Septic shock is caused by:

Septic shock is a life-threatening condition that occurs as a result of an infection. In septic shock, the body's response to the infection leads to systemic inflammation, which can result in a drop in blood pressure and insufficient blood flow to vital organs. This often follows an infection, typically from bacteria, but can also be caused by fungi or viruses. The body's immune response, in trying to fight the infection, ultimately causes widespread inflammation rather than localized damage, leading to the critical condition known as septic shock.

In this case, the other options do not directly relate to septic shock. A blood clot could lead to complications such as a stroke or pulmonary embolism, but it would not inherently cause septic shock. A heart attack results primarily from issues related to the heart and circulation, not infection. Finally, a severe allergic reaction (anaphylaxis) is also different, as it involves an exaggerated immune response to a substance, rather than an infection causing systemic issues. Thus, the presence of an infection is the key factor in the development of septic shock.