When identifying the odd one out, look for the category or field to which each item belongs. If most items belong to one category and one does not, that is the odd one out.
Let’s analyze the terms:
- Nephrology is the study of kidneys.
- Astrology is the study of stars and planets (it is not a medical science).
- Pathology is the study of diseases.
- Mycology is the study of fungi.
Among these, Astrology is different because it is not a medical science, unlike the other fields (Nephrology, Pathology, and Mycology), which are branches of biology or medicine.
Thus, Astrologyis the odd one out.
Therefore, the correct answer is \( \boxed{2} \).