Genetic Disorders and Biochemical Pathology Practice Questions

Q: A 54-year-old male with acute lymphocytic leukemia develops a blast crisis and is treated with intensive systemic chemotherapy. Following treatment, the patient will be at increased risk for the development of which of the following?

Uric acid kidney stones. **Explanation:** The correct answer is **D. Uric acid kidney stones.** This patient is experiencing **Tumor Lysis Syndrome (TLS)**, a high-yield oncological emergency in NEET-PG. When intensive chemotherapy is administered for high-grade malignancies like Acute Lymphocytic Leukemia (ALL) or Burkitt lymphoma, there is a massive, rapid breakdown of neoplastic cells. **Pathophysiology:** 1. **Nucleic Acid Breakdown:** As tumor cells lyse, their nuclei release large amounts of purines (adenine and guanine). 2. **Catabolism:** Purines are metabolized by **xanthine oxidase** into **uric acid**. 3. **Hyperuricemia:** The sudden surge in serum uric acid leads to its precipitation in the acidic environment of the renal collecting ducts. 4. **Urate Nephropathy:** This results in obstructive uropathy or the formation of radiolucent uric acid kidney stones. **Analysis of Incorrect Options:** * **A & B (Gallstones):** Bile pigment stones are associated with chronic hemolysis (e.g., Sickle Cell Anemia), while cholesterol stones are linked to obesity, female gender, and rapid weight loss. Chemotherapy-induced cell lysis specifically targets purine metabolism, not biliary components. * **C (Cystine stones):** These are caused by a congenital defect in the renal transport of COLA amino acids (Cystine, Ornithine, Lysine, Arginine), unrelated to malignancy or chemotherapy. **NEET-PG High-Yield Pearls:** * **Prevention of TLS:** Aggressive hydration and **Allopurinol** (xanthine oxidase inhibitor) or **Rasburicase** (recombinant urate oxidase that converts uric acid to soluble allantoin). * **Electrolyte Triad of TLS:** Hyperkalemia, Hyperphosphatemia, and **Hypocalcemia** (due to calcium-phosphate precipitation). * **Uric Acid Stones:** These are **radiolucent** on X-ray but visible on CT scans. They form best in **acidic urine**.

Q: All of the following are true about Wilson disease EXCEPT:

Raised ceruloplasmin level. **Explanation:** Wilson disease (Hepatolenticular degeneration) is an **autosomal recessive** disorder caused by a mutation in the **ATP7B gene** on chromosome 13. This gene encodes a copper-transporting ATPase responsible for incorporating copper into apoceruloplasmin and excreting excess copper into bile. **Why Option D is the Correct Answer (The Exception):** In Wilson disease, the defect in the ATP7B protein leads to a failure in the formation of holoceruloplasmin. Consequently, **serum ceruloplasmin levels are characteristically low** (<20 mg/dL), not raised. This occurs because apoceruloplasmin (the precursor) is unstable and rapidly degraded in the circulation. **Analysis of Other Options:** * **Option A:** It is indeed an **autosomal recessive** condition, making this a true statement. * **Option B:** **Kayser-Fleischer (KF) rings** are pathognomonic findings caused by copper deposition in the Descemet’s membrane of the cornea. They are present in 95% of patients with neurological symptoms. * **Option C:** While total serum copper is usually low (due to low ceruloplasmin), the **free (non-ceruloplasmin bound) serum copper is raised**, leading to tissue deposition in the liver, brain, and kidneys. (Note: In many clinical contexts, "raised serum copper" refers to this toxic free fraction). **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The most sensitive screening test is low serum ceruloplasmin; the gold standard is a **liver biopsy** (showing increased copper content). * **Urinary Findings:** 24-hour urinary copper excretion is **increased** (>100 μg/day). * **Treatment:** Drug of choice is **D-Penicillamine** (copper chelator). Alternatively, Trientine or Zinc (which inhibits intestinal copper absorption) can be used. * **Neuropsychiatric symptoms:** Often presents with tremors, dystonia, or parkinsonian features due to copper deposition in the **basal ganglia** (specifically the putamen).

Q: Menkes kinky hair disease is due to a deficiency of which element?

