Make Poison Antidote

To complete this quest, all you need to do is offer Razlo a Radscorpion tail.

If you've already completed Stop the Radscorpions without using explosives, you should have an ample supply available.

Talk to Dr. Razlo and there should be an option to give him the scorpion tail. In return you'll get a poison antidote and 250 XP.

Hints
For extra experience, go behind the curtin in Dr. Razlo's house to find a man laying on a futon. Go to your inventory, equipt the antidote and use it on the man laying down. This is Seth's (the captain of Shady Sand's guard) brother who was poisoned by a Radscorpion. In return for healing him you'll receive 400XP.

Another tip. If you're low on health after fighting the radscorpions, DON'T waste your stimpaks healing. Instead, when talking to Dr. Razlo (before giving him the tail) ask him to heal you, he'll give you a price for his services. SAY NO TO THIS PRICE. You should now be able to offer him a radscorpion tail instead of money. By doing this, Dr. Razlo will heal you to full health for free, while also giving you the antidote and 250XP.

Reward
250 XP, Radscorpion poison antidote.