**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**