Given the question: John is comparing his medical record with his father's and son's medical records. John is 40 years old; his father is 70 years old; and his son is 10 years old. It appears that both John and his father has Retinitis. But his son does not have Retinitis. Given the paragraph above, please answer correctly the following question: For whom Retinitis would be a leading cause of blindness, John or John's father?
The answer is:
John