Tranche thinness in structured credit products like MBS and CDOs is a significant concern because it implies that once a loss event breaches the equity tranche, which is usually the first to absorb losses, it can quickly affect the more senior tranches, like the mezzanine tranche. The thinness of these tranches means they have a limited capacity to absorb losses. Once breached, the losses that affect these tranches can be quite severe due to their relative size compared to the overall pool. This scenario can lead to substantial financial impacts for investors in these tranches.

A is incorrect because while market liquidity risk might be a concern for structured credit products, it is not directly related to the concept of tranche thinness and its impact on loss severity.
C is incorrect as the primary issue with tranche thinness is not the difficulty in pricing, but the risk of significant losses once the tranches are breached by loss events.
D is incorrect because the main concern is not about investor demand for the tranches but about the potential for large losses within these tranches in the event of defaults in the underlying asset pool.

Things to Remember

• In structured credit products, tranche thinness can lead to a rapid escalation of losses once initial equity tranches are breached, due to the limited loss-absorbing capacity of