This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Which of the following patient is most likely to harbor a Helicobacter pylori infection in the stomach? 
Select and explain. 
A. A 60-year-old middle-income American, B. A 25-year-old American in a low-income group, C. A 25-year-old Zairian, D. A 60-year-old Pakistani
D: A 60-year-old Pakistani
Answer: D: A 60-year-old Pakistani 
Explanation: The most important risk factors for H. pylori infection include older age, low income, and residence in a developing country. It is believed that infection generally is acquired in childhood. Although humans are the major reservoir of H. pylori, the route of infection is unclear, with fecal-oral and oral-oral spread both being possible. H. pylori is endemic in only 30% of Americans, but prevalence rates are as high as 80% in developing countries. Infection with a related species, H. heilmanii, is about 1% as common as H. pylori infection.