Paraneoplastic Syndromes: Exam Classics (MRCP Part 1)

TL;DR;

Paraneoplastic syndromes are a high-yield, pattern-recognition topic in MRCP Part 1, frequently tested via endocrine, neurological, dermatological, and metabolic clues rather than overt cancer features. If you can rapidly link syndrome → mechanism → tumour, you can secure straightforward marks. This article summarises the most tested associations, common traps, and a focused study checklist.

Why paraneoplastic syndromes matter for MRCP Part 1

Paraneoplastic syndromes are remote effects of malignancy not caused by local invasion or metastasis. They are mediated by ectopic hormone production, immune cross-reactivity, or cytokine release. Examiners favour them because they reward clinical reasoning: the malignancy is often hidden, while the biochemical or neurological abnormality is obvious.

In MRCP Part 1, these questions often appear in endocrinology, neurology, dermatology, and haematology sections and are designed to be answered quickly if you recognise the pattern.

For a broader framework of exam topics and weightings, see the MRCP Part 1 overview:https://www.crackmedicine.com/mrcp-part-1/

The 10 most tested paraneoplastic syndromes (exam classics)

Syndrome	Key clinical / lab feature	Most associated malignancy
SIADH	Euvolaemic hyponatraemia, concentrated urine	Small-cell lung carcinoma
Ectopic ACTH	Cushingoid features, hypokalaemic alkalosis	Small-cell lung carcinoma
Lambert–Eaton myasthenic syndrome	Proximal weakness, improves with use	Small-cell lung carcinoma
Hypercalcaemia (PTHrP-mediated)	High Ca²⁺, low PTH	Squamous cell lung carcinoma
Polycythaemia	Raised haematocrit, ↑ EPO	Renal cell carcinoma
Trousseau syndrome	Migratory thrombophlebitis	Pancreatic adenocarcinoma
Dermatomyositis	Proximal myopathy + rash	Ovarian, lung, GI cancers
Acanthosis nigricans (malignant)	Sudden, extensive hyperpigmentation	Gastric carcinoma
Subacute cerebellar degeneration	Rapid-onset ataxia	Ovarian, breast, lung
Hypertrophic osteoarthropathy	Clubbing, periostitis	Lung carcinoma

Exam focus: Small-cell lung carcinoma is disproportionately represented. If the stem involves a smoker with unexplained sodium, potassium, or neuromuscular symptoms, start there.

Five subtopics examiners repeatedly test

1. Endocrine paraneoplastic syndromes

• SIADH: low serum osmolality, inappropriately high urine osmolality

• Ectopic ACTH: Cushing syndrome with normal pituitary imaging

• Key discriminator: hormone secretion is autonomous, not feedback-regulated

2. Neuromuscular syndromes

• Lambert–Eaton: improves with repeated muscle use

• Contrast with myasthenia gravis (fatigable weakness, thymoma association)

3. Metabolic abnormalities

• Hypercalcaemia with suppressed PTH suggests PTHrP-secreting tumours

• Often tested without radiological evidence of bone metastases

4. Dermatological clues

• Dermatomyositis in adults → malignancy screen is mandatory

• Malignant acanthosis nigricans is abrupt and extensive

5. Thrombotic phenomena

• Recurrent or migratory thrombosis = paraneoplastic hypercoagulability

• Pancreatic cancer is the classic association

Mini-case (single best answer)

Question A 62-year-old man with a 35-pack-year smoking history presents with difficulty climbing stairs. Examination shows proximal leg weakness and reduced reflexes, which improve after repeated testing. CT brain is normal. What is the most likely underlying diagnosis?

Answer: Small-cell lung carcinoma

Explanation This presentation is typical of Lambert–Eaton myasthenic syndrome, a presynaptic calcium-channel disorder. In MRCP Part 1, this neuromuscular pattern plus smoking history strongly points to small-cell lung cancer.

Practise similar mixed SBAs in the Crack Medicine Free MRCP MCQs:https://www.crackmedicine.com/qbank/

Five common exam traps

• Mistaking SIADH for diuretic-induced hyponatraemia

• Assuming all cancer-related hypercalcaemia is due to bone metastases

• Confusing Lambert–Eaton with myasthenia gravis

• Forgetting dermatomyositis is malignancy-associated in adults

• Over-investigating locally instead of recognising the remote effect

Practical study checklist

• Can I match syndrome → tumour in under 5 seconds?

• Do I know whether the mechanism is hormonal or immune-mediated?

• Can I interpret sodium and calcium patterns confidently?

• Can I spot paraneoplastic clues in neurology questions?

• Have I practised these under timed conditions?

Use these checkpoints before attempting a full mock exam:https://www.crackmedicine.com/mock-tests/

Related reading (sibling posts)

• Electrolyte disorders for MRCP Part 1:https://www.crackmedicine.com/blog/electrolyte-disorders-mrcp-part-1/

• Endocrine syndromes high-yield revision:https://www.crackmedicine.com/blog/endocrine-syndromes-mrcp-part-1/

FAQs

What are paraneoplastic syndromes in MRCP Part 1?

They are systemic effects of malignancy unrelated to tumour spread. Exams test recognition through labs, neurology, or skin findings.

Which cancer causes the most paraneoplastic syndromes?

Small-cell lung carcinoma, especially SIADH, ectopic ACTH, and Lambert–Eaton syndrome.

How are these questions usually framed?

Indirectly—via biochemical abnormalities or neuromuscular symptoms rather than naming the cancer.

Do I need antibody details for MRCP Part 1?

Only at a basic level. Pattern recognition and tumour association matter more.

Ready to start?

Consolidate this topic with mixed endocrine and neurology questions in our "MRCP Part 1 overview" hub at /mrcp-part-1/ and reinforce pattern recognition using the /qbank/.

Sources

• MRCP(UK) Examination information and blueprint: https://www.mrcpuk.org

• NICE Clinical Knowledge Summaries: https://cks.nice.org.uk

• Kumar & Clark’s Clinical Medicine, Elsevier

• Harrison’s Principles of Internal Medicine, McGraw-Hill