Practice: Mortgage-Backed Security (MBS) Instrument and Market Features
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Fixed IncomeMortgage-Backed Security (MBS) Instrument and Market Features
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Question 1Exam level
When mortgage interest rates fall sharply, an investor holding a mortgage pass-through security is most likely exposed to:
How sure are you?
Correct: B. When rates fall, homeowners have an incentive to refinance their mortgages, which accelerates principal prepayments to the pass-through's investors. This is contraction risk: the security's average life shortens and the investor must reinvest the returned principal at the new, lower prevailing rates.
A. Extension risk is the opposite scenario: it occurs when rates RISE and homeowners have no incentive to refinance, so prepayments slow and the security's duration stretches out longer than expected. Falling rates produce the reverse effect, contraction, not extension.
C. A mortgage pass-through's cash flows are explicitly NOT fixed like a standard bond's: because the underlying mortgages can be prepaid at any time, the investor's actual principal and interest cash flows depend on the pace of prepayments, which is exactly the risk being tested here.
In a sequential-pay collateralized mortgage obligation (CMO) with tranches A, B, and C, the tranche most likely to bear the greatest extension risk is:
How sure are you?
Correct: B. In a sequential-pay structure, principal is distributed to tranche A first, then B, then C, in strict order. Tranche C waits longest to begin receiving principal, so if prepayments slow (rates rise), tranche C's average life stretches out the most of the three. Tranche A, receiving principal first, is the most exposed to contraction risk instead.
A. Tranche A is the one MOST exposed to contraction risk (prepayments arriving faster than expected), not extension risk, since it is first in line for whatever principal comes in. The question asks about extension risk, which lands hardest on the last tranche, not the first.
C. A CMO's tranching structure redistributes prepayment risk unevenly among the tranches; it never eliminates the risk itself. 'CMOs eliminate prepayment risk' is one of the most common traps on this topic; the correct concept is redistribution, not elimination.
Within a single CMO structure, compared to a support tranche, a PAC (planned amortization class) tranche most likely offers:
How sure are you?
Correct: B. A PAC tranche is structured to receive a predictable schedule of principal payments as long as actual prepayments stay within a specified band (the PSA range the structure was designed around); the support tranche absorbs the prepayment variability that keeps the PAC's cash flows stable. Investors accept a lower yield on the PAC in exchange for that predictability.
A. A higher yield for greater variability describes the SUPPORT tranche, not the PAC. The PAC is the more stable, lower-yielding tranche precisely because the support tranche is absorbing the prepayment risk on its behalf.
C. Receiving no principal until every other tranche is retired describes a sequential-pay tranche's position in the payment queue, not what distinguishes a PAC from a support tranche. A PAC tranche does receive a defined principal schedule of its own throughout the structure's life.
Compared to a mortgage-backed security guaranteed by GNMA (Ginnie Mae), one guaranteed by FNMA (Fannie Mae) most likely carries:
How sure are you?
Correct: B. GNMA is a U.S. government agency, so its guarantee carries the full faith and credit of the U.S. government, an explicit guarantee. FNMA and FHLMC are government-sponsored enterprises (GSEs), privately owned companies with a government charter; their guarantee is implicit, not backed directly by the government's full faith and credit.
A. GNMA is the one with the explicit, full-faith-and-credit government guarantee, because it is literally a federal government agency. FNMA is a GSE, a separate legal category with only an implicit guarantee, so the two are not identical on this point.
C. An FNMA MBS does carry credit support, just of the implicit, GSE-backed kind rather than no support at all; the distinction tested here is explicit versus implicit guarantee, not guarantee versus no guarantee.
The PSA prepayment benchmark's standard ramp assumes the conditional prepayment rate (CPR) most likely:
How sure are you?
Correct: A. The 100% PSA benchmark ramps the CPR up from 0.2% in the pool's first month, increasing by a fixed increment each month, until it reaches 6% in month 30, then holds at 6% for the remaining life of the pool. A 200% PSA assumption runs at twice that speed at every point on the ramp.
B. A constant 6% CPR from month 1 ignores the ramp-up period entirely; the PSA benchmark's defining feature is that prepayment speed starts low and builds up over the pool's first 30 months before leveling off, not a flat rate from day one.
C. This reverses the direction of the ramp. The PSA benchmark models prepayment speeds INCREASING over the pool's early life (as more homeowners become eligible or motivated to prepay), not decreasing.
A mortgage pass-through security's price appreciation when interest rates fall is most likely limited by:
How sure are you?
Correct: A. A pass-through MBS behaves like a bond with an embedded call option (the homeowners' prepayment option). As rates fall, prepayments accelerate, returning principal at par just when the investor would otherwise want to hold a high-coupon bond whose price is rising, capping further price appreciation. This is negative convexity, the opposite of a straight bond's positive convexity.
B. Positive convexity describes a straight, non-callable bond's price behavior, where price gains accelerate as rates fall. A pass-through MBS displays the opposite pattern, negative convexity, precisely because of the prepayment option embedded in the underlying mortgages.
C. There is no statutory price ceiling set at origination. The price-limiting effect comes from the economics of prepayment (the embedded call option), not from any regulatory or contractual cap on the security's price.
The primary purpose of transferring mortgage loans into a bankruptcy-remote special purpose entity (SPE) during securitization is most likely to:
How sure are you?
