Death caused by...?

[Coliosis]

Unfortunately my boyfriend's brother's girlfriend decided to buy a TINY bearded dragon from Petsmart instead of buying one of ours....it survived 8 days, just died today. The beardie was acting lethargic yesterday so I went to check on him to see what could be wrong. Everything looked fine except he looked a little dehydrated (wrinkly). After a drink he perked right up, no stress marks either. He's was eating fine as well since they got him. Then they found him dead this morning with yellow gunk coming out of his mouth and vent...what could have caused this?

 

[AHBD]

Poor little guy...no telling what was wrong. Was he kept on calci sand at the store ? He may have had an impaction, and once he had a drink + a soak it made him try to poo and he was not able to pass the calci sand. That's just a guess. Or maybe a toxic bug got in the tank. Sounds like he maybe ate something that made him sick.