Forensic Toxicology — MCQs

Forensic Toxicology Indian Medical PG Practice Questions and MCQs

Question 671: Carbon monoxide poisoning leads to which type of hypoxia?

A. Histotoxic hypoxia
B. Stagnant hypoxia
C. Hypoxic hypoxia
D. Anemic hypoxia (Correct Answer)

Explanation: **Explanation:** **1. Why Anemic Hypoxia is Correct:** Anemic hypoxia occurs when the blood's oxygen-carrying capacity is reduced, even though the arterial partial pressure of oxygen ($PaO_2$) remains normal. Carbon Monoxide (CO) has an affinity for hemoglobin that is **200–250 times greater** than that of oxygen. When CO binds to hemoglobin, it forms **Carboxyhemoglobin (COHb)**, which effectively "locks" the hemoglobin, preventing it from carrying oxygen. Furthermore, CO causes a **leftward shift of the Oxygen-Dissociation Curve**, meaning the remaining oxygen binds more tightly to hemoglobin and is not released to the tissues. Since the functional hemoglobin available for $O_2$ transport is decreased, it is classified as anemic hypoxia. **2. Why Other Options are Incorrect:** * **Histotoxic Hypoxia:** Occurs when tissues cannot utilize oxygen despite adequate delivery (e.g., **Cyanide poisoning**, which inhibits cytochrome oxidase). * **Stagnant (Ischemic) Hypoxia:** Occurs due to reduced blood flow or velocity (e.g., Heart failure, shock, or local embolism). * **Hypoxic Hypoxia:** Occurs when there is insufficient oxygen reaching the blood, leading to low $PaO_2$ (e.g., High altitude, drowning, or COPD). **3. Clinical Pearls for NEET-PG:** * **Cherry Red Discoloration:** A classic finding in CO poisoning seen in post-mortem staining, viscera, and blood. * **CT/MRI Finding:** Bilateral necrosis of the **Globus Pallidus** is a highly specific radiological sign. * **Treatment:** 100% Hyperbaric Oxygen (HBO) to reduce the half-life of COHb. * **Diagnosis:** Pulse oximetry is unreliable as it cannot distinguish between $O_2Hb$ and $COHb$; co-oximetry is required.

Question 672: Stomach wash is contraindicated in all except?

A. Carbolic acid (Correct Answer)
B. Hydrochloric acid
C. Sulphuric acid
D. Nitric acid

Explanation: **Explanation:** The core principle behind contraindicating gastric lavage (stomach wash) in corrosive poisoning is the risk of **esophageal perforation**. In most corrosive poisonings, the chemical causes liquefactive or coagulative necrosis, weakening the esophageal wall. Inserting a tube can lead to mechanical rupture. **1. Why Carbolic Acid (Option A) is the Correct Answer:** Carbolic acid (Phenol) is a unique corrosive. Unlike mineral acids, it has a **local anesthetic effect** on the gastric mucosa and causes "mummification" (fixation) of the tissues rather than deep, friable necrosis. Because it is rapidly absorbed and causes systemic toxicity (CNS depression, renal failure), removing the poison is critical. In carbolic acid poisoning, stomach wash is **not contraindicated**; in fact, it is recommended using warm water or olive oil to prevent systemic absorption. **2. Why the Other Options are Incorrect:** * **Hydrochloric acid (B), Sulphuric acid (C), and Nitric acid (D):** These are strong mineral acids. They cause intense coagulative necrosis, making the esophageal and gastric walls extremely soft and prone to perforation (the "softening" effect). Gastric lavage is strictly contraindicated in these cases to avoid mediastinitis or peritonitis. **High-Yield Clinical Pearls for NEET-PG:** * **Contraindications for Gastric Lavage:** Corrosive poisoning (except Carbolic acid), Comatose patients (unless intubated), Convulsant poisoning (may trigger seizures), and Volatile hydrocarbons (risk of aspiration pneumonia). * **Carbolic Acid Sign:** "Ochronosis" (pigmentation of cartilage) and "Carboluria" (urine turns green/black on standing). * **Antidote for Phenol:** There is no specific systemic antidote; treatment is symptomatic, but gastric lavage with olive oil/castor oil is a key decontamination step. * **Ewald’s Tube:** The wide-bore tube typically used for gastric lavage.

