Metab: Porphyrias: Acute Intermittent vs Cutanea Tarda for MRCP Part 1

TL;DR

Metab: Porphyrias: Acute Intermittent vs Cutanea Tarda is a classic MRCP Part 1 topic that tests haem biosynthesis, metabolic disease, neurology, dermatology, and hepatology. Acute Intermittent Porphyria (AIP) causes acute neurovisceral attacks without photosensitivity, whereas Porphyria Cutanea Tarda (PCT) presents with blistering photosensitivity and skin fragility. For examination purposes, focus on enzyme defects, characteristic clinical presentations, diagnostic metabolites, and treatment principles.

Why This Matters

Porphyrias appear regularly in MRCP-style questions because they test:

• Metabolic pathways

• Genetics

• Hepatology

• Neurology

• Dermatology

• Clinical reasoning

Candidates are often expected to identify a porphyria from a few characteristic clues, such as severe abdominal pain with normal abdominal findings or blistering photosensitivity affecting the hands.

A solid understanding of AIP and PCT can secure straightforward examination marks.

Haem Synthesis: The Essential Background

Haem synthesis occurs primarily in the liver and bone marrow through a series of enzymatic steps.

Defects in individual enzymes result in accumulation of specific intermediates. These intermediates are responsible for the characteristic symptoms of each porphyria.

For MRCP Part 1, candidates should focus on:

1. The defective enzyme

2. The accumulated metabolite

3. Clinical presentation

4. Diagnostic findings

5. Management

High-Yield Comparison Table

Feature	Acute Intermittent Porphyria (AIP)	Porphyria Cutanea Tarda (PCT)
Enzyme deficiency	Porphobilinogen deaminase (HMBS)	Uroporphyrinogen decarboxylase
Inheritance	Autosomal dominant	Usually sporadic, sometimes familial
Main system affected	Nervous system	Skin
Photosensitivity	No	Yes
Abdominal pain	Common	Rare
Neurological symptoms	Common	Absent
Psychiatric symptoms	Common	Absent
Skin blistering	Absent	Characteristic
Urinary findings	Increased ALA and PBG	Increased uroporphyrins
Common triggers	Drugs, alcohol, fasting	Alcohol, hepatitis C, iron overload
Treatment	Haemin and glucose	Venesection and low-dose hydroxychloroquine

The Five Most Tested Subtopics

1. Acute Intermittent Porphyria: Clinical Features

AIP typically presents with acute neurovisceral symptoms.

Key features include:

• Severe abdominal pain

• Nausea and vomiting

• Constipation

• Peripheral neuropathy

• Psychiatric disturbances

• Tachycardia

• Hypertension

A common examination clue is severe pain despite a relatively benign abdominal examination.

Examination Pearl

AIP causes neurological and abdominal symptoms but does not cause photosensitivity.

2. Triggers of Acute Intermittent Porphyria

Acute attacks are often precipitated by factors that increase hepatic haem synthesis.

Important triggers include:

• Barbiturates

• Antiepileptic drugs

• Rifampicin

• Alcohol

• Smoking

• Fasting

• Infection

• Menstruation

A favourite MRCP question describes a young woman developing severe abdominal pain shortly after starting a new medication.

3. Porphyria Cutanea Tarda: Clinical Features

PCT is the most common porphyria worldwide.

Typical manifestations include:

• Photosensitive blistering

• Fragile skin

• Hyperpigmentation

• Hypertrichosis

• Scarring of sun-exposed skin

The dorsal surfaces of the hands are classically affected.

Unlike AIP, neurological symptoms are absent.

4. Important Associations of Porphyria Cutanea Tarda

MRCP questions frequently test underlying risk factors.

Common associations include:

• Alcohol excess

• Chronic hepatitis C infection

• HIV infection

• Iron overload

• Haemochromatosis mutations

• Oestrogen therapy

Whenever PCT is suspected, think about liver disease and abnormal iron metabolism.

5. Diagnosis and Laboratory Findings

Acute Intermittent Porphyria

During an acute attack:

• Urinary porphobilinogen (PBG) is elevated

• Aminolaevulinic acid (ALA) is elevated

Urine may darken after standing because of oxidation of accumulated metabolites.

Porphyria Cutanea Tarda

Characteristic findings include:

• Elevated urinary uroporphyrins

• Increased plasma porphyrins

• Evidence of iron overload in some patients

Understanding which metabolite accumulates is a common examination theme.

