Today's chapter opens up with McDonald Miller who says...

Wait, acid? What?

Okay, you'll find that the base now has puddles of acid on the floor in certain places. Earlier a kid told Snake that was to help curb the rat infestation. Because, you know, there's just no other way to exterminate rats.

Well, according to a Google search: "Caustic soda is sometimes known by it's chemical name sodium hydroxide, this is a strong highly poisonous and corrosive alkali. It is often recommended for cleaning blocked drains."

You know what, game? I don't know if I have any of that, okay?

Okay, judging by my Google search, I'd wager - and I'm not a chemist - that there's probably not very much caustic soda/sodium chloride in chocolate. Unless chocolate is highly poisonous and corrosive. Lemme test this out by eating a chocolate biscotti right now. Nope, still alive!

And what secret agent has chocolate on them on a mission? What kind of pampered-ass soldier am I?

Thanks, MacGyver.

Ugh, Snake! Couldn't you have waited for the toilet?

Okay, now that we've gotten the cheap joke out of the way, in order to traverse the... sigh... sulphuric acid pools, simply equip the ration that has chocolate in it (it'll be in the description of the ration in your inventory) and walk over the puddle, which turns it brown. You have three puddles here. I only had one ration, which means I had to trek a long way back to get more.

As a final "fuck you", you also don't have the keycard for the door pictured here...

And as a bigger, "Fuck you in the ass for daring to give us money, play our games, and expect entertainment", Konami makes you go ALLLLLL the way back to the place where you fought Predator and search around the cornfields for the card! You just have to find the random spot. What? Games are supposed to be fun? Not in Japan! That's why they have dog-walking and train-conducting simulators. Really.

Finally, I found it. And beyond the door... the mission objective, Dr. Marv!