Question

poor sanitation and lack of access to clean water are examples of the effect of
lack of knowledge and education.
limited health care spending.
limited infrastructure.
environmental factors.

Explanation:

Poor sanitation and lack of access to clean water are often due to limited infrastructure. Infrastructure includes systems like water treatment plants, sewage disposal systems, etc. Without proper infrastructure, it's hard to ensure sanitation and clean water access. Limited healthcare spending would more directly affect medical services. Lack of knowledge/education might contribute but not as directly as infrastructure. Environmental factors are more about natural aspects like pollution sources (not the main cause here).

Answer:

limited infrastructure.