Ten High-Yield MRCP Facts

1. AIP is caused by porphobilinogen deaminase deficiency.

2. PCT is caused by uroporphyrinogen decarboxylase deficiency.

3. AIP causes neurovisceral symptoms.

4. Photosensitivity is absent in AIP.

5. PCT is the most common porphyria.

6. Blistering photosensitivity strongly suggests PCT.

7. Alcohol can exacerbate both disorders.

8. Urinary PBG is elevated during acute AIP attacks.

9. Venesection is a cornerstone treatment for PCT.

10. Intravenous haemin is used for severe AIP attacks.

Practical Example: MRCP-Style Single Best Answer

A 30-year-old woman presents with severe abdominal pain, constipation, and anxiety. She recently started an anticonvulsant medication. Examination reveals no significant abdominal findings. Urinary porphobilinogen is markedly elevated.

What is the most likely diagnosis?

A. Porphyria cutanea tarda

B. Acute intermittent porphyria

C. Wilson disease

D. Haemochromatosis

E. Variegate porphyria

Answer

B. Acute intermittent porphyria

Explanation

The combination of:

• Severe abdominal pain

• Neuropsychiatric symptoms

• Drug trigger

• Elevated urinary porphobilinogen

is highly characteristic of AIP.

The absence of photosensitivity further supports the diagnosis.

Practical Study-Tip Checklist

Before sitting MRCP Part 1, ensure you can confidently answer the following:

☑ State the enzyme defect in AIP.

☑ State the enzyme defect in PCT.

☑ Recall whether photosensitivity occurs in AIP.

☑ List at least three triggers of acute attacks.

☑ Identify the major diagnostic urinary marker for AIP.

☑ Recall common associations of PCT.

☑ Know the role of haemin in acute attacks.

☑ Understand why venesection is effective in PCT.

☑ Distinguish neurovisceral from cutaneous manifestations.

☑ Recognise the most common porphyria worldwide.

Five Common Examination Traps

1. Assuming All Porphyrias Cause Photosensitivity

Many candidates incorrectly believe all porphyrias produce skin manifestations. AIP does not.

2. Missing Drug Triggers

Questions frequently include anticonvulsants, barbiturates, or rifampicin as clues.

3. Confusing PBG with Uroporphyrin

Remember:

• AIP → elevated PBG

• PCT → elevated uroporphyrins

4. Forgetting Iron Overload Associations

PCT is strongly associated with haemochromatosis and excess iron stores.

5. Overlooking Psychiatric Symptoms

AIP may present with anxiety, depression, behavioural changes, or psychosis.

Exam-Focused Summary

When comparing AIP and PCT, remember the simple distinction:

Acute Intermittent Porphyria

• Neurovisceral disease

• Abdominal pain

• Neuropathy

• Psychiatric symptoms

• Elevated urinary PBG

• No photosensitivity

Porphyria Cutanea Tarda

• Cutaneous disease

• Photosensitive blistering

• Skin fragility

• Hypertrichosis

• Elevated uroporphyrins

• Associated with iron overload

This contrast alone answers the majority of MRCP Part 1 questions on porphyrias.

FAQs

Which porphyria is most commonly tested in MRCP Part 1?

Acute Intermittent Porphyria and Porphyria Cutanea Tarda are the most frequently examined because they represent the classic neurovisceral and cutaneous porphyria syndromes.

Does Acute Intermittent Porphyria cause photosensitivity?

No. AIP characteristically causes abdominal, neurological, and psychiatric symptoms without photosensitivity.

Why is venesection used in Porphyria Cutanea Tarda?

Venesection reduces excess iron stores, which contribute to disease activity and porphyrin accumulation.

What laboratory test helps diagnose Acute Intermittent Porphyria?

A markedly elevated urinary porphobilinogen level during an acute attack is a key diagnostic finding.

Which conditions are associated with Porphyria Cutanea Tarda?

Important associations include alcohol excess, chronic hepatitis C infection, HIV infection, and iron overload states such as hereditary haemochromatosis.

Ready to start

Strengthen your metabolic medicine preparation with:

• MRCP Part 1 Overview:https://www.crackmedicine.com/mrcp-part-1

• MRCP Question Bank: https://www.crackmedicine.com/qbank

• MRCP Lectures: https://www.crackmedicine.com/lectures

You may also find it useful to review related metabolic and hepatology topics, including haemochromatosis, Wilson disease, and inherited metabolic disorders.

Sources

1. MRCP(UK) Examination Information and Curriculum: https://www.mrcpuk.org/mrcpuk-examinations/part-1

2. NHS Overview of Porphyria: https://www.nhs.uk/conditions/porphyria/

3. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) – Porphyria: https://www.niddk.nih.gov/health-information/liver-disease/porphyria

4. British Porphyria Association: https://www.porphyria.org.uk/

5. European Porphyria Network: https://porphyria.eu/