Poison & Antidote: Sodium thiosulphate* MCQs With Answer

Poison & Antidote: Sodium thiosulphate* MCQs With Answer

Sodium thiosulphate is a key antidote studied in B.Pharm pharmacology for cyanide poisoning and several off‑label indications. Acting as a sulfur donor to the mitochondrial enzyme rhodanese, it converts toxic cyanide into less toxic thiocyanate for renal excretion. B.Pharm students should know its mechanism of action, intravenous administration, role within combination antidote kits, pharmacokinetics, adverse effects and monitoring for thiocyanate toxicity. Understanding formulation, dosing considerations and interactions with nitrites and hydroxocobalamin is essential for safe clinical use. These focused MCQs will deepen conceptual and practical knowledge for exams and clinical practice. ‘Now let’s test your knowledge with 50 MCQs on this topic.’

Q1. What is the primary clinical indication for intravenous sodium thiosulphate as an antidote?

• Treatment of organophosphate poisoning
• Treatment of cyanide poisoning
• Treatment of methanol poisoning
• Treatment of opioid overdose

Correct Answer: Treatment of cyanide poisoning

Q2. What is the principal biochemical role of sodium thiosulphate in cyanide detoxification?

• Oxidizes cyanide to carbon dioxide
• Donates sulfur for conversion of cyanide to thiocyanate via rhodanese
• Directly binds cyanide to form an inactive salt
• Induces renal excretion of free cyanide unchanged

Correct Answer: Donates sulfur for conversion of cyanide to thiocyanate via rhodanese

Q3. Which enzyme mediates the conversion of cyanide to thiocyanate using sulfur donors like thiosulphate?

• Cytochrome P450
• Rhodanese (thiosulphate sulfurtransferase)
• Glutathione peroxidase
• Carboxylesterase

Correct Answer: Rhodanese (thiosulphate sulfurtransferase)

Q4. Sodium thiosulphate is often given in combination with which class of agents in older cyanide antidote kits?

• Nitrites (e.g., sodium nitrite)
• Beta blockers
• Calcium channel blockers
• Loop diuretics

Correct Answer: Nitrites (e.g., sodium nitrite)

Q5. Which of the following is an alternative cyanide antidote that can be used instead of nitrite-based kits?

• Naloxone
• Hydroxocobalamin (vitamin B12a)
• Acetylcysteine
• Flumazenil

Correct Answer: Hydroxocobalamin (vitamin B12a)

Q6. What is the primary route of administration for sodium thiosulphate in acute cyanide poisoning?

• Oral
• Intravenous
• Intramuscular
• Topical

Correct Answer: Intravenous

Q7. Which metabolite results from sodium thiosulphate–mediated detoxification of cyanide?

• Cyanate
• Thiopurine
• Thiocyanate
• Thioglycolate

Correct Answer: Thiocyanate

Q8. How is thiocyanate primarily eliminated from the body after conversion from cyanide?

• Renal excretion
• Exhalation via lungs
• Fecal elimination
• Metabolism by hepatic CYP enzymes

Correct Answer: Renal excretion

Q9. Which patient population requires careful monitoring for thiocyanate accumulation due to reduced elimination?

• Patients with renal impairment
• Patients with asthma
• Patients with hyperthyroidism
• Patients with anemia

Correct Answer: Patients with renal impairment

Q10. Chronic elevated thiocyanate levels can cause which of the following toxic effects?

• Cardiac arrhythmias exclusively
• CNS effects like confusion and hypothyroidism
• Severe hypoglycemia
• Acute pancreatitis

Correct Answer: CNS effects like confusion and hypothyroidism

Q11. Which chemical species does the thiosulfate anion contain?

• SO4^2−
• S2O3^2−
• HSO3−
• SO3^2−

Correct Answer: S2O3^2−

Q12. In addition to cyanide poisoning, sodium thiosulphate has an off‑label clinical use in which condition related to dialysis patients?

• Calciphylaxis (vascular calcification and skin necrosis)
• Uremic pruritus
• Renal cell carcinoma
• Acute tubular necrosis

Correct Answer: Calciphylaxis (vascular calcification and skin necrosis)

Q13. Which property of sodium thiosulphate makes it useful against certain chemotherapy toxicities (e.g., cisplatin)?

• It is a strong acid that denatures platinum complexes
• It acts as a nucleophilic sulfur donor that can inactivate reactive platinum species
• It chelates iron to reduce oxidative stress
• It induces hepatic enzymatic metabolism of cisplatin

Correct Answer: It acts as a nucleophilic sulfur donor that can inactivate reactive platinum species

Q14. Which statement about sodium thiosulphate pharmacodynamics is correct?

