A profoundly depressed patient believes with complete conviction that she has committed a terrible, unforgivable sin and deserves to be punished. When asked how she came to this conclusion, she explains it follows naturally from how worthless and wicked her depression makes her feel about herself. Before revealing the answer: is this a primary delusion?

No, this is more consistent with a secondary (delusion-like) idea than a primary delusion. The belief here can be traced back to, and understood as arising from, her profound depressive mood — she herself explains it as following naturally from how her depression makes her feel — whereas a primary delusion cannot be explained in this way and arises with no other morbid experience behind it. The content (guilt, unworthiness) is a classic depressive theme, and its comprehensible origin in her mood state is what points toward a secondary rather than a primary delusion.