PAS-C01 Operations and Maintenance Practice Question
This PAS-C01 practice question tests your understanding of operations and maintenance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company uses AWS Systems Manager Patch Manager to patch a fleet of EC2 instances. After a recent patching operation, some instances failed with the error 'Unable to retrieve patch baseline'. The instances are in a private subnet with a VPC endpoint for SSM. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPC endpoint for SSM does not have a security group that allows HTTPS outbound to the endpoint
The error 'Unable to retrieve patch baseline' indicates that the EC2 instances cannot communicate with the AWS Systems Manager service to fetch the patch baseline configuration. Since the instances are in a private subnet with a VPC endpoint for SSM, the most likely cause is that the VPC endpoint's security group does not allow HTTPS outbound traffic (port 443) to the endpoint itself. Without this outbound rule, the SSM Agent on the instance cannot establish the TLS connection required to retrieve the patch baseline from the SSM API.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The VPC endpoint for SSM does not have a security group that allows HTTPS outbound to the endpoint
Why this is correct
Missing outbound HTTPS from the instance to the SSM endpoint prevents communication.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The CodeDeploy agent on the instances is outdated
Why it's wrong here
Patch Manager uses SSM Agent, not CodeDeploy Agent.
✗
The instances are not registered as managed instances with the SSM ManagedInstance role
Why it's wrong here
If they are already managed, this error is not related to registration.
✗
The instances do not have an IAM instance profile attached
Why it's wrong here
If the instance were missing an IAM role, it would not appear in the managed instances list at all.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume the error is due to missing IAM permissions (Option D) or instance registration (Option C), but the specific error message 'Unable to retrieve patch baseline' is a network connectivity error, not an authorization error, and the VPC endpoint security group misconfiguration is the classic cause in private subnet scenarios.
Detailed technical explanation
How to think about this question
VPC endpoints for Systems Manager (com.amazonaws.region.ssm) use AWS PrivateLink, which requires security group rules to allow inbound HTTPS from the instance's security group and outbound HTTPS to the endpoint's network interface. The SSM Agent uses HTTPS (TCP 443) to communicate with the SSM API; if the endpoint's security group lacks an outbound rule allowing traffic to the endpoint's private IP, the TCP handshake fails, resulting in the 'Unable to retrieve patch baseline' error. This is a common misconfiguration when using VPC endpoints with security groups that default to denying all outbound traffic.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Operations and Maintenance — This question tests Operations and Maintenance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VPC endpoint for SSM does not have a security group that allows HTTPS outbound to the endpoint — The error 'Unable to retrieve patch baseline' indicates that the EC2 instances cannot communicate with the AWS Systems Manager service to fetch the patch baseline configuration. Since the instances are in a private subnet with a VPC endpoint for SSM, the most likely cause is that the VPC endpoint's security group does not allow HTTPS outbound traffic (port 443) to the endpoint itself. Without this outbound rule, the SSM Agent on the instance cannot establish the TLS connection required to retrieve the patch baseline from the SSM API.
What should I do if I get this PAS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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