• It directly oxidizes cyanide to carbon dioxide
• It supplies sulfur for enzymatic conversion to less toxic thiocyanate
• It blocks cyanide absorption from the gut
• It forms a stable complex with hemoglobin to remove cyanide

Correct Answer: It supplies sulfur for enzymatic conversion to less toxic thiocyanate

Q15. A common adult dose used emergently for sodium thiosulphate in cyanide poisoning is often cited as:

• 500 mg IV bolus
• 12.5 g IV (25% solution)
• 50 g IV over 24 hours
• 100 mg orally twice daily

Correct Answer: 12.5 g IV (25% solution)

Q16. What is the pediatric dosing approach commonly recommended for sodium thiosulphate in cyanide poisoning?

• Fixed adult dose for all ages
• 250 mg/kg IV up to an adult maximum
• 10 mg/kg orally every 8 hours
• Not recommended in children

Correct Answer: 250 mg/kg IV up to an adult maximum

Q17. Which lab parameter is particularly important to monitor during and after treatment with sodium thiosulphate in cyanide poisoning?

• Plasma potassium only
• Serum thiocyanate levels and renal function
• Fasting blood glucose
• Serum ammonia

Correct Answer: Serum thiocyanate levels and renal function

Q18. Which of the following is NOT a recommended property of an effective antidote in acute cyanide poisoning?

• Rapid onset of action
• Ability to remove cyanide irreversibly
• Severe, long‑lasting immunosuppression as primary effect
• Favorable safety profile for emergency use

Correct Answer: Severe, long‑lasting immunosuppression as primary effect

Q19. Sodium thiosulphate’s action is best described as which type of detoxification process?

• Oxidative metabolism
• Conjugation by sulfur transfer to form less toxic metabolite
• Phase I CYP‑mediated hydroxylation
• Active transport into bile

Correct Answer: Conjugation by sulfur transfer to form less toxic metabolite

Q20. Which of the following adverse effects may occur with rapid intravenous administration of sodium thiosulphate?

• Transient hypotension and nausea
• Profound bradycardia only after oral dosing
• Immediate pulmonary embolism
• Acute pancreatitis within minutes

Correct Answer: Transient hypotension and nausea

Q21. In cyanide poisoning, why might sodium thiosulphate be given after administration of nitrites?

• Nitrites produce methemoglobin which binds cyanide; thiosulphate then enhances conversion to thiocyanate
• Nitrites deactivate thiosulphate unless given sequentially
• Thiosulphate must be given first to allow nitrites to work
• They should never be used together

Correct Answer: Nitrites produce methemoglobin which binds cyanide; thiosulphate then enhances conversion to thiocyanate

Q22. Which of the following is an important advantage of hydroxocobalamin over nitrite/thiosulphate kits?

• Hydroxocobalamin causes significant hypotension
• It does not induce methemoglobinemia and has fewer cardiovascular effects
• It is less effective at binding cyanide
• It requires oral administration

Correct Answer: It does not induce methemoglobinemia and has fewer cardiovascular effects

Q23. Which laboratory finding is commonly associated with acute cyanide poisoning that sodium thiosulphate helps to reverse indirectly?

• Severe metabolic acidosis with high lactate levels
• Marked hypoglycemia with low lactate
• Hyperkalemia with metabolic alkalosis
• Isolated high serum creatinine

Correct Answer: Severe metabolic acidosis with high lactate levels

Q24. Which statement about sodium thiosulphate stability and storage is most appropriate for pharmacists?

• 25% solutions are typically used and should be stored per manufacturer guidance to avoid microbial contamination
• It is stable indefinitely at room temperature once opened
• It must be frozen prior to use
• It is highly volatile and stored under nitrogen

Correct Answer: 25% solutions are typically used and should be stored per manufacturer guidance to avoid microbial contamination

Q25. Which contraindication is most relevant when considering sodium thiosulphate use?

• Known hypersensitivity to thiosulphate compounds
• History of seasonal allergies only
• Well-controlled hypertension
• Uncomplicated pregnancy with no exposures

Correct Answer: Known hypersensitivity to thiosulphate compounds

Q26. Which monitoring parameter indicates possible thiocyanate toxicity during prolonged therapy?

• Serum thiocyanate concentration
• Serum bilirubin only
• White blood cell count
• Serum magnesium

Correct Answer: Serum thiocyanate concentration

Q27. Sodium thiosulphate’s mechanism does NOT include which of the following actions?

