CV0-004 Troubleshooting Practice Question
A cloud administrator is investigating a sudden increase in latency for a microservices application. Distributed traces show that a single downstream service's response time grew from 20 ms to 2 seconds, and its CPU utilization remains low at 15 percent. The service makes calls to an external third-party API. Which of the following is the MOST likely cause?
⚠ Common exam trap
The trap here is assuming low CPU utilization means the service is healthy, when blocked threads waiting on an external dependency are the classic signature of this pattern.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The service is blocked waiting on the third-party API, and its thread pool is saturating under the increased wait time.
A slowdown in an external dependency pushes worker threads into long waits, so a fixed-size thread pool saturates and requests queue behind blocked workers. CPU remains low because the threads are waiting on I/O, not executing. The trace data localizing latency to the third-party call identifies the trigger, and the service's concurrency model explains the amplification.
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 service's container CPU limit is throttling it during request bursts.
Why it's wrong here
CPU throttling would show elevated CPU utilization near the limit and throttling metrics, and the service is only at 15 percent. Throttling also tends to produce latency spikes correlated with request volume rather than a sustained increase. With low CPU usage, the bottleneck is more likely waiting on an external dependency than local compute starvation.
- ✓
The service is blocked waiting on the third-party API, and its thread pool is saturating under the increased wait time.
Why this is correct
When a downstream dependency slows from milliseconds to seconds, worker threads spend most of their time waiting, so the pool fills and queued requests wait even longer. CPU stays low because threads are blocked on I/O, not computing. The growing external API latency is the trigger, and the thread pool saturation amplifies it across the service.
- ✗
The service's memory limit is too low, causing the kernel to swap pages to disk.
Why it's wrong here
Memory pressure would appear as rising memory usage, out-of-memory kills, or swap activity, none of which are described. Containers typically have swap disabled, and the symptom would be restarts or allocation failures rather than steady latency. Low CPU with high latency on an external call points away from memory constraints.
- ✗
The service's network interface is experiencing packet loss, causing TCP retransmissions.
Why it's wrong here
Packet loss would affect all connections from the service, including internal calls, and would typically produce variable latency with retransmission spikes rather than a uniform shift tied to one downstream dependency. The traces isolate the increase to the external API call, which indicates the dependency itself slowed. Retransmission behavior would also show up in network metrics.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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