Question 673: Methanol toxicity causes blindness due to the formation of?

A. Formic acid (Correct Answer)
B. Uric acid
C. Lactic acid
D. Pyruvic acid

Explanation: **Explanation:** Methanol (methyl alcohol) toxicity is a high-yield topic in forensic toxicology. The toxicity is not caused by methanol itself, but by its metabolic products. **Why Formic Acid is correct:** Methanol is metabolized in the liver by the enzyme **Alcohol Dehydrogenase** into Formaldehyde, which is then rapidly converted by **Aldehyde Dehydrogenase** into **Formic acid (Formate)**. 1. **Metabolic Acidosis:** Formic acid inhibits mitochondrial cytochrome c oxidase, leading to cellular hypoxia and a profound high anion gap metabolic acidosis. 2. **Ocular Toxicity:** Formic acid has a specific predilection for the optic nerve and retina. It causes optic disc edema, retinal ganglion cell damage, and demyelination, leading to the characteristic "snowstorm vision" and permanent blindness. **Why other options are incorrect:** * **B. Uric acid:** This is the end product of purine metabolism. While elevated in gout, it plays no role in methanol-induced ocular damage. * **C. Lactic acid:** While lactic acidosis can occur secondary to tissue hypoxia in methanol poisoning, it is a non-specific byproduct and not the primary toxin responsible for optic nerve atrophy. * **D. Pyruvic acid:** This is an intermediate in glycolysis. It does not cause toxicity in this context. **High-Yield Clinical Pearls for NEET-PG:** * **Antidote:** **Fomepizole** (inhibits alcohol dehydrogenase) is the drug of choice. Alternatively, **Ethanol** is used as it has a higher affinity for alcohol dehydrogenase. * **Adjunctive Therapy:** **Folic acid** (Leucovorin) helps accelerate the breakdown of formic acid into $CO_2$ and $H_2O$. * **Putamen Necrosis:** On MRI, bilateral symmetrical necrosis of the **putamen** is a pathognomonic finding of methanol poisoning. * **Fatal Dose:** Approximately 30–100 ml; **Fatal Period:** 10–36 hours.

Question 674: Krait poisoning is primarily characterized by which type of toxicity?

A. Vasculotoxic
B. Neurotoxic (Correct Answer)
C. Cardiotoxic
D. Haemotoxic

Explanation: **Explanation:** **Krait (Common Krait - *Bungarus caeruleus*)** venom is primarily **neurotoxic**. It contains potent **pre-synaptic neurotoxins** (like $\beta$-bungarotoxin) that prevent the release of acetylcholine from nerve endings at the neuromuscular junction. This leads to progressive muscular paralysis, typically starting with cranial nerves (ptosis, diplopia) and potentially leading to fatal respiratory failure. A unique clinical feature of Krait bites is that they are often painless and occur at night, sometimes presenting as "silent" abdominal pain. **Analysis of Incorrect Options:** * **Vasculotoxic/Haemotoxic (Options A & D):** These terms are often used interchangeably in toxicology. This type of toxicity is characteristic of **Vipers** (e.g., Russell’s Viper, Saw-scaled Viper). Viper venom contains enzymes like phospholipase $A_2$ and proteases that cause local tissue destruction, coagulation abnormalities (VICC), and hemorrhage. * **Cardiotoxic (Option C):** While some cobra venoms have cardiotoxic components that can affect heart rhythm, it is not the primary classification for Krait poisoning. **High-Yield Clinical Pearls for NEET-PG:** * **Krait vs. Cobra:** Both are Elapids (Neurotoxic). However, Krait venom is **pre-synaptic** (irreversible/poor response to Neostigmine), while Cobra venom is **post-synaptic** (reversible/good response to Neostigmine). * **The "Early Morning Neuroparalysis":** A classic Krait presentation where a patient wakes up with paralysis or respiratory distress after a painless bite during sleep. * **Management:** High doses of Polyvalent Anti-Snake Venom (ASV) and mechanical ventilation are the mainstays of treatment.

Question 675: Ball is used as an antidote for poisoning by which substance?