• Serving as a sulfur donor
• Direct chelation of lead ions
• Enhancing enzymatic conversion of cyanide to thiocyanate
• Reducing cyanide’s inhibition of cytochrome c oxidase indirectly

Correct Answer: Direct chelation of lead ions

Q28. In which industrial or exposure scenario is sodium thiosulphate most likely to be indicated?

• Carbon monoxide exposure from faulty heaters
• Cyanide exposure from burning plastics or metal processing
• Benzodiazepine overdose
• Chronic lead exposure from paints

Correct Answer: Cyanide exposure from burning plastics or metal processing

Q29. Which chemical property of thiosulphate contributes to its reactivity with cyanide in the presence of rhodanese?

• Its oxidizing potential
• Its ability to donate a sulfur atom (sulfur transfer)
• Its strong acid dissociation
• Its hydrophobicity

Correct Answer: Its ability to donate a sulfur atom (sulfur transfer)

Q30. When treating cyanide poisoning, which sequence is often recommended in older protocols?

• Administer thiosulphate first, then nitrite
• Administer nitrite to induce methemoglobin, then thiosulphate
• Only give thiosulphate orally
• Give dopamine before any antidote

Correct Answer: Administer nitrite to induce methemoglobin, then thiosulphate

Q31. Which of the following best describes thiocyanate’s toxicity profile in high concentrations?

• Primarily hepatic necrosis
• Neurological symptoms, confusion and possible hypothyroidism
• Immediate anaphylaxis in all patients
• Profound coagulopathy

Correct Answer: Neurological symptoms, confusion and possible hypothyroidism

Q32. For a pharmacist, which counselling point is most relevant for emergency use of sodium thiosulphate?

• It can be self-administered at home for mild poisoning
• It is for intravenous use in medical settings and requires monitoring of renal function
• It should be mixed with acidic beverages for oral absorption
• No monitoring is required after administration

Correct Answer: It is for intravenous use in medical settings and requires monitoring of renal function

Q33. Which formulation concentration is commonly available for sodium thiosulphate used as an antidote?

• 0.9% solution only
• 25% solution (commonly cited for emergency use)
• 100% pure crystalline injectable
• 0.01% solution for nebulization

Correct Answer: 25% solution (commonly cited for emergency use)

Q34. Which statement about sodium thiosulphate and methemoglobinemia is true?

• Sodium thiosulphate directly produces methemoglobin
• Sodium nitrite, not thiosulphate, is the agent that induces methemoglobinemia in antidote kits
• Both nitrite and thiosulphate equally cause methemoglobinemia
• Neither nitrite nor thiosulphate affect hemoglobin

Correct Answer: Sodium nitrite, not thiosulphate, is the agent that induces methemoglobinemia in antidote kits

Q35. In the context of B.Pharm practice, why is understanding sodium thiosulphate’s interaction with renal function important?

• Because thiocyanate is renally excreted and accumulates in renal impairment
• Because it is eliminated only via hepatic metabolism
• Because it causes irreversible renal failure in all patients
• Because renal impairment enhances its antidotal potency

Correct Answer: Because thiocyanate is renally excreted and accumulates in renal impairment

Q36. Which monitoring is crucial when sodium thiosulphate is used for prolonged off‑label treatment of calciphylaxis?

• Frequent audiometry
• Renal function and serum electrolytes
• Serum amylase levels
• Ophthalmic pressure

Correct Answer: Renal function and serum electrolytes

Q37. What effect does sodium thiosulphate have directly on cytochrome c oxidase inhibited by cyanide?

• Directly reactivates the inhibited enzyme
• Provides sulfur that allows enzymatic detoxification of cyanide, indirectly restoring enzyme function
• Permanently degrades cytochrome c oxidase
• Blocks oxygen binding to cytochrome c oxidase

Correct Answer: Provides sulfur that allows enzymatic detoxification of cyanide, indirectly restoring enzyme function

Q38. Which clinical sign would most strongly suggest cyanide poisoning where sodium thiosulphate might be lifesaving?

• Brisk red venous blood with severe lactic acidosis and hypotension
• Gradual weight loss over months
• Mild headache that resolves
• Localized skin rash only

Correct Answer: Brisk red venous blood with severe lactic acidosis and hypotension

Q39. Which of the following best describes a limitation of sodium thiosulphate when used alone in severe cyanide poisoning?

