"Azo-what, exactly?" asked Rarity.
"Azoturic sclerosis," said the doctor. "It's a progressive disease that affects a pony's muscles and the nerve fibers that connect them. It's extremely rare. Very little is known about it, and it doesn't typically occur in ponies your age. We wouldn't even have performed the tests if the Princess hadn't insisted."
Rarity pursed her lips for a moment. "Wait, now I remember... That's the name of the wasting disease you identified, isn't it Twilight?" she said, turning to Twilight Sparkle.
Twilight nodded. "Yes. The treatment I developed should keep the symptoms from affecting you for at least twenty years," she said. "Probably much longer, actually. Hopefully I'll find a cure before then. If not, you'll be in for a rapid decline at some point."
"Well... with all due respect, Princess," said the doctor, "we can't really promise Ms. Rarity results of that sort. The treatments you've discovered do seem to work well, but they're still experimental. We've only been using them for six years, so we have no way of predicting what her condition will look like when she's forty, much less fifty."
"Twilight doesn't make promises she can't keep," said Rarity, solemnly. "That's why you decided to research this disease, isn't it? Another prophetic vision from your change?"
"I knew it was going to happen to you, yes. That's why I started work on it immediately after my transformation," she said, then turned to the doctor. "That bit of information is not to be shared with anypony, not even a husband or wife. Ever. Am I clear?"
"Yes, absolutely, my Princess," the doctor said nervously, then took a deep breath. "Well, then I suppose you're in luck, Ms. Rarity. According to Princess Twilight's research, treatment should wait until the onset of pre-symptomatic indicators which our tests suggest are still several months out. When treatment does begin, you will require monthly injections, some tissue transplants every few years, and a daily regimen of pills three times per day," the doctor said. "So there's quite a bit to it, I'm afraid."
Rarity laughed brightly. "Darling, that's no worse than the rigmarole I go through every morning to look this fabulous," she said, and winked at the doctor.
"I'll... go get the paperwork," the doctor said, blushing as he exited the room.
Rarity waited until the door had closed. "Alright dear, give it to me straight. How long do I really have?" she asked.
"I was serious. You've got at least twenty years, probably much more. In the meantime, I'll be doing more research," said Twilight. "I'll do my best to find a cure."
Rarity sighed and bit at her lower lip. "I'm so grateful for all you've done for me, Twilight. I really, truly am. But, something is nagging at me, and I must be blunt," she said. "Twenty years as a worst-case scenario isn't bad by any stretch, but it isn't exactly a lifetime, either. Is it selfish for me to keep dating him? I'm certain I love him, but I don't want to break his heart by dying young."
"He knows what he's in for, Rarity. He was already going to outlive you," said Twilight. "You don't have to feel guilty for having a shortened life expectancy. That's ridiculous. And even in the worst case, twenty years is still a long time."
Rarity nodded, and put on a brave face. "Then I'll tell him tonight. And Twilight?"
"Yes?"
"Cheer up, for Heavens' sake. You and I have a long time together too, Little Miss Maudlin Immortal," she scowled jokingly, then grinned.
Twilight smiled very wide. "Yeah, we do. And I wouldn't miss it for anything."