There appears to be much speculation about whether Iran will (and can) block the straight of Hormuz. For this to be a credible counter-measure against nuclear sanctions, Iran's leadership has to be prepared to lose all external as well as domestic support. To understand this point you have to appreciate;
1. 25% of Iran's GDP, and virtually all of its hard currency, is derived from oil exports. Losing those exports entirely would mean a collapse in the domestic economy, which would result in greater domestic unrest.
2. Combine this with a war with the whole world, which will likely be ready to bear arms against the loss of oil supply.
3. And the fight would not merely be waged by the distant west, who will be trigger-ready with their long range missiles. Iran's close neighbours and cartel partners who rely on the Straight of Hormuz to sustain their own economies and keep their own people well-fed in these times of sudden "arab springs" would fight tooth-and-nail against Iran.
Can it still happen? Iran is full of very clever people, and I imagine their leaders would be very aware of the yawning precipice should they contemplate what in effect would be a combination of domestic unrest and war with the rest of the world. Tenable?