Question:
A case of poisoning was brought to the casualty, a gastric lavage was done, and the lavage turned black when it was heated after being treated with silver nitrate. The poisoning is most likely to have been due to which of the following?
A case of poisoning was brought to the casualty, a gastric lavage was done, and the lavage turned black when it was heated after being treated with silver nitrate. The poisoning is most likely to have been due to which of the following?
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Silver nitrate turning black on heating with gastric lavage fluid is the classic test for phosphine from aluminium phosphide (Celfos) poisoning.
Updated On: Jul 8, 2026
- Tik-20
- Celfos
- Malathion
- Parathion
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Verified By Collegedunia
The Correct Option is B
Solution and Explanation
Step 1: Understanding the Question:
The question describes a bedside test on gastric lavage fluid, silver nitrate is added and the mixture turns black on heating. We need to identify which poison this test points to.
Step 2: Key Concept:
This blackening with silver nitrate on heating is a classic test for phosphine gas, which is released from metal phosphide compounds such as aluminium phosphide in the presence of moisture or stomach acid.
Step 3: Detailed Explanation:
Aluminium phosphide is a solid fumigant and rodenticide that, once it meets moisture or gastric hydrochloric acid, breaks down and releases phosphine gas.
Phosphine is a strong reducing agent, and when the lavage fluid containing it is treated with silver nitrate and heated, the phosphine reduces the silver ions to metallic silver, which deposits as a black precipitate, this is the basis of the silver nitrate paper or solution test used to detect phosphide poisoning.
Celfos is a well known trade name for aluminium phosphide used widely as a grain fumigant and rodenticide in India, and it is a common agent of both accidental and suicidal poisoning, which fits a case reaching the casualty as an emergency.
Step 4: Why the other options are wrong:
Tik-20 is also associated with phosphide type compounds in casual naming, but Celfos is the specific and most standard aluminium phosphide brand linked with this exact silver nitrate blackening test in toxicology teaching, so it is taken as the intended answer here.
Malathion is an organophosphate insecticide that works by inhibiting acetylcholinesterase, causing a cholinergic toxidrome with excess salivation, pinpoint pupils and muscle fasciculations, it does not release phosphine gas and will not blacken silver nitrate on heating.
Parathion is likewise an organophosphate compound with the same cholinergic mechanism as malathion, and it also does not produce phosphine or a positive silver nitrate blackening test.
Step 4: Final Answer:
The blackening of silver nitrate on heating after gastric lavage points to phosphine release, most likely due to Celfos, an aluminium phosphide compound.
The question describes a bedside test on gastric lavage fluid, silver nitrate is added and the mixture turns black on heating. We need to identify which poison this test points to.
Step 2: Key Concept:
This blackening with silver nitrate on heating is a classic test for phosphine gas, which is released from metal phosphide compounds such as aluminium phosphide in the presence of moisture or stomach acid.
Step 3: Detailed Explanation:
Aluminium phosphide is a solid fumigant and rodenticide that, once it meets moisture or gastric hydrochloric acid, breaks down and releases phosphine gas.
Phosphine is a strong reducing agent, and when the lavage fluid containing it is treated with silver nitrate and heated, the phosphine reduces the silver ions to metallic silver, which deposits as a black precipitate, this is the basis of the silver nitrate paper or solution test used to detect phosphide poisoning.
Celfos is a well known trade name for aluminium phosphide used widely as a grain fumigant and rodenticide in India, and it is a common agent of both accidental and suicidal poisoning, which fits a case reaching the casualty as an emergency.
Step 4: Why the other options are wrong:
Tik-20 is also associated with phosphide type compounds in casual naming, but Celfos is the specific and most standard aluminium phosphide brand linked with this exact silver nitrate blackening test in toxicology teaching, so it is taken as the intended answer here.
Malathion is an organophosphate insecticide that works by inhibiting acetylcholinesterase, causing a cholinergic toxidrome with excess salivation, pinpoint pupils and muscle fasciculations, it does not release phosphine gas and will not blacken silver nitrate on heating.
Parathion is likewise an organophosphate compound with the same cholinergic mechanism as malathion, and it also does not produce phosphine or a positive silver nitrate blackening test.
Step 4: Final Answer:
The blackening of silver nitrate on heating after gastric lavage points to phosphine release, most likely due to Celfos, an aluminium phosphide compound.
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