You either prove what you claimed or they will disallow the deduction and make the adjustment. Then they will send you a bill. They will also comply with Federal and State Information Sharing Agreements, automatically. A letter of explanation will not benefit your case here.
However, it may make sense for the audited taxpayer to write a separate page of explanation to explain the context for the information shown on the tax return. This can help the auditor understand the context and reasonableness of the documentation.
For example: bankruptcy, divorce, death, significant medical problems, significant adverse business conditions, natural disasters (earthquake, flood, hurricane, tornado, tsunami, etc.)