A Time Bomb in the Arm: Rare Delayed Presentation of a Post-traumatic True Brachial Artery Aneurysm

True aneurysms of the upper extremity arteries are rare, particularly in the brachial and ulnar arteries. We present the case of a 52-year-old male with a painful, progressively enlarging swelling over the left antecubital fossa, ultimately diagnosed as a true brachial artery aneurysm. Surgical excision and end-to-end arterial anastomosis were performed successfully. Histopathology confirmed the diagnosis of a true aneurysm. Incidentally, imaging also revealed an asymptomatic true aneurysm of the contralateral ulnar artery, for which conservative surveillance was advised. This case highlights the importance of considering vascular etiologies in upper limb swellings and underscores the potential for bilateral or multiple aneurysms in atypical locations. It also emphasizes the need for thorough systemic evaluation and long-term follow-up in such patients.