“My knee blew up out of nowhere — I haven’t even injured it”

A hot, swollen, acutely painful single joint with no real injury must ALWAYS have SEPTIC ARTHRITIS excluded first — a joint-destroying emergency — so urgent assessment ± joint aspiration is the safe pathway for a genuinely hot/septic-looking joint, not NSAIDs/drainage for a "twisted knee". The likely diagnosis here is REACTIVE ARTHRITIS: an acute (oligo)arthritis (often a knee) days–weeks after a GI (dysentery) or STI trigger, ± enthesitis (Achilles/heel), conjunctivitis and urethritis ("can’t see, can’t pee, can’t climb a tree"), HLA-B27-associated and usually self-limiting. The examinable hidden agenda is the unvoiced sexual trigger — unprotected sex with a new partner the patient won’t disclose — which is directly relevant (chlamydia) and needs a non-judgemental sexual history, an STI screen and partner notification. Take a thorough history eliciting BOTH the GI infection and the sexual history (and the eye/urinary symptoms), exclude sepsis, screen/treat the STI and notify partners, manage the arthritis (NSAIDs ± intra-articular steroid), and safety-net septic features.