You hold
and RHO opens 1♠.
This is a mildly interesting problem: any of double, 1NT, 2H or pass could be right and would probably gain votes in a bidding competition.
However, you choose to pass. The auction continues
1♠ Pass 4♠ 5♥
5♠
Once you have picked yourself up off the floor, it's your go. What now?

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