Section A Directions:In this
section, there is a short passage with 5 questions or incomplete statements.
Read the passage carefully.
When the heart breaks down, it beats irregularly or not at
all. A bone can chip or snap. But when the complex network of neurons in our
brain fails to work, the result can be a near-endless variety and combinations
of mental illnesses. It is normal to sometimes be sad, happy,
anxious, confused, forgetful or fearful, but when a person’s emotions, thoughts
or behavior frequently trouble them, or disrupt their lives, they may be subject
to mental illness. According to the World Health Organization (WHO), about 450
million people worldwide are affected by mental, neurological or behavioral
problems at any time. However, determining that someone has a
mental illness, and which one, is one of the challenges psychiatrists
face. Historically, some symptoms of mental illness, such as
eccentric behavior and hearing voices, have been taken as evidence of heavenly
communication or demonic (恶魔的) possession. More recently, brain
scans have directly linked these conditions with changes in levels of
neurotransmitters—chemicals that convey messages across neurons—or alterations
in the number or structure of neurons in different brain areas. For instance,
people suffering from depression often display lowered levels of the
neurotransmitter serotonin (血清素). In a few cases, the immediate
cause of the malfunction has been identified. Alzheimer’s disease, a major
source of dementia and memory loss in the elderly, is caused by the accumulation
of protein plaques which choke neurons in the brain. Some
infectious diseases can also develop into a mental illness. Untreated HIV
infection can cause dementia, as can the uncontrolled replication of the germ
that causes syphilis (梅毒). In many cases the precise cause is
unclear and experts suspect that many different factors are involved. The chance
that identical twins both develop schizophrenia (精神分裂症) is much higher than that
for fraternal twins or siblings, proving the strong role of inherited genes. But
scientists are accumulating a growing list of other risk factors that predispose
people to this condition, including prenatal exposure to famine conditions,
certain infections or exposure to lead. The season of their birth also seems
important—birth in winter or early spring increases the risk, as does an older
father and, controversially, child abuse. Genes are also thought
to influence many other mental health problems, including: anorexia, autism,
Alzheimer’s disease and bipolar disorder. Some researchers
believe that smoking cigarettes and taking recreational drugs may elevate a
user’s risk of mental illnesses. According to the passage, Alzheimer’s disease is the main cause of the elderly people’s ______.