A 69 year old male presents with an episode of slurring of speech which lasted for 12 hours and then resolved.He is heavy smoker having smoked 60 cigarettes per day for 40 years. He had a single episode of haemoptysis 4 weeks previously and has underlying lung cancer with brain metastases and is referred for an urgent CT chest. An ECG was performed which showed new AF. All blood tests including cardiac biomarkers were normal. Chest CT is shown. Which of the following is most probable diagnosis?

**Core Concept**
The patient's new onset atrial fibrillation (AF) and resolved episode of slurring of speech suggest an embolic event, which is a common complication of AF. The presence of underlying lung cancer with brain metastases and a history of haemoptysis may indicate hypercoagulability, increasing the risk of thrombus formation.

**Why the Correct Answer is Right**
The chest CT is likely to show a left atrial thrombus, which is a common cause of embolic events in patients with AF. The thrombus may be visualized as a filling defect within the left atrium, which is a critical finding that requires prompt recognition. The patient's history of lung cancer with brain metastases and hypercoagulability may contribute to the development of a left atrial thrombus.

**Why Each Wrong Option is Incorrect**
**Option A:** Left atrial myxoma is a rare, benign tumor of the left atrium that may present with embolic events, but it is less likely than a left atrial thrombus given the patient's history of AF and hypercoagulability.
**Option C:** Infective endocarditis is a potential cause of embolic events, but it typically presents with fever, new heart murmur, or positive blood cultures, which are not mentioned in the patient's history.
**Option D:** Rhabdomyoma is a benign tumor of the heart, but it is more commonly associated with tuberous sclerosis complex and is not a likely diagnosis in this patient.

**Clinical Pearl / High-Yield Fact**
A left atrial thrombus is a common cause of embolic events in patients with AF, particularly those with underlying hypercoagulability or malignancy. Prompt recognition of a left atrial thrombus on CT is critical to prevent further embolic events.

**✓ Correct Answer: B. Left atrial thrombus**

✓ Correct Answer: B. Left atrial thrombus