"But we have no idea about the clinical manifestations of poisoning with this substance, so we must trouble you."

The meeting room was so quiet that the ticking of the clock on the wall could be heard.

Zhaxi’s mind buzzed. 1987, 1992, Zhou Huaixin, organophosphate insecticides; these fragments spun rapidly in his head, forming a picture he dared not imagine.

Yang Ping’s voice broke the silence: "Are you saying that this compound might be the organophosphate insecticide that Professor Zhou helped develop thirty years ago?"

Director Zhang nodded: "It’s possible, but we can’t be certain yet. To determine the structure, we need more samples. And the samples can only be found in Professor Zhou’s body tissues; there might still be residues in his liver, kidney, and adipose tissues. We need to do more precise extraction and purification, which will take at least two weeks."

Officer Yu asked: "If this compound is indeed the insecticide from thirty years ago, how toxic is it?"

Director Zhang remained silent for a few seconds, then said: "According to an internal report from 1992, the compound codenamed A-8 has an oral LD50 of 0.8 milligrams per kilogram of body weight. Based on this figure, a 60-kilogram adult would need only 48 milligrams to be lethal. Moreover..." he emphasized, "the report specifically mentions that the dermal toxicity of A-8 is also high. Its dermal LD50 is 2.5 milligrams per kilogram of body weight. This means that through skin absorption, around 150 milligrams could be lethal. Additionally, A-8 is easily soluble in organic solvents like toluene and xylene, and once in solution, its percutaneous absorption rate greatly increases."

Zhaxi’s hands were trembling as he quickly wrote in his notebook: A-8, organophosphate insecticide, LD50 oral 0.8mg/kg, dermal 2.5mg/kg. He looked up and saw that Yang Ping’s expression had not changed, but he knew Yang Ping’s mind was certainly racing.

Yang Ping asked a question that no one had thought of: "Director Zhang, does the 1992 internal report mention the chronic toxicity of A-8? For example, the manifestations of long-term low-dose exposure?"

Director Zhang flipped through the report and found a page: "Yes! The report conducted a 90-day subchronic toxicity experiment using rats. The results showed that rats with long-term low-dose exposure to A-8 experienced weight loss, reduced appetite, decreased activity, coarse fur, and grayish-brown pigmentation on their toes and nose tips. Blood tests showed a continual decrease in cholinesterase activity, but not necessarily to lethal levels. Pathological examinations revealed mild hepatocellular degeneration and focal necrosis of myocardial fibers."

He finished reading, looked up at Yang Ping: "Professor Yang, these manifestations highly align with Professor Zhou’s condition."

A thought flashed through Zhaxi’s mind: weight loss, pigmentation on the fingers, continuous decline in cholinesterase; all of these were present in Professor Zhou. The health report from four months ago showed cholinesterase levels at 2100, already half of the lower normal limit. Additionally, he indeed had grayish-brown pigmentation on his fingertips.

’But we have no knowledge of how pathological and pathophysiological knowledge connects toxicology to clinical manifestations with this new toxin.’

Yang Ping was silent for a long time, then he spoke in a quiet voice: "Professor Zhou had been in long-term contact with this compound for at least half a year before his death. It wasn’t a single poisoning incident, but chronic and sustained exposure."

Director Zhang nodded: "Correct! Moreover, if it’s eventually confirmed that what was detected in his body is indeed A-8, then the cause of his death is clear: an acute exacerbation on top of chronic poisoning, possibly due to a particularly large exposure at some point, which exceeded the body’s compensatory capacity, leading to an acute cholinergic crisis and respiratory inhibition."

"Thank you so much, Professor Yang. Without your reminder at that time, we would have never thought in this direction, and evidently, in the face of such high-intelligence crime, our breadth of knowledge is insufficient." Director Zhang turned to look at Dr. Yuan, who nodded in agreement.

Yang Ping pondered for a moment and said:

"The current situation is this: A rare organophosphate compound was detected in Professor Zhou’s body, with the molecular formula C₁₈H₂₀NO₄P, which is likely the insecticide codenamed A-8 that he helped develop thirty years ago. This compound has high dermal toxicity, is soluble in organic solvents, and long-term low-dose exposure leads to chronic poisoning, weight loss, pigmentation, and a continual decline in cholinesterase. His death is an acute exacerbation on top of chronic poisoning."

