Congenital adrenal hyperplasia (CAH) questions love to bait you with blood pressure + electrolytes + androgen effects. If you can run a fast mental “flowchart” (what’s high? what’s low? what does that do?), you can usually nail the enzyme deficiency in under 30 seconds.
The 10-second steroid pathway map (what to memorize)
Think of the adrenal cortex as 3 zones making 3 products:
- Glomerulosa → Aldosterone (“Salt”)
- Fasciculata → Cortisol (“Sugar”)
- Reticularis → Androgens (“Sex”)
And 3 key enzymes for CAH:
- 21-hydroxylase: needed for aldosterone + cortisol
- 11β-hydroxylase: needed for final steps to cortisol (and for aldosterone pathway at the 11-step)
- 17α-hydroxylase: needed for cortisol + sex steroids
Visual mnemonic device: “21 → salt + sugar; 17 → sex; 11 → BP up via DOC”
Picture three “doors” on the steroid assembly line:
- Door 21 opens to Salt (aldo) and Sugar (cortisol)
- Door 17 opens to Sex (androgens)
- Door 11 is the “pressure door” because DOC gets stuck and acts like a mineralocorticoid → HTN
One-liner:
CAH enzyme deficiencies are pattern recognition: check BP + K⁺ + androgens + key intermediates (17-OHP or DOC).
Step-by-step diagnostic flowchart (USMLE-style)
Step 1: Start with blood pressure
- If BP is LOW (hypotension) → think 21-hydroxylase deficiency (salt-wasting)
- If BP is HIGH (hypertension) → think 11β-hydroxylase or 17α-hydroxylase
Step 2: If BP is LOW → 21-hydroxylase deficiency
Check:
- K⁺: tends to be HIGH (hyperkalemia)
- Renin: HIGH (low aldosterone → RAAS activation)
- Androgens: HIGH (shunting toward reticularis)
- Key lab: ↑ 17-hydroxyprogesterone
Clinical clue set
- Ambiguous genitalia in XX infant (virilization)
- Precocious puberty in XY
- Salt-wasting crisis: vomiting, dehydration, shock, hypoglycemia
Step 3: If BP is HIGH → decide between 11β vs 17α using androgens
A) Hypertension + HIGH androgens → 11β-hydroxylase deficiency
Why: cortisol is low → ACTH high → adrenal hyperplasia → androgens rise. Also DOC rises and acts like a mineralocorticoid.
Check:
- K⁺: tends to be LOW (hypokalemia)
- Renin: LOW (volume expansion)
- Aldosterone: LOW (suppressed by HTN/low renin)
- Key lab: ↑ 11-deoxycortisol and ↑ DOC
Clinical clue set
- HTN (key differentiator from 21)
- Virilization/precocious puberty (like 21, but BP flips)
- Low cortisol → possible hypoglycemia, fatigue (variable)
B) Hypertension + LOW androgens → 17α-hydroxylase deficiency
Why: can’t make cortisol or sex steroids → ACTH rises → steroid precursors shunt into DOC/corticosterone → mineralocorticoid effect → HTN.
Check:
- K⁺: LOW (hypokalemia)
- Renin: LOW
- Androgens: LOW
- Aldosterone: often LOW (suppressed; DOC is driving mineralocorticoid activity)
- Sex hormones: low DHEA/testosterone/estradiol
Clinical clue set
- XY (46,XY): undervirilized (ambiguous/female external genitalia, undescended testes)
- XX (46,XX): delayed puberty, primary amenorrhea, lack of secondary sex characteristics
- Low cortisol with high ACTH (hyperplasia) but no androgen excess
Mini-table: fastest way to differentiate (memorize this)
| Enzyme deficiency | Cortisol | Aldosterone effect | BP | K⁺ | Androgens | Key buildup |
|---|---|---|---|---|---|---|
| 21-hydroxylase | ↓ | ↓ (true low aldo) | ↓ | ↑ | ↑ | ↑ 17-hydroxyprogesterone |
| 11β-hydroxylase | ↓ | DOC acts like aldo | ↑ | ↓ | ↑ | ↑ DOC, ↑ 11-deoxycortisol |
| 17α-hydroxylase | ↓ | DOC/corticosterone ↑ | ↑ | ↓ | ↓ | ↑ DOC, ↑ corticosterone |
The “single question stem” pattern matches
21-hydroxylase (most common)
- Newborn with vomiting + dehydration + hyperkalemia + hypotension
- XX infant with ambiguous genitalia
- Lab buzzword: ↑ 17-hydroxyprogesterone
11β-hydroxylase
- Child with early puberty/virilization + hypertension
- Lab buzzword: ↑ DOC (mineralocorticoid effect) and ↑ 11-deoxycortisol
17α-hydroxylase
- Teen with hypertension + hypokalemia + delayed puberty
- XY with female/ambiguous external genitalia
- Buzzword: “no sex steroids” + mineralocorticoid excess (via DOC)
High-yield USMLE pearls (the traps and tie-breakers)
- All CAH = ↑ ACTH → adrenal hyperplasia (because cortisol is low).
- Aldosterone may be low in HTN CAH (11β, 17α) because DOC suppresses renin/aldo—don’t let “mineralocorticoid excess” trick you into thinking aldosterone must be high.
- 21-hydroxylase screening: newborn screen classically targets 17-hydroxyprogesterone.
- Electrolytes are your shortcut:
- HyperK + hypotension = 21
- HypoK + hypertension = 11β or 17α → then check androgens (virilized vs undervirilized)
- Sex development clue:
- Virilization (androgens high) → 21 or 11β
- Undervirilization / delayed puberty (androgens low) → 17α
Quick “flowchart mantra” to say in your head
BP low? 21. BP high? DOC problem: 11 or 17. Virilized? 11. Not virilized? 17.