Final answer:
The diagnosis for the described symptoms, including tenderness and swelling below the medial canthal tendon, is likely dacryocystitis, an inflammation of the lacrimal sac.
Step-by-step explanation:
The diagnosis for a 36-year-old female presenting with a left eye watering, fever, redness, pain in the left medial canthal region, and a tender swelling just below the medial canthal tendon could be dacryocystitis. This condition is an inflammation of the lacrimal sac, often occurring when a nasolacrimal duct is blocked. Such infections may present with purulent discharge, pain, swelling, and redness, as seen in acute bacterial conjunctivitis; however, the specific location of the tenderness and swelling near the medial canthal tendon is more indicative of dacryocystitis rather than conjunctivitis.