He paused: "Now there are three problems, first, where did A-8 come from? The insecticide project from thirty years ago was long discontinued, and such a compound shouldn’t exist anywhere. Second, how did Professor Zhou come into contact with A-8? Was it through the skin, or through the digestive or respiratory tract? Third, who made him come into contact with A-8?"

He looked at Officer Yu: "These three problems need us to answer together."

Officer Yu nodded: "Professor Yang, what do you need us to do?"

Yang Ping said: "First, check all the laboratory records, reagent usage records, and waste disposal records at Professor Zhou’s laboratory. A-8 couldn’t have appeared out of thin air; it must have been synthesized. If Professor Zhou synthesized A-8 himself, there would definitely be records."

He turned to Director Zhang: "Second, confirm the exact structure of this compound as soon as possible. Once we have the structure, we can determine its origin, whether it’s indeed the A-8 from back then, or some sort of analogue, I can also deduce more detailed possible symptoms."

He then turned to Dr. Yuan: "Third, re-examine Professor Zhou’s clothing, especially his underwear and close-fitting clothes. If he came into contact with A-8 through the skin over a long period, there should be residues on his underwear. Also, all the books in his study and laboratory need quantitative analysis for surface residues. I need to know the concentration of A-8 on each book."

He finished speaking and looked at everyone: "Everyone split up, and communicate progress at any time."

Officer Yu, Director Zhang, and Dr. Yuan stood up, gathered their things, and prepared to leave. At the door, Officer Yu suddenly stopped, turned back, and looked at Yang Ping: "Professor Yang, there’s one more thing. We’re investigating the source of the threatening letters to Professor Zhou, but we have no leads yet."

Yang Ping nodded: "Let me know as soon as you find any clues."

Officer Yu breathed a sigh of relief; with Professor Yang’s help in this high-intelligence crime, he felt much more at ease.

After they left, only Yang Ping and Zhaxi were left in the meeting room.

Yang Ping asked Zhaxi: "Have you sorted out any leads?"

Zhaxi hesitated, then said: "I’ve been thinking, if A-8 is something from thirty years ago, why is it only appearing now? Why did Professor Zhou suddenly start contacting it after his retirement?"

Yang Ping nodded: "Continue."

Zhaxi continued: "Also, those books. If A-8 was applied to the books, the perpetrator would need to know which books Professor Zhou would flip through and how often. This person must be very familiar with Professor Zhou’s habits. Moreover, the perpetrator would need access to those books, be it the books in Professor Zhou’s study or the ones in the laboratory."

Yang Ping said: "So who do you think fits these conditions best?"

Zhaxi thought for a moment, then said: "Li Xiong."

Yang Ping neither confirmed nor denied, instead he said: "Tell me your reasons."

Zhaxi opened his notebook and read aloud what he had organized: "First, Li Xiong has been with Professor Zhou for twelve years and is the person closest to him, the one most familiar with his habits. Second, Li Xiong can access Professor Zhou’s laboratory and study; he said that Professor Zhou’s study is usually locked, but if he wanted the key, he should be able to get it. Third, Li Xiong helped Professor Zhou buy toluene; those two bottles of toluene could have been used to dissolve A-8. Fourth, Li Xiong is from the chemistry department and has enough chemistry knowledge to know the toxicity and usage methods of A-8. As Officer Yu said, this crime inadvertently narrows down the range of suspects."

After reading, he looked up at Yang Ping.

Yang Ping was silent for a while, then said: "You’re right, Li Xiong meets those conditions, but he’s not the only one."

Zhaxi was stunned: "Who else?"

Yang Ping said: "Have you considered Professor Zhou himself?"

Zhaxi’s mouth dropped open: "Himself? You mean Professor Zhou poisoned himself?"

Yang Ping shook his head: "Not poisoning, but an accident. If Professor Zhou synthesized A-8 in the laboratory himself, and accidentally came into contact with a high concentration of the solution during the process, or inhaled aerosol, leading to chronic poisoning. He might not even have been aware."

Zhaxi’s mind was racing: "But what about those books? How do you explain the A-8 residues on the books?"

Yang Ping said: "If he synthesized A-8 in the lab, got the solution on his hands, and then went to flip through books, residues would be left on the pages. Over time, the books he frequently flipped through would become contaminated."

Zhaxi was silent; he had never considered this possibility.

"But he should have known the toxicity of such a substance and should have been extremely cautious."

"Well, I’m just saying it’s a possibility."