In the following question, four words have been given, out of which three are…

Concept: In an odd-one-out classification question, three words share a common category while the fourth breaks that pattern. Words ending in "-logy" generally name a field of study, but the shared category here narrows further to genuine natural/life sciences — fields that investigate a defined living organism or bodily system using the scientific method.

Application: Nephrology is the branch of medicine that studies the kidneys and their diseases. Entomology is the branch of zoology that studies insects. Mycology is the branch of biology that studies fungi. All three examine a defined biological subject through scientific investigation, so they belong to the same category of natural/life-science disciplines. Astrology, however, is the practice of interpreting the positions of stars and planets to predict human affairs — it studies celestial bodies, not a living organism, and it is not recognised as a science.

• Nephrology — studies the kidneys (a biological system) → fits the shared life-science group

• Entomology — studies insects (living organisms) → fits the shared life-science group

• Mycology — studies fungi (living organisms) → fits the shared life-science group

• Astrology — studies the positions of stars and planets to predict human affairs, not a living organism, and is not a recognised natural science → breaks the life-science pattern

Cross-check: Nephrology, Entomology, and Mycology are all recognised natural-science disciplines listed in general-science references, while Astrology never appears in that list — confirming it as the outlier.

Hence, Astrology is the odd one out among the four.