Which word means "to gather together"?

In this context, the word "etiology" refers to the cause or origin of something, in particular, the cause of the flu epidemic of 1918.

It implies the study or understanding of the underlying factors or reasons that led to the occurrence of the epidemic. The options "transmission," "mortality rate," and "pathology" do not accurately reflect the meaning of "etiology" as it is used in this sentence.

Surveillance refers to the act of closely monitoring or observing someone or something, typically for the purpose of gathering information or ensuring safety. It involves systematic observation, often done in a discreet or secretive manner. The options "isolation," "cognition," and "assessment" do not specifically convey the meaning of placing someone or something under observation.

Two-sided.

In medical terminology, "bilateral" refers to something that affects or is present on both sides. In the context of the sentence, it indicates that the weakness observed in the patient's ambulation is affecting both sides or both legs. The word "bilateral" is used to describe a condition or characteristic that is symmetrical or occurring on both sides of the body. Therefore, option A, "Two-sided," is the correct answer.

Convex.

The term "concave" refers to a surface or shape that curves inward or is rounded inward, like the inside of a bowl. In the context of the sentence, it indicates that the child's chest had a shape that was curved inward rathe r than being flat or rounded outward. The opposite of concave is convex, which means curved outward or bulging.

Therefore, option B, "Convex," is the correct answer.