A pediatrics MCQ that pairs autosomal-recessive inheritance with prenatal dexamethasone is likely testing 21-hydroxylase-deficient congenital adrenal hyperplasia (CAH). Before choosing “25% recurrence risk,” ask: 25% of what, and under which parental-genotype assumption? It means a 25% chance of an affected child at each conception when both parents are carriers—not a promise that exactly one of four children will be affected.
Start with the parental-genotype assumption
The standard board calculation assumes each biological parent carries one pathogenic variant in CYP21A2, the gene responsible for 21-hydroxylase deficiency. Each parent passes one copy of the gene to the child. If both are carriers, the familiar outcomes are:
| Child’s inherited copies | Chance at each conception | Board interpretation |
|---|---|---|
| Two pathogenic variants | 25% | Affected with 21-hydroxylase-deficient CAH |
| One pathogenic variant | 50% | Carrier of a familial variant |
| Neither familial pathogenic variant | 25% | Not affected by this familial condition and not a carrier of these variants |
The calculation is often taught as a simple autosomal-recessive cross: one carrier parent contributes either a typical or a pathogenic copy, and so does the other. A child must inherit a pathogenic copy from both parents to have the recessive condition.
In a classic board stem, an affected child usually implies that each parent contributed a pathogenic variant. Real genetic counseling should confirm the family’s variants and parental results rather than treating that simplified pedigree as guaranteed: 21-hydroxylase testing can be technically complex, and uncommon genetic situations can change the interpretation.
Keep probability separate from family size
The 25% is a per-pregnancy probability, not a cumulative quota. If a couple has two unaffected children, the chance in the next pregnancy remains 25% under the same carrier-parent assumptions. Prior outcomes do not change which copy either parent can pass on in a new conception.
A common wrong answer converts the ratio into a prediction: “One child affected, two carriers, and one unaffected in every four.” That is the expected proportion over many conceptions, not a guaranteed sequence for one family. Another common error is choosing 50% as the risk of an affected child; 50% is the chance of inheriting one familial variant and being a carrier.
Separate inheritance from fetal phenotype
The 25% risk concerns inheriting the condition; it is not a 25% risk of a virilized 46,XX fetus. In classic 21-hydroxylase deficiency, prenatal androgen exposure can virilize the external genitalia of an affected 46,XX fetus. If an exam explicitly asks for the chance of an affected 46,XX fetus, that adds a fetal-sex condition: under the usual simplified assumption of a 50:50 chance of an XX or XY fetus, the probability is 1 in 8 per conception. Do not add that extra calculation when the question asks only for the recurrence risk of CAH.
Prenatal dexamethasone belongs to a separate part of the reasoning. Its proposed purpose is to reduce androgen-related virilization in an affected 46,XX fetus; it does not change the inheritance pattern or prevent the fetus from inheriting CAH. Specialty guidance continues to regard prenatal treatment as experimental, with consideration limited to approved protocols at centers able to collect outcomes. In a genetics question, dexamethasone is context—not a reason to change the Mendelian risk.
A focused revision exercise for this MCQ format
For a missed inheritance item, set a six-minute timer and close the explanation. On a blank page, write the parental genotypes as carrier × carrier, then draw the four equally likely combinations. Label each one affected, carrier, or inherits neither familial variant. Say the denominator out loud: each conception.
Now test whether you understand the rule rather than just recall the number. Rewrite the stem three ways:
- The couple has already had two unaffected children. State the risk for the next conception and explain why it has not changed.
- The question asks for the chance the next child is a carrier. Choose the correct outcome and explain why it is not the affected-child risk.
- The question asks specifically for an affected 46,XX fetus. Add the sex assumption before calculating; do not silently substitute that answer for the risk of CAH in any child.
Finish by correcting one distractor in a sentence: “Prenatal dexamethasone may affect virilization, but it does not alter the chance of inheriting the pathogenic variants.” This last step links the genetics to the clue in the stem without confusing a proposed fetal treatment with inheritance.
Practical takeaways
- For the standard 25% / 50% / 25% calculation, both parents are assumed to be heterozygous carriers.
- The affected-child risk is 25% per conception; it does not shrink after unaffected siblings.
- The 50% outcome describes a carrier child, not an affected child.
- Prenatal dexamethasone does not change the genetic recurrence risk and is not routine treatment.
