In mammals, including humans, nitric oxide ($NO$) is an important cellular signaling molecule involved in many physiological processes. Research into its function led to the 1998 Nobel Prize for discovering its role in cardiovascular function. One specific role of nitric oxide in cardiac function is the dilation of blood vessels, a process called vasodilation. Vasodilation of the arteries lowers blood pressure and decreases the force that the heart muscle needs to exert to pump blood.
The cell signaling mechanism begins when $NO$ diffuses into the smooth muscle cells of the blood vessel and activates guanylyl cyclase. The complete signaling mechanism is illustrated in Figure 1 below:
In addition to its role as a vasodilator, $NO$ is produced by macrophages, phagocytic cells of the immune system. Macrophages use $NO$ to counteract DNA replication in infectious microorganisms, and they produce compounds called peroxynitrites that are toxic to many bacteria and fungi.
Some infections can result in a medical condition known as septic shock, which can be fatal. The main characteristic of septic shock is a dangerous lowering of blood pressure, which impairs blood flow at the microscopic level of the capillaries (smallest blood vessels), resulting in hypoxic (low oxygen) conditions in tissues and cells.
Which of the following is a possible cause for the symptoms of septic shock?