Short-selling firm Citron Research's Thursday afternoon tweet sent shares of pharmacy benefit management company Express Scripts (ESRX) tanking, and someone looked to be ideally positioned for the move.

On Thursday morning, one trader appeared to buy a large number of bearish options on Express Scripts.

Bought with the stock above $74.50, the trade would only pay off if the stock fell below $72.45 in just over a week.

Specifically, 5,395 December 74-strike put contracts were purchased for $1.55 per share, for a total cost of about $840,000.

Not only was this the largest trade in the stock, but it was one of the 20 largest single options trades on any single stock as of Thursday afternoon, based on the number of contracts.

Then, less than three hours later, the Citron Research Twitter account blasted: "$ESRX is Philidor of the pharma industry. @therealdonaldtrump promises to fix drug pricing? Two words: EXPRESS SCRIPTS." This was followed up by: "Watch Andrew Left from Citron on @cnbcfastmoney today at 5:30 EST to explain why $ESRX is on its way to $45."

As a result, Express Scripts shares tanked, and the option bought for $1.55 was trading for as much as $6.50 — for a swift profit of about $2.7 million. By the end of the day, the put price moderated to $4.70.

It is unclear, of course, who this options buyer was, and what information they may have been privy too. But suffice it to say that if they got lucky, they got extremely lucky.

When asked about the trade Thursday on CNBC's " Fast Money ," Citron founder Andrew Left said, "That wasn't me," adding that he didn't know about the trade, and is upset that "someone did that off my work."

Express Scripts responded to the tweets in a brief statement, "Express Scripts is a market force that uniquely puts medicine within reach by driving down cost of care and improving health outcomes."

Left and Citron are best known for their report taking on Valeant Pharmaceuticals (Toronto Stock Exchange: VRX-CA).