Protozoan and helminthic diseases are difficult to treat because:

• A. They lack genetic material, so drugs cannot affect them.
• B. They are resistant to all forms of physical sterilization.
• C. They reproduce exclusively outside the human body.
• D. Their cells are structurally similar to human cells, making selective targeting challenging.

Answer: (D)

Verified step by step guidance

1. Understand the nature of protozoan and helminthic organisms: They are eukaryotic parasites, meaning their cellular structure is similar to that of human cells.
2. Recognize that many antimicrobial drugs target features unique to prokaryotic cells (like bacterial cell walls or ribosomes), which are absent or different in eukaryotic cells.
3. Analyze why this similarity in cell structure makes it difficult to develop drugs that selectively target the parasites without harming human host cells.
4. Consider the implications of this similarity on drug toxicity and side effects, which limits the options for effective treatment.
5. Conclude that the main challenge in treating protozoan and helminthic diseases is the difficulty in finding drugs that can distinguish between parasite cells and human cells due to their structural similarities.