Saccular aneurysm

A 40-year-old man with a history of autosomal dominant polycystic kidney disease presents to the emergency department for evaluation of a sudden onset severe headache. An hour ago, the patient woke up suddenly with an excruciating headache that reached maximum intensity within seconds. His additional medical history includes hypertension and a 20 pack year smoking history. Temperature is 37.0°C (98.6°F), pulse is 110/min, and blood pressure is 150/90 mmHg. On physical examination, nuchal rigidity is noted. A non-contrast CT of the head is obtained and is unremarkable. A lumbar puncture is performed. Which of the following is most likely to be found on further evaluation of this patient’s cerebrospinal fluid?