- Oranges / lemons are a kind of food
- Eating oranges / lemons can prevent scurvy
- Hence, eating a kind of food can prevent scurvy

- Eating a kind of food can prevent scurvy
- Scurvy is a kind of disease
- So, eating a kind of food can prevent a disease

- Eating a kind of food can prevent a disease
- Eating / digestion is when an organism takes in nutrients from food into itself by eating
- So, taking in nutrients by eating a kind of food can prevent a disease

- Taking in nutrients by eating a kind of food can prevent a disease
- If something can prevent a something else then that something else is probably caused by a deficiency of that something
- So, if a disease can be prevented by eating a certain kind of food then that disease is probably caused by a nutritional deficiency

Therefore, if a disease can be prevented by eating a certain kind of food then that disease is probably caused by a nutritional deficiency
Question
Scurvy is a disease that sailors often got on long voyages. It was discovered that scurvy could be prevented by eating oranges and lemons. This suggests that scurvy is a disease caused by

Context
- Scurvy is a kind of disease 
- Oranges / lemons are a kind of food 
- Eating / digestion is when an organism takes in nutrients from food into itself by eating 
- Eating oranges / lemons can prevent scurvy 
- If something can prevent a something else then that something else is probably caused by a deficiency of that something