Final answer:
The next step in the management of the pruritic rash on the young man's lower trunk and upper thighs would be further evaluation to determine the cause of the rash.
Step-by-step explanation:
The next step in the management of this young man's pruritic rash would be to consider the possibility of an insect bite reaction or an infection. Given his recent vacation to Puerto Rico and being bitten by insects during the hike, it is important to evaluate the rash further.
The patient may need to undergo further diagnostic tests, such as a skin biopsy or blood tests, to determine the cause of the rash and guide the appropriate treatment.