A. Morphine
B. Aconite
C. Phenol
D. Mercury (Correct Answer)

Explanation: **Explanation:** **BAL (British Anti-Lewisite)**, also known as **Dimercaprol**, is a chelating agent used as an antidote for heavy metal poisoning. It contains two sulfhydryl (-SH) groups that compete with the sulfhydryl groups in human tissue enzymes for binding with the metal. By forming a stable, non-toxic, heterocyclic ring complex with the metal, it allows the toxin to be excreted safely in the urine. It is the treatment of choice for acute poisoning by **Mercury**, Arsenic, Gold, and Antimony. **Analysis of Options:** * **Mercury (Correct):** BAL is highly effective for inorganic mercury salts. However, it is contraindicated in chronic poisoning by elemental or organic mercury (like methylmercury) as it may redistribute the metal to the brain. * **Morphine (Incorrect):** The specific antidote for opioid/morphine toxicity is **Naloxone** (a pure opioid antagonist). * **Aconite (Incorrect):** There is no specific antidote for Aconite (the "Queen of Poisons"). Treatment is symptomatic, focusing on managing cardiac arrhythmias with drugs like Amiodarone or Flecainide. * **Phenol (Incorrect):** Phenol is a corrosive. Management involves gastric lavage with olive oil or water and supportive care; there is no specific chemical antidote. **Clinical Pearls for NEET-PG:** * **Route:** BAL must be administered via **deep Intramuscular (IM)** injection because it is dispensed in peanut oil. * **Contraindication:** Avoid in patients with **G6PD deficiency** (causes hemolysis) and **peanut allergies**. * **Other Chelators:** For Lead poisoning, BAL is often used in combination with EDTA. For chronic mercury or lead poisoning, oral **Succimer (DMSA)** is preferred.

Question 676: Saturnine encephalopathy is caused by intoxication of:

A. Arsenic
B. Antimony
C. Lead (Correct Answer)
D. Mercury

Explanation: **Explanation:** **Saturnine encephalopathy** is a severe neurological manifestation of chronic **Lead (Plumbism)** poisoning. The term "Saturnine" originates from alchemy, where the planet Saturn was the symbol for lead. 1. **Why Lead is Correct:** Chronic lead poisoning affects multiple systems. In the CNS, lead disrupts the blood-brain barrier and causes cerebral edema, leading to encephalopathy. This is more common in children and presents with irritability, ataxia, convulsions, and coma. Other classic signs of lead poisoning include **Burtonian lines** (blue-purple line on gums), **Basophilic stippling** of RBCs, and **Wrist drop/Foot drop** due to peripheral neuropathy. 2. **Why Other Options are Incorrect:** * **Arsenic:** Acute poisoning causes "rice-water stools," while chronic exposure leads to **Raindrop pigmentation**, hyperkeratosis, and Mees' lines on nails. It does not cause Saturnine encephalopathy. * **Antimony:** Its toxicity mimics arsenic (gastrointestinal distress) but is generally less potent. It is not associated with specific encephalopathic syndromes. * **Mercury:** Chronic mercury poisoning (Hydrargyrism) is characterized by **Erethism** (pathological shyness), **Mercurial Lentis** (brownish discoloration of the lens), and **Danbury tremors** (Hatter's shakes). **High-Yield Clinical Pearls for NEET-PG:** * **Treatment of Lead Poisoning:** The drug of choice for Saturnine encephalopathy is **BAL (British Anti-Lewisite)** followed by **EDTA**. For asymptomatic children with high levels, **Succimer (DMSA)** is preferred. * **Radiology:** Look for "Lead lines" (increased metaphyseal density) in the long bones of children. * **Screening:** Blood lead levels are the gold standard; however, **Erythrocyte Protoporphyrin (EP)** levels are used for screening.

Question 677: Anaemia, punctate basophilia, constipation, blue line, and abdominal colic are characteristic of which condition?

