Why might people in the United States be more likely to be malnourished as compared to undernourished? What is the difference between the two and why is this important to know?
By many definitions the two words are synonyms. However I think the distinction you're supposed to find is that malnourished usually means not enough nutrients in the food that is eaten. Undernourished means not enough food. Many people in the United States are malnourished because they don't eat enough fruits and vegetables and don't get enough nutrients. In the U.S., a person could be obese, yet malnourished.
http://www.merriam-webster.com/dictionary/undernourished
http://www.thefreedictionary.com/malnourished