Which is a primary and immediate cause of malnutrition?

The choice of presence of illness as a primary and immediate cause of malnutrition is grounded in the fact that illness often directly impacts the body's ability to absorb nutrients and utilize them effectively. When an individual is ill, particularly with diseases that affect the gastrointestinal system, such as diarrhea or infections, the body may not only require more nutrients to combat the illness, but also may struggle to process the nutrients consumed. This leads to malnutrition even if food intake appears adequate.

Furthermore, illness can increase metabolic demands, further stressing the body's nutritional needs. For example, fever or infections can escalate metabolism, requiring higher calorie intake that may not be met if the individual is also experiencing reduced appetite or inability to consume food.

In contrast, while inadequate food supply, overpopulation, and poor sanitation are important factors that can contribute to malnutrition over time, they do not serve as immediate causes to the same extent as illness does. An individual could have access to food but still become malnourished if an acute illness interrupts their ability to eat or absorb nutrients efficiently. Thus, the immediate physiological effects of illness on the body's nutritional status highlight why it is the correct answer in identifying a primary and immediate cause of malnutrition.