A. Opium addiction
B. Arsenic poisoning
C. Mercuric poisoning
D. Lead poisoning (Correct Answer)

Explanation: **Explanation:** The clinical presentation described—**anaemia, punctate basophilia, constipation, blue line, and abdominal colic**—is the classic pentad of **Chronic Lead Poisoning (Plumbism)**. 1. **Why Lead Poisoning is correct:** * **Anaemia & Punctate Basophilia:** Lead inhibits enzymes like *ALAD* and *Ferrochelatase*, disrupting heme synthesis. This leads to microcytic hypochromic anaemia. **Punctate basophilia (basophilic stippling)** occurs due to the inhibition of the enzyme *pyrimidine 5'-nucleotidase*, causing the aggregation of ribosomes in RBCs. * **Abdominal Colic & Constipation:** Known as "Lead Colic," this is caused by the spasmodic contraction of intestinal muscle. * **Blue Line (Burtonian Line):** A characteristic bluish-black line on the gums at the gingival margin, caused by the deposition of lead sulphide. 2. **Why other options are incorrect:** * **Opium addiction:** Characterized by miosis (pinpoint pupils), drowsiness, and respiratory depression, not hematological changes. * **Arsenic poisoning:** Presents with "rice water stools" (acute) or hyperpigmentation ("raindrop pigmentation") and hyperkeratosis of palms/soles (chronic). * **Mercuric poisoning:** Features include tremors (Danbury tremors), erethism (personality changes), and acrodynia (Pink disease). **High-Yield Clinical Pearls for NEET-PG:** * **Wrist drop/Foot drop:** Due to radial/peroneal nerve palsy (extensor muscle weakness). * **Facial Pallor:** The earliest sign of lead poisoning (circumoral pallor). * **Radiology:** "Lead lines" (increased density) at the metaphyses of long bones in children. * **Treatment:** **Succimer (DMSA)** is the oral chelator of choice; **Ca-EDTA** or **BAL** are used for severe cases/encephalopathy.

Question 678: Methanol causes eye damage by converting into which substance?

A. Formic acid (Correct Answer)
B. Acetaldehyde
C. Pyridine
D. Acetic acid

Explanation: ### Explanation **Methanol Metabolism and Toxicity** Methanol (methyl alcohol) is metabolized in the liver via a two-step oxidation process. It is first converted into **formaldehyde** by the enzyme *alcohol dehydrogenase*. Formaldehyde is then rapidly converted into **formic acid** (formate) by *aldehyde dehydrogenase*. **Why Formic Acid is the Correct Answer:** Formic acid is the primary toxic metabolite responsible for the clinical manifestations of methanol poisoning. It inhibits mitochondrial cytochrome c oxidase, leading to cellular hypoxia. In the eye, formic acid specifically targets the **optic nerve and retinal ganglion cells**, causing optic disc edema, retinal demyelination, and permanent blindness (often described as "feeling like being in a snowstorm"). It also causes profound metabolic acidosis with an elevated anion gap. **Analysis of Incorrect Options:** * **B. Acetaldehyde:** This is the intermediate metabolite of **Ethanol** (ethyl alcohol), not methanol. It is responsible for the "hangover" symptoms and the Disulfiram-like reaction. * **C. Pyridine:** This is a basic heterocyclic organic compound used as a denaturant for alcohol to make it unpalatable; it is not a metabolite of methanol. * **D. Acetic Acid:** This is the final metabolite of **Ethanol** (formed from acetaldehyde). **High-Yield Clinical Pearls for NEET-PG:** * **Antidote:** **Fomepizole** (inhibits alcohol dehydrogenase) is the preferred antidote. **Ethanol** is used as an alternative because it has a higher affinity for alcohol dehydrogenase than methanol. * **Treatment:** Sodium bicarbonate is used to treat acidosis, and **Folic acid** (Leucovorin) is given to enhance the degradation of formic acid into $CO_2$ and $H_2O$. * **Putamen Necrosis:** On MRI, bilateral necrosis of the **putamen** is a characteristic finding in severe methanol poisoning.

Question 679: What is the fatal dose of arsenic?

