This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anti Microbial, Subject name: Pharmacology
A mother is breast-feeding her 2-month-old infant. Which one of the following drug situations involving the mother is unlikely to cause effects in the nursing infant? 
Select and explain. 
A. Ciprofloxacin for a UT infection, B. Amphetamine for weight loss, C. Nystatin for a yeast infection, D. Triazolam as a "sleeping pill"
C: Nystatin for a yeast infection
Answer: C: Nystatin for a yeast infection 
Explanation: Drugs that are capable of crossing the blood-brain barrier penetrate most body tissues and can appear in the milk of the lactating mother. Though concentrations of such drugs may be low in breast milk, they may cause effects in an infant who perhaps weighs just a few kilograms. Fluoroquinolones also penetrate tissues, and because they are contraindicated in children, it seems appropriate not to risk infant exposure via breast milk. The "safest" drug situation concerns nystatin, which is used only via the topical route and, as a polyene, does not cross membrane barriers.