Apple is set to report a stunning 50% jump in quarterly sales.

Stock index futures pointed to a mixed open on Wall Street on Tuesday.

However, the company may face more pressing questions about the health of chief executive Steve Jobs after the company said its Chief Executive was taking medical leave for the third time since 2004, reviving fears over the long-term future of the iPhone and iPad maker and sending the company's stock traded in Frankfurt tumbling more than 8%.

Apple said on Monday that Cook, the chief operating officer, will take charge of the iPhone and iPad maker as Jobs focuses on his health. Unlike Jobs' half year medical leave in 2009, during which he specified he'd return to work at the end of June and stuck to it, Apple did not say when, if ever, Jobs would return as CEO.

That means Cook 50, considered a logical eventual successor to Jobs 55, could be in charge for a long time, perhaps permanently. Jobs has been Apple's public face and pitchman, making gadgets such as the iPod, the iPhone and the iPad household names. Jobs pulled Apple back from the brink of financial disaster after returning to the company in 1997 following a 12-year hiatus. He has become the heart of Apple, such that investors send shares tumbling on every bit of news or rumor of his ailing health.

Cook has run Apple twice before: first for two months in 2004 while Jobs battled pancreatic cancer, and once again for five-and-a-half months in 2009 while Jobs received a liver transplant.

The first time Cook was in charge, things went so well that Apple promoted him from executive vice president to chief operating officer in 2005. Cook has since become Apple's top-paid executive, with a salary of $900,000 for the current fiscal year.

During Cook's most recent tenure, which lasted from mid-January to the end of June 2009, Apple released a new version of the iPhone and updated laptop computers on schedule.

The company also announced that its iTunes app store hit a major milestone: More than one billion apps were downloaded within the first nine months of its existence.

According to StarMine's SmartEstimate, which places more weight on recent forecasts by top-rated analysts, Apple should post EPS of $5.47 on revenue of $24.5 billion.

IT research firm Gartner, Inc. said last week that according to its preliminary results, worldwide PC shipments increased 3.1% year-over-year in the fourth quarter of 2010, although below its outlook for a 4.8% growth, reflecting steady growth in the professional market driven by replacement purchases.

Research Company Ovum forecasts Apple to lose its market dominance over the next four years.

In the year-ago quarter, the company reported net income of $3.38 billion or $3.67 per share on revenues of $15.68 billion. In the preceding quarter, Apple's profit jumped 70%, driven by strong sales of Macintosh computers, iPhones and iPad multimedia devices. Fourth-quarter net income was $4.31 billion or $4.64 per share on net sales that surged 67% to a record $20.34 billion.

Deutsche Bank said it expects expanded iPad distribution, and additional international penetration to drive strong demand into fourth quarter and beyond.

An out-sized surprise in Apple results has become an article of faith among investors. The company has beaten Wall Street's estimate by an average of 29% over the past two years, and bested on revenue by 9% on average.

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