Key TakeawaysShares in McDonald’s slumped 6% in extended trading on Tuesday after health authorities said they were investigating an E. coli outbreak possibly linked to the fast-food chain’s Quarter Pounder burgers.The projected news-driven drop may lead to a retest of a prior 16-month trading range before the stock potentially continues its longer-term uptrend.investors should watch important retracement levels on the McDonald's chart around $300, $280, and $260.The measuring principle, which calculates th ...