:** The inheritance pattern of Lysosomal Storage Disorders (LSDs) is a high-yield topic for NEET-PG. Most LSDs follow an **Autosomal Recessive (AR)** inheritance pattern. However, there are two notable exceptions that are **X-linked Recessive (XLR)**: **Fabry disease** and **Hunter syndrome**. 1. **Why Fabry Disease is Correct:** Fabry disease is caused by a deficiency of the enzyme **$\alpha$-galactosidase A**, leading to the accumulation of ceramide trihexoside. The gene responsible is located on the X chromosome, making it an **X-linked Recessive** disorder. Clinical hallmarks include angiokeratomas, peripheral neuropathy (acroparesthesia), and late-stage renal/cardiac failure. 2. **Why Other Options are Incorrect:** * **Krabbe Disease:** An **AR** disorder caused by galactocerebrosidase deficiency. It is characterized by "globoid cells" and severe demyelination. * **Gaucher Disease:** The most common LSD, inherited as **AR**. It results from glucocerebrosidase deficiency. Look for "crinkled paper" cytoplasm in macrophages and hepatosplenomegaly. * **Tay-Sachs Disease:** An **AR** disorder caused by Hexosaminidase A deficiency. It presents with a cherry-red spot on the macula and progressive neurodegeneration, but notably lacks hepatosplenomegaly (unlike Niemann-Pick). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for XLR LSDs:** *"The **Hunter** shot a **Fabry** bird with his **X**-bow."* (Hunter syndrome and Fabry disease are X-linked). * **Hunter vs. Hurler:** Both are Mucopolysaccharidoses, but Hunter is XLR and lacks corneal clouding, while Hurler is AR and has corneal clouding. * **Enzyme Replacement Therapy (ERT)** is now available for Gaucher, Fabry, and Hunter/Hurler diseases.

Q: A patient presents with growth retardation, delayed milestones, and kinky hair. What is the most likely diagnosis?

Menke's disease. ### Explanation **Correct Answer: A. Menke’s Disease** Menke’s disease (also known as Menkes Kinky Hair Syndrome) is an **X-linked recessive** disorder caused by a mutation in the **ATP7A gene**. This mutation leads to defective copper transport and absorption, resulting in systemic copper deficiency. * **Clinical Triad:** The classic presentation includes **growth retardation**, **progressive neurological deterioration** (delayed milestones and seizures), and characteristic **"kinky" or "steely" hair** (pili torti). * **Pathophysiology:** Copper is a vital cofactor for enzymes like *lysyl oxidase* (cross-linking of collagen/elastin) and *tyrosinase* (melanin synthesis). Deficiency leads to connective tissue defects, brittle hair, and depigmentation. **Why the other options are incorrect:** * **B. Trisomy 21 (Down Syndrome):** While it presents with growth retardation and delayed milestones, the characteristic features are almond-shaped eyes, flat nasal bridge, Simian crease, and hypotonia, rather than kinky hair. * **C. Lesch-Nyhan Syndrome:** This is an X-linked disorder of purine metabolism (HGPRT deficiency). It is characterized by **self-mutilation**, choreoathetosis, and hyperuricemia (orange sand in diapers), not hair abnormalities. * **D. Wilson Disease:** This involves a mutation in the **ATP7B gene**, leading to copper **overload** (toxicity). It typically presents in older children or adolescents with hepatic failure, Kayser-Fleischer (KF) rings, and basal ganglia symptoms. **NEET-PG High-Yield Pearls:** * **Gene involved:** ATP7A (Menkes - "A"bsorption defect) vs. ATP7B (Wilson - "B"iliary excretion defect). * **Microscopy of hair:** Shows **Pili torti** (hair twisted 180° on its axis). * **Biochemical markers:** Low serum copper and low serum ceruloplasmin. * **Radiology:** May show "wormian bones" in the skull and metaphyseal spurring.

Q: Which of the following is NOT a single gene disorder?