A. 10 mg
B. 100 grams
C. 1 gram (Correct Answer)
D. 10 grams

Explanation: **Explanation:** Arsenic (specifically Arsenic Trioxide, also known as "Sankhiya") is a classic irritant poison frequently tested in NEET-PG. **Why 1 gram is the correct answer:** The fatal dose of Arsenic Trioxide for an adult is typically cited between **120 mg to 200 mg** (roughly 2 grains). In the context of the provided options, **1 gram** is the most clinically significant and accurate threshold that ensures fatality. While smaller doses (around 100-200 mg) can be lethal, 1 gram represents a massive, undoubtedly fatal dose in forensic toxicology. **Analysis of Incorrect Options:** * **A. 10 mg:** This is far below the lethal threshold. While chronic exposure to low doses leads to "Arsenicosis," it will not cause acute death. * **B. 100 grams:** This is an extreme quantity. Arsenic is highly potent; a few hundred milligrams are sufficient to cause death, making 100 grams an unrealistic figure for a "minimum fatal dose." * **D. 10 grams:** While certainly fatal, it is significantly higher than the established minimum lethal dose (120–200 mg). **High-Yield Clinical Pearls for NEET-PG:** * **Fatal Period:** Usually 12 to 48 hours. * **MCL (Maximum Contaminant Level):** In drinking water is **0.01 mg/L** (WHO). * **Clinical Presentation:** Acute poisoning mimics **Cholera** (rice-water stools, but distinguished by the presence of tenesmus and blood in arsenic poisoning). * **Chronic Poisoning Signs:** Raindrop pigmentation, hyperkeratosis of palms/soles, and **Mee’s lines** on nails. * **Antidote:** British Anti-Lewisite (BAL/Dimercaprol) is the drug of choice. * **Post-mortem:** Sub-endocardial hemorrhages (putty-like heart) are a characteristic finding.

Question 680: Which of the following is not a phase of organophosphorus poisoning?

A. Acute cholinergic phase
B. Intermediate syndrome
C. OPC induced delayed polyneuropathy
D. Late onset proximal myopathy (Correct Answer)

Explanation: Organophosphorus (OP) compounds inhibit the enzyme acetylcholinesterase (AChE), leading to an accumulation of acetylcholine at synapses. This toxicity typically manifests in three distinct, well-defined chronological phases. **Why "Late onset proximal myopathy" is the correct answer:** There is no clinical entity known as "late onset proximal myopathy" in the standard progression of OP poisoning. While the **Intermediate Syndrome** involves proximal muscle weakness, it is not "late onset" (occurring within 24–96 hours) and is a specific neuromuscular junction disorder rather than a primary myopathy. **Explanation of Incorrect Options (Phases of OP Poisoning):** * **A. Acute Cholinergic Phase:** Occurs within minutes to hours. It is characterized by **SLUDGE** (Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis) and **DUMBELS** (Diarrhea, Urination, Miosis, Bronchospasm, Emesis, Lacrimation, Salivation) due to overstimulation of muscarinic and nicotinic receptors. * **B. Intermediate Syndrome (Type II Paralysis):** Occurs 24–96 hours after the acute crisis. It involves muscle weakness of the proximal limbs, neck flexors, and respiratory muscles. It is thought to be due to post-synaptic neuromuscular junction dysfunction. * **C. OP Induced Delayed Polyneuropathy (OPIDN):** Occurs 2–3 weeks after exposure. It is caused by the inhibition of **Neuropathy Target Esterase (NTE)**, leading to "dying-back" axonal degeneration. It presents as distal "stocking and glove" paresthesia and motor weakness (foot drop/wrist drop). **High-Yield Clinical Pearls for NEET-PG:** * **Antidote of choice:** Atropine (reverses muscarinic effects; titrated until secretions dry). * **Pralidoxime (PAM):** A cholinesterase reactivator; most effective if given before "aging" of the enzyme occurs (usually within 48 hours). * **Diagnosis:** Best initial test is measuring **Pseudocholinesterase** (Butyrylcholinesterase) levels. * **Smell:** Characteristic **garlic-like** odor of breath/vomitus.

Practice by Chapter

General Principles of Toxicology

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Corrosive Poisons

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Metallic Poisons

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Non-Metallic Poisons

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Organic Irritant Poisons

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Neurotic Poisons

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Cardiac Poisons

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Asphyxiant Poisons

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Food Poisoning

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Drug Abuse and Dependence

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Analytical Toxicology Methods

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Interpretation of Toxicology Results

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