This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nasolacrimal Disorders, Subject name: Ophthalmology
The most common causative organism of canaliculitis is 
Select and explain. 
A. HSV, B. Candida albicans, C. Actinomyces israelii, D. Nocardia asteroides
C: Actinomyces israelii
Answer: C: Actinomyces israelii 
Explanation: Chronic canaliculitis is an uncommon condition, frequently caused by Actinomyces israelii, anaerobic Gram-positive bacteria. Occasionally scarring and canalicular obstruction may result. Presentation is with unilateral epiphora associated with chronic mucopurulent conjunctivitis refractory to conventional treatment. There is pericanalicular redness and oedema, and mucopurulent discharge on pressure over the canaliculus. A 'pouting' punctum may be a diagnostic clue in mild cases. In contrast to dacryocystitis, there is no lacrimal sac involvement. Concretions (sulfur granules) are metabolic products of Actinomyces and other hydrogen sulfide-utilizing bacteria, and classically are expressed on canalicular compression or following canaliculotomy