Out of the four alternatives, choose the one word that can be substituted for the given phrase "a remedy for all diseases". This vocabulary item tests your knowledge of precise one-word substitutions used in general English and medical or metaphorical contexts.

Introduction / Context:
One-word substitution questions are designed to test how precisely you understand and can use English vocabulary. The phrase given here is "a remedy for all diseases". The task is to find the single English word that means exactly this, a concept that often appears in literature, discussions about medicine, and even in political or economic debates as a metaphor.

Given Data / Assumptions:

The phrase is "a remedy for all diseases".
The correct word should refer to a cure or solution for every possible problem or illness.
We are working in standard English vocabulary used in exams and textbooks.
Some options are genuine medical terms but do not indicate a cure for everything.

Concept / Approach:
The word panacea comes from Greek roots and literally refers to a universal remedy, a cure-all. In modern English, panacea is used both literally (in older medical ideas) and metaphorically (for any supposed solution to all problems). Antiseptic is a substance that kills germs on the surface of the body or objects. An antibiotic fights bacterial infections, and a vaccine prevents specific diseases. Narcotic refers to a drug that dulls the senses or relieves pain but is not a universal cure. Therefore, only panacea matches the idea of a remedy for all diseases.

Step-by-Step Solution:
Step 1: Focus on the key part of the phrase: for all diseases. This means not just many diseases but every disease. Step 2: Recall that panacea is a well known word meaning cure-all or universal remedy. Step 3: Compare antiseptic and antibiotic. Both are important medical terms but are used only against certain germs rather than every possible disease. Step 4: Note that narcotic is mainly related to pain relief and sedation, not curing all illnesses. Step 5: Realise that vaccine refers to prevention of particular diseases and is not used as a general cure for everything. Step 6: Conclude that panacea is the only option that exactly fits the phrase given in the question.

Verification / Alternative check:
To verify, place the word in a sentence. The politician claimed that his new policy was a panacea for all economic problems uses panacea in a clearly metaphorical way, meaning a cure for all issues. If we replace panacea with antibiotic or antiseptic, the sentence becomes illogical because these words do not carry the idea of solving every problem. Similarly, in older medical theories, people sometimes looked for a panacea that would cure all diseases, which directly matches the phrase in the question.

Why Other Options Are Wrong:
Option A Antiseptic prevents the growth of germs on the skin or objects but is not a universal cure for all diseases.
Option B Antibiotic works against bacteria but not against viruses or every possible illness, so it cannot be called a cure-all.
Option C Narcotic is a drug that dulls pain and may induce sleep; it does not cure underlying diseases and is certainly not a universal remedy.
Option E Vaccine helps the body develop immunity to specific diseases; it prevents rather than cures and is always disease-specific, not universal.

Common Pitfalls:
Many students are tempted by technical-sounding medical words when they see the phrase remedy for disease. However, examiners often use this type of question to see if you know special literary or classical words like panacea. A good strategy is to remember that panacea is almost always used with phrases like for all problems or for all diseases. When you see a phrase that mentions all or every in this context, panacea is a strong candidate.

Final Answer:
The one word that means "a remedy for all diseases" is panacea.