Correct: A. The SPE is structured to be bankruptcy-remote: the transferred mortgage loans are legally isolated from the originator, so if the originator later becomes insolvent, its creditors cannot reach the collateral backing the securities already sold to investors. That legal isolation is the core purpose of the SPE step in securitization.
B. The whole point of the SPE transfer is that the originator no longer legally owns the loans; ownership moves to the SPE. The originator may still be retained as loan servicer for a fee, but legal ownership is exactly what is transferred away.
C. An SPE isolates the collateral from the originator's OTHER creditors; it does not eliminate the credit risk inherent in the mortgage pool itself. Investors in the securities still bear the risk that the underlying homeowners default.
Compared to residential mortgage-backed securities, commercial mortgage-backed securities (CMBS) are most likely structured with:
How sure are you?
Correct: A. Because commercial mortgage borrowers are typically large, sophisticated entities, CMBS pools are commonly structured with call protection at both the loan level and the structure level, such as a lockout period during which prepayment is prohibited, defeasance (the borrower substitutes government securities as replacement collateral), or yield maintenance charges that compensate investors for lost interest on a prepayment. This makes CMBS cash flows more predictable than residential MBS.
B. CMBS, like residential CMOs, are commonly tranched by seniority and credit risk, with junior/subordinate tranches absorbing losses before senior tranches; pooled pro rata loss sharing with no tranching at all is not the standard CMBS structure.
C. CMBS prepayment behavior is deliberately made LESS like residential MBS prepayment through the call protection features described above; individual commercial borrowers do not prepay the way a large, diversified pool of individual homeowners does, and the structure is built specifically to blunt that difference in behavior, not replicate it.
An investor comparing a mortgage pass-through security to the underlying pool of individual mortgage loans is most likely to observe that the pass-through security:
How sure are you?
Correct: A. A mortgage pass-through security channels the pool's principal and interest collections directly to security holders on a pro-rata (proportional) basis, after the servicer and guarantor deduct their fees. Each investor's monthly cash flow therefore varies with the pool's actual prepayment experience rather than following a fixed bond-like schedule.
B. A pass-through's cash flow to investors is explicitly NOT fixed: it moves with however much principal and interest the underlying mortgage pool actually collects each month, which includes any unscheduled prepayments.
C. Principal is returned to pass-through investors continuously, as it is collected from the underlying pool (including prepayments), not held back until one final stated maturity date the way a bullet-maturity bond repays principal.
A non-agency (private-label) mortgage-backed security, compared to a GNMA-guaranteed security, most likely requires:
How sure are you?
Correct: A. Non-agency MBS are issued without any GNMA, FNMA, or FHLMC guarantee, so the deal structure itself has to provide credit support to attract investors, commonly through subordination (junior tranches absorb losses first) or overcollateralization (the collateral pool exceeds the securities issued against it).
B. Private-label issuers are not inherently lower risk than a government-backed pool; the opposite is true, which is exactly why non-agency deals need their own structural credit enhancement in place of a government guarantee.
C. Non-agency MBS by definition carry no government guarantee at all, explicit or implicit; that is what makes them 'non-agency' in the first place, and it is why they need structural credit enhancement instead.
An investor holds a CMO support tranche during a period when interest rates fall sharply, causing prepayments to run far above the PSA band the structure was designed around. Combining the role of a support tranche with the PAC tranche's protection band, this investor should most likely expect:
How sure are you?
Correct: B. The support tranche exists specifically to absorb the prepayment variability that keeps the PAC tranche's cash flows stable within its designed band. When prepayments run far above that band (as they would with a sharp rate decline), the support tranche absorbs the EXCESS principal being returned, shortening its own average life dramatically (contraction risk) precisely so the PAC tranche can continue to receive its planned, stable schedule. This is the defining trade-off of the PAC/support structure.
A. CMO tranching exists precisely because different tranches do NOT receive identical protection; the PAC tranche is structured for stability specifically because the support tranche absorbs the variability instead, which is the whole mechanism this question tests.
C. A support tranche's entire purpose is to be LESS stable than the PAC tranche, not functionally identical to it; describing them as equivalent misses the core PAC/support risk-redistribution relationship.
An investor is choosing between a GNMA-guaranteed pass-through and a similar-coupon, similar-maturity non-agency (private-label) pass-through with no credit enhancement beyond the quality of the underlying loans. Combining the guarantee structure of each with the concept of required yield compensation for risk, the non-agency security should most likely:
How sure are you?
Correct: B. GNMA carries the explicit, full-faith-and-credit guarantee of the US government, eliminating credit risk for the investor. A non-agency security with no credit enhancement bears the underlying loans' actual credit risk directly; investors require additional yield compensation for taking on that risk that the GNMA security's investors do not bear at all. All else equal (coupon, maturity), the non-agency security must offer a HIGHER yield to attract investors given its greater credit risk.
A. Competitive pricing among issuers does not eliminate the fundamental credit risk difference between a government-guaranteed security and one with no credit enhancement; investors still require compensation for that real, un-guaranteed credit risk regardless of how issuers price against each other.
C. Being 'mortgage-backed' in both cases does not make the two securities equivalent in risk; the guarantee structure (explicit government guarantee vs. none at all) is exactly the differentiating factor that drives a required yield difference between them.