In the subject analogy "Nephrology : kidney :: ichthyology : ____", which living group is studied in ichthyology?

Introduction / Context:
This analogy tests your general knowledge of specialised branches of science and the subjects they study. Nephrology is a medical specialty that focuses on a particular organ of the human body. Ichthyology, on the other hand, is a branch of zoology. To complete the analogy correctly, you must know what nephrology studies and then match that relationship with ichthyology and its subject.

Given Data / Assumptions:

• Nephrology is a branch of medicine.
• Nephrology focuses on kidneys and related diseases.
• Ichthyology is a sub discipline within zoology.
• Options include fish, soil, algae, insects and birds.

Concept / Approach:
The pair "Nephrology : kidney" tells us that the structure is "name of a specialised field : primary object of study". Nephrology deals with the kidneys, including their structure, function and diseases. Ichthyology uses the same pattern but within zoology. It is the scientific study of fishes, including their classification, biology, habitat and behaviour. Thus, we must pair ichthyology with fish to maintain the same type of relationship as in the first pair.

Step-by-Step Solution:

Recognise that nephrology is a medical specialisation focused on kidneys. Identify the pattern "field : what it studies". Recall from basic biology that ichthyology is the branch of zoology that studies fishes. Compare each option: fish are animals with gills, living mainly in water. Select fish as the object of study that matches ichthyology.

Verification / Alternative check:
Check the remaining options against common scientific branches. Soil is studied under disciplines like pedology and soil science. Algae are studied in phycology. Insects are studied in entomology. Birds are studied in ornithology. None of these fields uses the name ichthyology. Therefore, only fish align with the correct specialised term, ensuring that the analogy accurately mirrors the pattern of the first pair.

Why Other Options Are Wrong:
Although all other options are valid subjects in different branches of science, they do not correspond to ichthyology. Choosing any of them would mix up scientific terminology and break the one to one relationship between field name and primary subject. As a reasoning question, the analogy expects precise matching, not just any living organism.

Common Pitfalls:
Some candidates may confuse ichthyology with other biologically sounding terms and guess insects or algae. This usually happens when terms like entomology, phycology and ichthyology are not clearly distinguished in memory. Regular revision of these standard combinations is useful not only for reasoning sections but also for general knowledge segments in competitive exams.

Final Answer:
The correct completion is "Nephrology : kidney :: Ichthyology : fish".