The planning phase is crucial for any audit engagement and must be completed and approved before starting fieldwork. This ensures that the engagement has a clear scope, objectives, and methodology, which are necessary for conducting an effective and efficient audit. Without proper planning, the audit team may miss critical areas or waste resources on low-priority risks.
IIA References:
* IIA Standard 2200: Engagement Planning requires that internal auditors develop and document a plan for each engagement, including the engagement's objectives, scope, timing, and resource allocations, before starting fieldwork.
* The Practice Guide on Planning emphasizes that completing and approving the planning phase ensures that all necessary preparations are made, and potential risks are identified and mitigated before fieldwork begins.

Comprehensive and Detailed Explanation:
Even though this is an advisory engagement, internal auditors must still evaluate significant control gaps discovered during the engagement (Standard 2120 - Risk Management). Since a key procurement control is missing, the auditor should not ignore the risk (D). Expanding the advisory engagement scope (C) would be outside the engagement mandate unless approved. Deferring review to next year's plan (A) delays needed assurance. The most appropriate step is to perform a root cause analysis and test whether the approved workaround is effective (B). This ensures that risks are mitigated in the interim until a permanent control is implemented, providing management with actionable feedback.

Audit criteria should be appropriate and specific to each audit assignment, considering the unique context and objectives of each engagement. Consistency across all audit assignments (Option A) is not always feasible or desirable, as it could lead to inappropriate assessments. Instead, criteria should be flexible to allow the identification of nonadherence, represent reasonable standards, and align with good management practices relevant to each specific audit.
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IIA Standard 2201: Planning Considerations.
IIA Practice Guide on Audit Planning.

Confirmation letters are generally considered the most reliable source of documentary evidence because they involve direct verification of information with an independent third party. This type of evidence is less prone to bias or manipulation compared to internal documents like policy statements or remittance advices.
IIA References:
* IIA Standard 2310: Identifying Information requires that internal auditors gather sufficient, reliable, relevant, and useful information to support engagement results. Third-party confirmations, such as confirmation letters, are considered highly reliable because they come from independent sources.
* The Practice Guide on Audit Evidence highlights that external confirmations provide strong evidence of the accuracy of the information being audited.

Audit workpaper preparation is an essential aspect of an internal auditor's work that contributes significantly to their professional development. Preparing workpapers helps auditors develop their skills in documenting audit evidence, supporting audit conclusions, and enhancing their understanding of the audit process. This process ensures that the auditor gains a thorough understanding of the engagement and strengthens their analytical and documentation skills.
IIA References:
* IIA Standard 2330: Documenting Information requires internal auditors to prepare workpapers that document relevant information to support the conclusions and engagement results. This process is integral to their professional development, as it hones their ability to capture and organize audit evidence effectively.
* The Practice Guide on Audit Documentation emphasizes that workpaper preparation helps auditors develop critical thinking and ensures that they can justify their findings and conclusions based on documented evidence.