CAH Basics for MRCP Part 1

TL;DR

Congenital Adrenal Hyperplasia (CAH) Basics are a high-yield, predictable topic in MRCP Part 1, centred on enzyme deficiencies affecting cortisol synthesis. Focus on 21-hydroxylase deficiency, electrolyte patterns, and raised 17-hydroxyprogesterone. Most exam questions rely on recognising clinical and biochemical patterns rather than detailed memorisation. Mastering a few core pathways can secure easy marks.

Why this matters

CAH is important because:

• It links adrenal physiology with clinical medicine

• Questions are predictable and recurring

• It tests pattern recognition rather than rote learning

A strong grasp of CAH can help answer multiple endocrine MCQs correctly.

Core sections

1. What is CAH?

CAH refers to a group of autosomal recessive disorders caused by defects in enzymes required for cortisol synthesis in the adrenal cortex.

Key mechanism:

• ↓ Cortisol → ↑ ACTH → adrenal hyperplasia

• Accumulated precursors are diverted into androgen pathways

2. The Most Important Type: 21-Hydroxylase Deficiency

This accounts for over 90% of CAH cases and is the most tested variant.

Feature	Finding
Cortisol	↓
Aldosterone	↓ (in severe forms)
Androgens	↑
17-Hydroxyprogesterone	↑↑

Clinical features:

• Neonates: salt-wasting crisis (shock, dehydration)

• Females: ambiguous genitalia due to virilisation

• Males: normal genitalia at birth, later precocious puberty

3. Other Enzyme Deficiencies (Exam-Relevant)

11β-Hydroxylase Deficiency

• ↓ Cortisol

• ↑ Androgens

• ↑ 11-deoxycorticosterone → hypertension

17α-Hydroxylase Deficiency

• ↓ Cortisol

• ↓ Androgens

• ↑ Mineralocorticoids → hypertension + hypokalaemia

4. The Key Concept: “Block → Buildup → Diversion”

This is the single most important exam principle.

• Enzyme block → precursor accumulation

• Pathway diversion → excess androgen or mineralocorticoid production

👉 Nearly all CAH MCQs can be solved using this concept.

5. Clinical Presentations You Must Recognise

• Salt-wasting crisis: vomiting, dehydration, hypotension, hyponatraemia, hyperkalaemia

• Virilisation: ambiguous genitalia in XX infants

• Precocious puberty: early androgen effects

• Hypertension: suggests 11β or 17α deficiency

6. Diagnosis

• Raised 17-hydroxyprogesterone → hallmark of 21-hydroxylase deficiency

• Electrolyte abnormalities:

  • Hyponatraemia + hyperkalaemia → salt-wasting CAH

• ACTH stimulation test confirms diagnosis

7. Management Principles

• Glucocorticoids → suppress ACTH and reduce androgen excess

• Mineralocorticoids → replace aldosterone if deficient

• Salt supplementation in neonates

8. High-Yield Exam Clues

1. Ambiguous genitalia + electrolyte imbalance → CAH

2. Hypotension + hyperkalaemia → 21-hydroxylase deficiency

3. Hypertension + virilisation → 11β deficiency

4. Hypertension + sexual infantilism → 17α deficiency

5. Raised 17-OHP → diagnostic marker

Practical examples / mini-cases

MCQ Example:

A newborn female presents with vomiting, dehydration, and ambiguous genitalia. Blood tests show hyponatraemia and hyperkalaemia.

What is the most likely diagnosis?

A. 11β-hydroxylase deficiencyB. 17α-hydroxylase deficiencyC. 21-hydroxylase deficiencyD. Addison’s disease

Answer: C. 21-hydroxylase deficiency

Explanation: This is classic salt-wasting CAH:

• ↓ aldosterone → hyponatraemia, hyperkalaemia

• ↑ androgens → virilisation

• Most common cause: 21-hydroxylase deficiency

Practise more questions via Free MRCP MCQs or simulate exam conditions with a Start a mock test.

Common pitfalls (5 bullets)

• Confusing hypertension (11β, 17α) with hypotension (21)

• Forgetting aldosterone deficiency in 21-hydroxylase deficiency

• Mixing up enzyme pathways without understanding flow

• Missing 17-hydroxyprogesterone as the key diagnostic test

• Assuming all cases present in infancy (non-classic CAH presents later)

Study-tip checklist

• ✅ Memorise the three key enzyme deficiencies

• ✅ Focus on electrolyte patterns

• ✅ Learn the “block → buildup → diversion” concept

• ✅ Practise MCQs repeatedly

• ✅ Revise endocrine pathways visually

👉 Reinforce your learning through the MRCP Part 1 overview and deepen understanding via https://www.crackmedicine.co.uk/lectures/

FAQs

1. What is the most common cause of CAH?

21-hydroxylase deficiency accounts for over 90% of cases and is the most commonly tested form in MRCP Part 1.

2. Why does CAH cause virilisation?

Cortisol pathway blockage diverts precursors into androgen production, leading to excess androgen effects.

3. How is CAH diagnosed in exams?

Look for elevated 17-hydroxyprogesterone with clinical features such as electrolyte imbalance or virilisation.

4. Which types of CAH cause hypertension?

11β-hydroxylase and 17α-hydroxylase deficiencies cause hypertension due to excess mineralocorticoid activity.

5. Can CAH present later in life?

Yes, non-classic CAH may present in adolescence or adulthood with mild androgen excess symptoms.

Ready to start?

CAH is a predictable and high-yield topic in MRCP Part 1. Master the enzyme patterns and practise regularly to maximise your score.

👉 Start practising today with Free MRCP MCQs and assess your readiness using a Start a mock test.

Sources

• MRCP(UK) Examination Content: https://www.mrcpuk.org/mrcpuk-examinations

• NICE Clinical Knowledge Summaries (Adrenal disorders): https://cks.nice.org.uk/

• Kumar & Clark Clinical Medicine, 10th Edition

• Oxford Handbook of Endocrinology and Diabetes