Copper. **Explanation:** **Menkes Kinky Hair Disease** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**. This gene encodes a P-type ATPase responsible for the absorption of dietary copper from the intestine and its transport into the bloodstream. In this condition, copper is trapped within intestinal mucosal cells, leading to severe systemic **copper deficiency**. Copper is a vital cofactor for several enzymes. Its deficiency results in: * **Lysyl oxidase failure:** Leads to defective collagen cross-linking (causing arterial tortuosity and skeletal issues). * **Tyrosinase failure:** Leads to hypopigmentation. * **Cytochrome c oxidase failure:** Leads to neurodegeneration and hypotonia. * **Characteristic finding:** "Steely" or "kinky" hair (pili torti) due to defective keratin disulfide bond formation. **Analysis of Incorrect Options:** * **B. Iodine:** Deficiency leads to Goiter and Hypothyroidism (Cretinism in infants). * **C. Iron:** Deficiency causes Microcytic Hypochromic Anemia, not structural hair defects. * **D. Selenium:** Deficiency is associated with **Keshan disease** (cardiomyopathy) and Kashin-Beck disease (osteoarthropathy). **High-Yield Pearls for NEET-PG:** 1. **ATP7A vs. ATP7B:** Remember **A**TP7**A** is for **A**bsorption (Menkes - deficiency), while **ATP7B** is for **B**iliary excretion (Wilson’s disease - toxicity). 2. **Diagnosis:** Low serum copper and low serum ceruloplasmin levels. 3. **Inheritance:** X-linked Recessive (primarily affects males). 4. **Hair Microscopy:** Shows *Pili torti* (twisted hair).

Q: A 30-year-old female presents with bluish-black discoloration of the sclera and pinna for the last 4 months. Her urine turns black on standing. Which of the following tests is not used for the diagnosis of this condition?

Guthrie test. ### Explanation The clinical presentation of **bluish-black discoloration of the sclera (ochronosis)** and **urine turning black on standing** is characteristic of **Alkaptonuria**. This autosomal recessive disorder is caused by a deficiency of the enzyme **homogentisate oxidase**, leading to the accumulation of homogentisic acid (HGA). **Why Option D is the Correct Answer:** The **Guthrie test** is a semi-quantitative bacterial inhibition assay used for the neonatal screening of **Phenylketonuria (PKU)**, not Alkaptonuria. It detects elevated levels of phenylalanine in the blood using *Bacillus subtilis*. **Analysis of Other Options (Diagnostic Tests for Alkaptonuria):** * **Benedict’s Test (A):** Homogentisic acid is a strong reducing agent. It gives a positive result (yellow/orange precipitate) and a characteristic dark supernatant. * **Ferric Chloride Test (B):** When added to urine containing HGA, it produces a **transient deep blue/green color**. * **Silver Nitrate Test (C):** HGA reduces silver nitrate to metallic silver in an alkaline medium, causing the solution to turn black (Ammoniacal Silver Nitrate test). ### High-Yield Clinical Pearls for NEET-PG: * **Enzyme Defect:** Homogentisate 1,2-dioxygenase (HGO). * **Pathophysiology:** Accumulation of HGA leads to the formation of **alkapton bodies** (polymers) that deposit in connective tissues (Ochronosis). * **Clinical Triad:** Alkaptonuria (black urine), Ochronosis (pigmentation), and Arthritis (large joint involvement). * **Gold Standard Diagnosis:** Quantitative measurement of HGA in urine via **Gas Chromatography-Mass Spectrometry (GC-MS)**. * **Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase).

Q: What is the most likely diagnosis if a urine sample collected for routine testing darkens on standing?

Alkaptonuria. **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 tyrosine and phenylalanine. Its absence leads to the accumulation of **Homogentisic Acid (HGA)**. When urine containing high levels of HGA is left standing or is exposed to an alkaline environment, the HGA undergoes oxidation and polymerization to form a melanin-like pigment, causing the urine to turn **brownish-black**. **Analysis of Incorrect Options:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. Urine typically has a characteristic **"mousy" or "musty" odor** due to phenylacetate, but it does not darken on standing. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. The urine has a distinct **burnt sugar or maple syrup smell**, not a color change upon standing. * **Tyrosinemia:** Characterized by a **"boiled cabbage-like" or "rancid" odor** (especially in Type I) due to the accumulation of methionine metabolites and succinylacetone. **High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The deposition of black pigment in connective tissues (cartilage, sclera, ears), often leading to severe arthritis in later life. * **Diagnostic Test:** Addition of Benedict’s reagent to urine gives a **strongly positive (yellow-orange) result** for reducing sugars, but the supernatant turns black. Ferric chloride test also gives a transient deep blue/green color. * **Dietary Management:** Low protein diet (restriction of Phenylalanine and Tyrosine) and high doses of Vitamin C (to prevent HGA oxidation). **Nitisinone** is a newer drug used to inhibit HGA production.

