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What is What would be the most reliable test for an acutely injured knee of a 27-year-old athlete - and explain in detail?
The best test for an ACL injury is Lachman's test

"When the preoperative examination (no anaesthesia) was analysed for patients with isolated ACL injuries, the authors found that 98% of the Lachmann tests, 54% of anterior drawer tests, and 27% of the pivot shift tests were positive. When the examination under anaesthesia was analysed for these isolated injuries, authors found that 100% Lachmann tests, 81% of anterior drawer tests, and 100% of the pivot shift tests were positive". -- Evidence base sports medicine