Correct Answer: B. Latanoprost
Latanoprost is a prostaglandin F2α analogue that selectively increases uveoscleral (unconventional) aqueous humour outflow by remodelling extracellular matrix in the ciliary muscle bundles and widening intermuscular spaces. This mechanism is distinct from conventional trabecular outflow. In Indian glaucoma management, latanoprost is a first-line agent for open-angle glaucoma and ocular hypertension because it achieves 25–35% intraocular pressure (IOP) reduction with once-daily dosing (evening), excellent tolerability, and no systemic side effects. The uveoscleral route accounts for ~10% of aqueous drainage at baseline but becomes the dominant pathway when prostaglandin analogues are used. This makes latanoprost particularly effective in patients with compromised trabecular function (as in primary open-angle glaucoma). The drug's efficacy is mediated through FP-receptor activation on ciliary muscle cells, triggering remodelling that increases uveoscleral conductance without affecting aqueous production or conventional outflow significantly.
Why the other options are wrong
A. Pilocarpine — Pilocarpine is a muscarinic agonist that increases conventional (trabecular) outflow by ciliary muscle contraction, not uveoscleral flow. It causes miosis and accommodation, making it unsuitable for long-term glaucoma therapy in India due to poor compliance and side effects (brow ache, myopia). NBE traps students who confuse 'increased outflow' with 'uveoscleral outflow'—pilocarpine does the opposite. C. Carbonic anhydrase inhibitor — Carbonic anhydrase inhibitors (acetazolamide, dorzolamide) reduce aqueous humour production by inhibiting carbonic anhydrase in the ciliary body epithelium; they do not increase uveoscleral outflow. While effective for IOP reduction, they work via a fundamentally different mechanism and carry systemic side effects (metabolic acidosis, hypokalaemia) when used orally. This is a common distractor for students who remember 'glaucoma drugs' without mechanism specificity. D. Timolol — Timolol is a non-selective β-blocker that reduces aqueous production by decreasing ciliary body blood flow and secretion; it does not increase uveoscleral outflow. While a standard second-line agent in Indian glaucoma protocols, timolol's mechanism is production-focused, not outflow-focused. NBE pairs this with the question to test whether students conflate 'IOP reduction' with 'uveoscleral flow enhancement'.
High-Yield Facts
- Prostaglandin analogues (latanoprost, travoprost, bimatoprost) are first-line agents for glaucoma in India; they increase uveoscleral outflow by 50–100% via FP-receptor activation.
- Uveoscleral (unconventional) outflow accounts for ~10% of aqueous drainage at baseline but becomes dominant (>50%) when prostaglandin analogues are used.
- Evening dosing of latanoprost is optimal because IOP is highest in morning/early afternoon; once-daily administration improves compliance in Indian primary care settings.
- Conventional (trabecular) outflow is increased by pilocarpine and cholinergic agonists; carbonic anhydrase inhibitors and β-blockers reduce aqueous production, not outflow.
- Latanoprost side effects include conjunctival hyperaemia (30–50% of patients), iris darkening, and eyelash growth—all reversible and cosmetically manageable in Indian populations.
Mnemonics
PGA = Prostaglandin Analogues → Uveoscleral Prostaglandin Analogues → Uveoscleral outflow. Latanoprost, travoprost, bimatoprost all work via this pathway. Remember: PGA ≠ production reduction. COAT for Glaucoma Mechanisms Carbonic anhydrase inhibitors (production ↓), Outflow agonists (pilocarpine → trabecular), Adrenergic blockers (timolol → production ↓), Topical PGAs (latanoprost → uveoscleral ↑). Use to differentiate mechanism in MCQs.
NBE Trap
NBE conflates 'IOP reduction' with 'uveoscleral outflow enhancement'—all four drugs lower IOP, but only latanoprost specifically increases uveoscleral flow. Students who memorise "latanoprost works" without understanding how may confuse it with production-reducing agents.
Clinical Pearl
In Indian outpatient glaucoma clinics, latanoprost is the default first-line agent because it achieves target IOP reduction with once-daily evening dosing, requires no systemic monitoring (unlike acetazolamide), and has minimal drug interactions—critical in a polypharmacy-heavy population. A patient on latanoprost who develops conjunctival redness should be reassured it is reversible and cosmetic, not a reason to discontinue.
_Reference: Robbins & Cotran Pathologic Basis of Disease (Ch. 29, Glaucoma); Harrison's Principles of Internal Medicine (Ch. 413, Glaucoma); KD Tripathi Essentials of Medical Pharmacology (Ch. 16, Antiglaucoma agents)_