A/N: This is a pretty science-heavy chapter. I had planned on this being the last chapter, but then there were lots of words and my brain decided I was done. I believe there's one chapter to go, but we'll see.
It had been an exhausting week, but they'd managed it. By working 'round the clock for roughly 170 hours, Sherlock, John, and Molly had managed to complete toxicology tests that normally took upwards of a month. 170 hours of staring into microscopes, of careful pipetting, of whirring centrifuges and Western blots and a slow elimination of possibilities.
And it had all been for nothing. (Wrong wrong it must be wrong.)
Sherlock stared at the results of the last test, the very last, the last possible solution to the murder of Tim Dorset. (Wrong wrong how could I be wrong?)
Negative. (Wrong wrong oh god I'm wrong.)
Sherlock narrowed his eyes. "This is impossible. You must have messed up the test." John barely restrained an angry sigh. "We've run it three times now, Sherlock. All of them. There's nothing."
"Then you must have missed something!" Sherlock bellowed, now pacing the lab frenetically, hands scrubbing through his hair. (I must have missed something, I missed something, I'm wrong, I'm wrong, oh god, I'm wrong.)
Neither John nor Molly seemed the slightest bit put off by Sherlock's sudden explosion. They had been at this for a week, with barely any downtime. They were all running on crisps and coffee at this point. Sherlock hadn't left the hospital, and John and Molly had only left for an hour, each going to their respective flats to bring back toiletries and fresh clothes. A spare gurney that had been wheeled into the lab was the closest thing any of them had seen to a bed the whole week. They slept in shifts, one at a time, two people always awake, always busy, always searching for something, anything.
But still, nothing.
But they all knew there had to be something. Sherlock's reasons for doing so were obvious, but John and Molly weren't just following him blindly. There was something off about all this. The two doctors knew Dorset shouldn't have had a stroke and a heart attack. If it had been one or the other, they would have been able to write it off as idiopathic – one of those strange flukes that struck mortals down every day. But both? At the same time? While in the presence of Richardson, whose behavior was, to put it lightly, so peculiar? It hardly seemed probable.
John nodded. "Right. So nothing in the blood or tissues. Nothing in his medical history. Nothing in the stomach contents. Where do we look now?"
Sherlock stopped his pacing and half-turned toward John, one arm crossed over his chest to hold his own elbow, worrying his fingernails with his teeth. "Hmm?"
"Where do we look?" John repeated himself. "We're all in agreement here; the man certainly didn't die of natural causes. So where do we look to find proof?"
"What if we stopped looking for something specific?" Molly had lost some of her usual hesitancy to speak somewhere along the past seven days. It had been awkward – she had been awkward – at first, but by now she was just too tired to care that both men had swung around to look at her, or to shrink away from the piercing look Sherlock was directing at her.
"We've got it in our heads that whatever we're looking for is on that list," She gestured to a whiteboard listing over a hundred chemical compounds, all of which had been crossed off. "But maybe it broke down before we could test for it, or maybe it's something else."
"Something else? It's an incredibly extensive list, Molly. What exactly would this something else be?" Sherlock snapped. But Molly wasn't fazed. "I don't know. But what if we just looked at the samples, like under a microscope, and maybe we'll find something off? You know, observe and all that."
Sherlock found himself in the uncomfortable position of realizing that Molly had an excellent idea, and worse, she had thought of it before he had. He shook his head, willing the thoughts away. "Alright, fine." With that, they divvied up the small amount of Dorset's blood, liver, and stomach contents they had left, and each went to their microscopes.
Thirty-seven minutes and 5 slides of Dorset's last meal later, Sherlock suddenly took his slide over to the scanning electron scope that would allow him a much closer view of the evidence. "Find something?" John asked. "Maybe... not sure yet." Sherlock muttered distractedly, fiddling with the dials of the scope.
He finished his adjustment and peered into the eyepiece, examining the individual molecules of his slide, making a mental list. (Caffeine – from coffee, obviously. Sucrose – sugar from the cake, possibly coffee as well. Egg protein – cake. Gluten – flour from cake. Theobromine – chocolate cake, then. Sodium chloride – table salt, that goes in cakes, doesn't it? Lactose – milk. Soy protein and tyramine – from tofu. What is tofu doing in a cake?"
Sherlock voiced his thoughts aloud. "John, you bake. What would tofu be doing in a cake?"
"Huh? Tofu in a – I do not bake! I cook. There's a difference." John glared at Sherlock, who rolled his eyes. "Okay, fine, you cook. But the question stands: Does tofu go in cakes?"
"Umm, no," Molly chirped. "Obviously." She laughed nervously, suddenly realizing that the detective may not like the fact that she had turned his favorite retort against him. But John spoke before Sherlock could respond. "What else is in there?"
"Hm? Oh, the expected things – caffeine from the coffee, sucrose, lactose, milk and egg protein, tyramine, traces of flour and sodium chloride, theobromine – all from the cake – and of course stomach acid."
John furrowed his brow. "Tyramine... why does that ring a bell?" Sherlock rolled his eyes. "Oh, I don't know, maybe because it's an amino acid and therefore has probably been mentioned to you once or twice in your medical training. Or one would hope."
John glared at him. "No there's some sort of thing, about not eating tofu and too much tyramine and some sort of reaction..."
The men froze. Their eyes met, and realization dawned on both. "Oh, brilliant, John, that is brilliant!" Sherlock's frustration had vanished, and he had morphed into a perfect impersonation of a child on Christmas morning. John smiled wide. "It really is, isn't it?" John agreed.
"What is?" Molly looked between the two men, thoroughly confused.
"Tyramine, Molly!" Sherlock exaulted. Molly looked at him blankly. Sherlock stepped forward gleefully and put his arms on her shoulders. "Think! We know Dorset and Richardson gave each other professional advice. And what is Richardson? He's a psychiatrist! And what condition makes up a significant portion of a psychiatrist's practice?"
Molly stared for a second, not sure if she was supposed to answer, or if Sherlock was just being especially dramatic. "C'mon, Molly, think!" Molly hazarded a guess. "Uh, depression?"
"Yes!" cried Sherlock. "And what does a psychiatrist do for depression? Well, he prescribes anti-depressants of course. Most likely an SSRI like Prozac, but what if he had it in for one of his patients? What if instead of something safe, he prescribed Dorset an MAOI, a depression medication that carries the risk of severe side effects, possibly even death, if the patient eats even a small amount of anything rich in tyramine – for example, tofu? What if Richardson, blaming Dorset for losing the malpractice case, slipped tofu into an otherwise innocuous cake, knowing full well it would set off a devastating reaction in Dorset's brain that would cause his blood pressure to rocket until his blood vessels burst and his heart gave out? Don't you see? It was murder! Brilliant, beautiful, clever murder!"
"But.. but.. there's no record of depression in Dorset's file, and he wasn't on any medication," Molly said, internally cursing her voice's waver.
"Oh, that's just a detail! Now that we know what to look for, it will hardly be difficult to find record of Richardson giving Dorset the medication!" Sherlock twirled around in place, then swooped over to the make-shift bed in the corner to grab his coat and scarf as John shrugged his jacket back on. Suddenly all exhaustion was forgotten in the thrill of the chase.
Molly goggled at the men as they rushed out of the room. Some people just had all the fun.