• Its onset may be slower than agents that directly bind cyanide (e.g., hydroxocobalamin)
• It is always faster than hydroxocobalamin
• It cannot be given intravenously
• It causes irreversible methemoglobinemia

Correct Answer: Its onset may be slower than agents that directly bind cyanide (e.g., hydroxocobalamin)

Q40. In toxicology, which concept explains why thiosulphate requires the enzyme rhodanese to detoxify cyanide effectively?

• Non‑enzymatic spontaneous oxidation
• Enzyme‑catalyzed sulfur transfer to cyanide for thiocyanate formation
• Active transport into mitochondria
• Direct covalent binding without enzymatic assistance

Correct Answer: Enzyme‑catalyzed sulfur transfer to cyanide for thiocyanate formation

Q41. Which of the following best characterizes thiocyanate’s effect on the thyroid at high levels?

• Stimulates increased thyroid hormone synthesis
• May interfere with iodine uptake causing hypothyroid features
• Has no effect on thyroid function
• Causes hyperthyroidism exclusively

Correct Answer: May interfere with iodine uptake causing hypothyroid features

Q42. From a pharmaceutical standpoint, sodium thiosulphate is classified chemically as:

• An organophosphate
• An inorganic sulfur salt
• A tertiary amine
• A peptide antibiotic

Correct Answer: An inorganic sulfur salt

Q43. In cases where hydroxocobalamin is unavailable, what is a reasonable antidotal approach for suspected severe cyanide poisoning?

• Supportive care only with no antidote
• Use nitrite plus sodium thiosulphate as per kit protocols if indicated
• Administer naloxone empirically
• Give activated charcoal intravenously

Correct Answer: Use nitrite plus sodium thiosulphate as per kit protocols if indicated

Q44. Which pharmacokinetic characteristic is most relevant for dosing adjustments of sodium thiosulphate in renal failure?

• Hepatic clearance is the major pathway
• Renal excretion of thiocyanate requires dose consideration to avoid toxicity
• It is bound strongly to plasma proteins and unaffected by renal function
• It is metabolized to carbon dioxide and exhaled

Correct Answer: Renal excretion of thiocyanate requires dose consideration to avoid toxicity

Q45. Which adjunctive therapy addresses the metabolic consequences (lactic acidosis) of cyanide poisoning while antidotes work?

• High-flow oxygen and supportive hemodynamic care
• Immediate insulin infusion
• Enteral bicarbonate only
• Delayed fluid restriction

Correct Answer: High-flow oxygen and supportive hemodynamic care

Q46. Regarding drug interactions, sodium thiosulphate is least likely to interact with which drug class in acute use?

• Renally cleared drugs where thiocyanate competes for excretion
• Agents causing methemoglobinemia when given concurrently
• Long‑term lithium therapy affecting renal handling
• Topical corticosteroids applied to skin

Correct Answer: Topical corticosteroids applied to skin

Q47. For exam preparation, which practical laboratory skill related to sodium thiosulphate might B.Pharm students be expected to understand?

• How to compound and label intravenous solutions correctly following aseptic technique and concentration calculations
• How to prepare oral tablets from thiosulphate crystals for routine use
• How to synthesize thiosulphate chemically in the lab
• How to administer the antidote intramuscularly in community pharmacy

Correct Answer: How to compound and label intravenous solutions correctly following aseptic technique and concentration calculations

Q48. In toxicology lectures, why is it important to teach both mechanism and clinical logistics (e.g., dosing, monitoring) for antidotes like sodium thiosulphate?

• Because theory alone is sufficient for clinical care
• Because understanding mechanism helps select appropriate antidote and logistics ensure safe administration and monitoring
• Because logistics are more important than mechanism
• Because antidotes never require monitoring

Correct Answer: Because understanding mechanism helps select appropriate antidote and logistics ensure safe administration and monitoring

Q49. Which of the following is true regarding the time window for effective antidotal therapy in acute cyanide toxicity?

• Antidotal therapy is ineffective after 24 hours only
• Early administration of antidotes like thiosulphate improves outcomes; delays increase mortality
• Timing is irrelevant because cyanide effects are permanent
• Antidotes should only be given after 48 hours

Correct Answer: Early administration of antidotes like thiosulphate improves outcomes; delays increase mortality

Q50. Which study or clinical consideration would be most helpful for a B.Pharm student to evaluate when learning about sodium thiosulphate use in calciphylaxis?

• Randomized trials, case series, dosing regimens and renal safety data for off‑label use
• Only animal studies unrelated to human dosing
• Only in vitro chemical stability without clinical outcomes
• Advertising brochures from non‑regulated sources

Correct Answer: Randomized trials, case series, dosing regimens and renal safety data for off‑label use