Bacterial encephalitis and meningitis are difficult to treat because:

• A. bacteria in the brain are resistant to all known antibiotics
• B. bacterial infections in the brain do not cause inflammation
• C. the immune system cannot recognize bacteria in the central nervous system
• D. many antibiotics cannot cross the blood-brain barrier effectively

Answer: (D)

Verified step by step guidance

1. Understand the role of the blood-brain barrier (BBB) as a selective barrier that protects the brain by preventing many substances, including certain antibiotics, from entering the central nervous system (CNS).
2. Recognize that bacterial encephalitis and meningitis involve infections within the CNS, where effective treatment requires antibiotics to reach sufficient concentrations in the brain tissue and cerebrospinal fluid.
3. Analyze why some antibiotics fail to treat these infections effectively: many cannot cross the BBB in adequate amounts, limiting their therapeutic effect despite the presence of bacteria.
4. Consider other options given in the problem and evaluate their validity: bacteria in the brain are not inherently resistant to all antibiotics, bacterial infections do cause inflammation, and the immune system can recognize bacteria in the CNS but is limited by the BBB.
5. Conclude that the main challenge in treating bacterial encephalitis and meningitis is the difficulty many antibiotics have in crossing the blood-brain barrier, which restricts their access to the site of infection.