Q: Gyrate atrophy of the retina is caused by the accumulation of which substance?

Ornithine. **Explanation:** **Gyrate atrophy of the choroid and retina** is a rare, autosomal recessive metabolic disorder caused by a deficiency of the mitochondrial enzyme **Ornithine Aminotransferase (OAT)**. 1. **Why Ornithine is correct:** The OAT enzyme normally catalyzes the conversion of **ornithine** into glutamate-gamma-semialdehyde (a precursor for proline and glutamate). When this enzyme is deficient, ornithine cannot be metabolized, leading to severe **hyperornithinemia** (10–20 times normal levels). This accumulation is toxic to the retinal pigment epithelium and the choroid, leading to progressive vision loss and characteristic circular (gyrate) chorioretinal lesions. 2. **Why other options are incorrect:** * **Arginine:** While ornithine is derived from arginine via the urea cycle (arginase enzyme), arginine itself does not accumulate to toxic levels in this specific condition. * **Citrulline:** Accumulation of citrulline is seen in **Citrullinemia Type I** (deficiency of Argininosuccinate synthase), which presents with hyperammonemia and neurological distress, not retinal atrophy. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Clinical Triad:** Progressive night blindness, constricted visual fields, and "gyrate" (scalloped) chorioretinal atrophy. * **Treatment:** A subset of patients responds to **Vitamin B6 (Pyridoxine)**, which is a cofactor for the OAT enzyme. Arginine-restricted diets are also used to lower ornithine levels. * **Biochemical Link:** It is one of the few urea cycle-related disorders that does **not** typically present with hyperammonemia.

Question 1

A 54-year-old male with acute lymphocytic leukemia develops a blast crisis and is treated with intensive systemic chemotherapy. Following treatment, the patient will be at increased risk for the development of which of the following?

Accepted Answer

Uric acid kidney stones

Answer

Bile pigment gallstones

Answer

Cholesterol gallstones

Answer

Cystine kidney stones

Question 2

All of the following are true about Wilson disease EXCEPT:

Accepted Answer

Raised ceruloplasmin level

Answer

Autosomal recessive inheritance

Answer

Kayser-Fleischer rings

Answer

Raised serum copper level

Question 3

Arthritis occurs in which of the following conditions?

Accepted Answer

Alkaptonuria

Answer

Cystinosis

Answer

Maple syrup urine disease

Answer

Homocystinuria

Question 4

All of the following are true about Sickle cell disease, EXCEPT:

Accepted Answer

A 'sticky patch' is generated due to the replacement of a nonpolar residue with a polar residue.

Answer

A single nucleotide change results in the substitution of Glutamine with Valine.

Answer

Restriction Fragment Length Polymorphism (RFLP) is a result of a single base change.

Answer

Hemoglobin S (HbS) confers resistance against malaria in heterozygotes.

Question 5

Menkes kinky hair disease is due to a deficiency of which element?

Accepted Answer

Copper

Answer

Iodine

Answer

Iron

Answer

Selenium

Question 6

Which of the following is NOT a mitochondrial disorder?

Accepted Answer

Fragile X Syndrome

Answer

Pearson Syndrome

Answer

MERRF (Myoclonic Epilepsy Ragged Red Fibre)

Answer

MELAS (Mitochondrial Encephalopathy Lactic Acidosis and Stroke-like episode)

Question 7

A 30-year-old female presents with bluish-black discoloration of the sclera and pinna for the last 4 months. Her urine turns black on standing. Which of the following tests is not used for the diagnosis of this condition?

Accepted Answer

Guthrie test

Answer

Benedict's test

Answer

Ferric chloride test

Answer

Silver nitrate test

Question 8

What is the most likely diagnosis if a urine sample collected for routine testing darkens on standing?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Maple syrup disease

Answer

Tyrosinemia

Question 9

A young adult presents with progressive intellectual deterioration, weakness, ataxia, and seizures. Laboratory tests demonstrate an abnormality of an important mitochondrial enzyme. Which of the following conditions is this person most likely suffering from?

Accepted Answer

Leigh's disease

Answer

Adrenoleukodystrophy

Answer

Central pontine myelinolysis

Answer

Krabbe's disease

Question 10

Gyrate atrophy of the retina is caused by the accumulation of which substance?

Accepted Answer

Ornithine

Answer

Arginine

Answer

Citrulline

Answer

None of the above

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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