WASHINGTON — There have been more than a smattering of bad days for the Mets so far this year, but they were all dress rehearsals for the one that finally, mercifully, closed the door on an April from hell.

The Mets lost, 23-5. Nationals third baseman Anthony Rendon went 6-for-6, with three home runs and 10 RBIs. Then there was poor Kevin Plawecki, catcher by trade, tossing up 80-mph meatballs that the National Park scoreboard euphemistically called “changeups.”

Yet those were simply the footnotes to a display of dyspepsia that we’ll long remember, even measured against the long and colorful history of the Mets being … well, the Mets. These horrific adventures are always inexcusable; what makes this one worse is that it was perfectly avoidable.

Noah Syndergaard was staked to a 1-0 lead, and promptly gave five runs back thanks to a batch of 100- and 99-mph heaters the Nats treated every bit as rudely as Plawecki’s pillows later on. But even that awful first inning was mere prelude to the sight of Thor walking off the mound, grabbing his armpit after throwing a changeup to Bryce Harper.

The preliminary diagnosis on the player: probable lat strain.

The permanent diagnosis on the franchise: gross malpractice.

Again.

“It may or may not be related to his previous complaint,” Mets GM Sandy Alderson said, referring to the biceps pain that bothered Syndergaard a few days earlier. His ace will have an MRI performed Monday morning at 7; you hope the Mets have handcuffs, leg irons and straitjackets at the ready to make sure he actually slips into the tube.

So we are back to standard default position with the Mets, rolling our eyes at how they so expertly dump kerosene on their own heads, how they never, ever learn from history and are forever doomed to repeat it.

If you believe Syndergaard’s lat has nothing to do with what was bothering him before? You surely believe you can purchase both the Whitestone AND Throgs Neck bridges at a yard sale in Ozone Park.

This doesn’t just defy logic and explanation; it would also run counter to the Mets’ vast chronology of chronically underplaying injuries, something they’ve been doing for parts of two decades and two front-office regimes.

This time they aren’t alone because it was the player’s stubbornness that conspired alongside the Mets’ standard M.O. of close-your-eyes-and-hope-for-the-best. Syndergaard refused to take an MRI last week, and Anderson’s reaction to that Saturday instantly entered the Top 10 of Mets quotes of all time:

“I can’t tie him down and throw him in the tube.”

Maybe that would have been a bit much. But it’s clear in retrospect that Alderson should have been the grownup in this situation, informed Syndergaard: “It’s the MRI tube or the disabled list. Your choice.”

It’s hard to believe the GM wouldn’t have won in the court of public opinion on that one, and it would have sent a message that the Mets are done with their ages-old habit of tinkering with injuries that almost always — and we’re being generous adding the “almost” — cause maximum trouble.

Once upon a time, Matt Harvey thought he knew better than the Mets brass and their doctors, too, and once upon a time it was Harvey who sat where Syndergaard now sits — king of the hill, top of the heap, impervious to the wishes of his bosses. Harvey paid a hard price for that hubris, and the humbler version of No. 33 who inhabits the Mets clubhouse now is proof of that.

This is no way to run a baseball club, with ballplayers calling their own shots on matters of such import. It was certainly enough to drive a manager nuts: Terry Collins was angrily stewing in the dugout — no doubt thanks to a terrible cocktail of blowout loss and the relentless buzz of the injury bug.

He carried that into his postgame remarks, his face reddening by the second, and he finally blew up at an innocuous question before cutting the session short. Collins, of course, pointing to the radar-gun readings, also passed along the company line: “This has nothing to do with his arm.”

But the manager can afford to blind himself with boundless optimism; if he doesn’t believe, who will?

The front office? The player? Their responsibility is different, and they both ultimately decided to play Russian roulette with a priceless franchise asset. And discovered, too late, that the chamber wasn’t empty. The Mets will tell you that’s bad luck. Everyone else would call it standard operating procedure.