Which of the following is the most common cause of neonatal conjunctivitis in the first 24 hours of life?

The most common cause of neonatal conjunctivitis in the first 24 hours of life is chemical irritation. This typically occurs due to the use of silver nitrate or other antiseptic eye drops that may be administered prophylactically to prevent gonococcal infections. Chemical conjunctivitis usually presents within the first day after birth and is characterized by a mild, self-limiting inflammatory reaction that is triggered by these agents.

In contrast, gonococcal infection typically manifests later, usually presenting in the first week of life, and is associated with more severe symptoms, including purulent discharge. Chlamydial infection also occurs later, often presenting within the first 1 to 2 weeks after delivery, and is associated with conjunctival inflammation and discharge. Viral conjunctivitis tends to have an even later onset and is less common as an early cause of conjunctivitis in neonates.

Thus, the immediate inflammatory response due to chemical irritants makes this the most common cause of conjunctivitis within the first 24 hours after birth.