William's syndrome. **Explanation:** The distinction between single-gene (Mendelian) disorders and chromosomal microdeletion syndromes is a high-yield topic in NEET-PG Biochemistry and Genetics. **Why William’s Syndrome is the correct answer:** Unlike the other options, **William’s Syndrome** is not caused by a mutation in a single gene. It is a **microdeletion syndrome** involving the contiguous loss of approximately 26–28 genes on the long arm of **chromosome 7 (7q11.23)**. The most notable gene lost in this region is the *ELN* (elastin) gene, which accounts for the cardiovascular features, but the overall phenotype results from the loss of multiple neighboring genes. **Analysis of Incorrect Options (Single Gene Disorders):** * **Haemochromatosis:** An **Autosomal Recessive** disorder typically caused by a single mutation in the *HFE* gene (most commonly C282Y), leading to iron overload. * **Cystic Fibrosis:** An **Autosomal Recessive** disorder caused by mutations in the *CFTR* gene on chromosome 7. The most common mutation is the deletion of phenylalanine at position 508 (ΔF508). * **Huntington’s Disease:** An **Autosomal Dominant** trinucleotide repeat disorder (CAG repeats) in the *HTT* gene on chromosome 4. **Clinical Pearls for NEET-PG:** * **William’s Syndrome Triad:** "Elfin" facies, Intellectual disability with "cocktail party personality" (extreme friendliness), and **Supravalvular Aortic Stenosis (SVAS)**. * **Diagnosis:** The gold standard for microdeletion syndromes like William's is **FISH** (Fluorescence In Situ Hybridization) or Microarray, not standard karyotyping. * **Hypercalcemia:** Patients often present with idiopathic infantile hypercalcemia due to increased sensitivity to Vitamin D.

Question 1

Deficiency of which of the following enzymes leads to toxicity of 5-fluorouracil?

Accepted Answer

Dihydropyrimidine dehydrogenase

Answer

Purine nucleoside phosphorylase

Answer

PRPP synthase

Answer

Adenosine deaminase

Question 2

What is characteristically seen in Hartnup disorder?

Accepted Answer

Pellagra-like rash

Answer

Burned feet rash

Answer

Alopecia

Answer

Vitiligo

Question 3

All of the following are characteristic of I-cell disease except?

Accepted Answer

Buphthalmos

Answer

Coarse facial features

Answer

Corneal clouding

Answer

Failure to thrive

Question 4

A 19-year-old male presents for a routine physical examination for sports activities. His fasting blood glucose is 7.5 mM. C-peptide and insulin levels are near normal under fasting conditions. Postprandially, his blood glucose levels are only slightly elevated before stabilizing at fasting levels. He denies excessive thirst or urination but recalls his mother had gestational diabetes. What type of inheritance best characterizes this alteration in glucose homeostasis?

Accepted Answer

Autosomal dominant

Answer

Autosomal recessive

Answer

Sex linked

Answer

Mitochondrial

Question 5

In which of the following conditions is the formation of a calcium-containing kidney stone NOT likely?

Accepted Answer

Lesch-Nyhan syndrome

Answer

Parathyroid adenoma

Answer

Multiple myeloma

Answer

Sarcoidosis

Question 6

Which of the following is a Y-linked disease?

Accepted Answer

Leri-Weil dyschondrosteosis

Answer

Adreno leukodystrophy

Answer

Fabry disease

Answer

Diabetes insipidus

Question 7

Which of the following lysosomal storage disorders is NOT inherited as an autosomal recessive trait?

Accepted Answer

Fabry disease

Answer

Krabbe disease

Answer

Gaucher disease

Answer

Tay-Sachs disease

Question 8

A patient presents with growth retardation, delayed milestones, and kinky hair. What is the most likely diagnosis?

Accepted Answer

Menke's disease

Answer

Trisomy 21

Answer

Lesch-Nyhan syndrome

Answer

Wilson disease

Question 9

Which of the following is an exception to mitochondrial inheritance?

Accepted Answer

Colour blindness

Answer

Leigh disease

Answer

Mitochondrial encephalopathy

Answer

MELAS

Question 10

Which of the following is NOT a single gene disorder?

Accepted Answer

William's syndrome

Answer

Haemochromatosis

Answer

Cystic fibrosis

Answer

Huntington's disease

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

On this page

Practice by Chapter

Want unlimited practice?