This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Least predictable reaction to drug: 
Select and explain. 
A. Toxicity, B. Side effect, C. Idiosyncrasy, D. Allergy
C: Idiosyncrasy
Answer: C: Idiosyncrasy 
Explanation: Unpredictable (Type B or Bizarre) reactions
These are based on peculiarities of the patient and not on drug's known actions; include allergy and idiosyncrasy. They are less common, often non-dose related, generally more serious and require withdrawal of the drug. Some of these reactions can be predicted and prevented if their genetic basis is known and suitable test to characterize the individual’s phenotype is performed.
Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 92