Platelet activating factor (PAF)

Platelet activating factor (PAF) has all the following actions except 

A. promotes aggregation of platelets. 
B. promotes degranulation of platelets. 
C. stimulates the release of vasoactive substances 
from platelets. 
D. causes vasoconstriction of arterioles.

The answer is D.

 Platelet-activating factor (PAF) acts not only on platelets, but also on leukocytes and the microvasculature, where it causes vasodilatation and enhanced vascular permeability.

The effects of PAF include the following:
A. platelets
(1) aggregation     

(2) degranulation                 

(3) release of serotonin and histamine
B. polymorphonuclear leukocytes (1) stimulation of motility of leukocytes        

(2) superoxide production 

(3) degranulation
C. Blood vessels (1) vasodilatation               

(2) increased permeability