Correct! The left colic flexure is a watershed region between the superior and inferior mesenteric vessels. Watershed is a generic term for a region at the most distal area between two anastomotic vascular beds. As this point is most distal, a drop in circulation of those vessels and resultant drop in oxygen/nutrient availability results in ischemia of the watershed regions before other regions.

As ischemia progresses, other organs and regions subsequently become ischemic. 

Incorrect. The transverse colon receives a good supply from the middle colic artery (branch of the superior mesenteric artery), while this may be reduced by the atherosclerosis the transverse colon is not the most vulnerable point.

Incorrect. The sigmoid colon receives a good supply from the sigmoidal arteries (branches of the inferior mesenteric artery), while this may be reduced by the atherosclerosis the sigmoid colon is not the most vulnerable point. 

Incorrect. The right colic flexure sits between the territory of hte middle and right colic arteries (both branches of the superior mesenteric artery), while this may be reduced by the atherosclerosis the right colic flexure is not the most vulnerable point.

Incorrect. While this area is of concern as it is the watershed between the superior and middle/inferior rectal arteries, there is sufficient small vascular connectivity through this region that it is less vulnerable during ischemic colitis than another part of the GI system.

Incorrect. The liver is part of the foregut supplied by the celiac trunk. Thus, as the obstructions are in the superior and inferior mesenteric arteries, the foregut has adequate supply.