Which of the following is not a side effect commonly associated with Galantamine, Donepezil, and Rivastigmine?

Galantamine, Donepezil, and Rivastigmine are acetylcholinesterase inhibitors commonly used to treat Alzheimer's disease and other forms of dementia. These medications increase the levels of acetylcholine in the synapses, which can lead to various side effects due to the enhancement of cholinergic activity.

The first option, syncope, can occur due to the cardiovascular effects of these medications, which may lower heart rate and subsequently affect blood pressure regulation. Gastrointestinal distress is a well-known side effect for these drugs because of increased cholinergic activity in the GI tract, leading to symptoms such as nausea, vomiting, and diarrhea. Headaches can also occur as a side effect due to central nervous system effects or changes in cerebral blood flow caused by cholinergic stimulation.

In contrast, high blood pressure is not a commonly associated side effect with these medications. Instead, the cholinergic effects can lead to vasodilation and possibly a reduction in blood pressure, making the incidence of hypertension unlikely. This distinction makes it clear why high blood pressure is the correct answer: it is not a side effect typically associated with the use of Galantamine, Donepezil, or Rivastigmine.