All is not what is seems in U.S. markets this spring. Thursday's tiny close higher is a perfect example; while the S&P 500 managed to end the session up a point and change, most of the stocks in the big index actually closed lower on the day.

Zooming out to the full year, there's a similar disconnect between what's going on in the individual stocks and the big market averages. The good news is that a significant chunk of the market is actually working better than most investors realize right now. The corollary is that an equally big slice of the S&P looks downright toxic to your portfolio right now.

Year-to-date, almost a fifth of S&P components are down 8% or more. In some cases, they're down a lot more than that. These laggards have been a huge performance drag in 2016 for anyone that owns them. The challenge for investors now is that a different group of stocks are likely to turn toxic from here.

To find the stocks waving red flags right now, we're turning to the charts today for a technical look at five big stocks that could be toxic to own.

For the unfamiliar, technical analysis is a way for investors to quantify qualitative factors, such as investor psychology, based on a stock's price action and trends. Once the domain of cloistered trading teams on Wall Street, technicals can help top traders make consistently profitable trades and can aid fundamental investors in better entry and exit points.

Just so we're clear, the companies I'm talking about today are hardly junk. By that, I mean they're not next up in line at bankruptcy court -- and many of them have very strong businesses. But that's frankly irrelevant to what happens to their stocks; from a technical analysis standpoint, sellers are shoving around these toxic stocks right now. For that reason, fundamental investors need to decide how long they're willing to take the pain if they want to hold onto these firms in the weeks and months ahead. And for investors looking to buy one of these positions, it makes sense to wait for more favorable technical conditions (and a lower share price) before piling in.

So without further ado, let's take a look at five toxic stocks to sell.

Mednax

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The first stock we're taking a look at today is $6 billion health care services company Mednax(MD) - Get Report . Mednax has certainly been toxic for shareholders in recent months. Since the end of last summer, this stock has shed more than 20% of its market value, significantly underperforming the S&P's breakeven stance over that stretch. The bad news is that the price action indicates that Mednax could be in store for more downside from here.

Mednax is currently forming a descending triangle pattern, a bearish continuation pattern that's formed by horizontal support down below shares at $62, and downtrending resistance to the upside. Basically, as shares of Mednax bounce between those two technically significant price levels, they've been getting squeezed closer and closer to a breakdown through this stock's $62 price floor. If and when that happens, we've got a distinct sell signal in this stock.

Relative strength, which measures Mednax's price performance versus the broad market, is an extra red flag to watch here. Our relative strength line is still holding onto its downtrend from last fall, which tells us that Mednax is continuing to underperform the rest of the market even now. If $62 gets violated, this stock becomes a sell.

Waddell & Reed Financial

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We're seeing the exact same setup in shares of asset management firm Waddell & Reed Financial(WDR) - Get Report . Like Mednax, Waddell & Reed has been looking bearish for a while now; shares have been halved in value in the trailing 12 months. A textbook descending triangle setup is pointing to the risk of more downside ahead for this stock.

For Waddell & Reed, the sell signal comes on a violation of support down at $21.

Why all of the significance at that $21 level? It all comes down to buyers and sellers. Price patterns, such as this descending triangle in Waddell & Reed, are a good quick way to identify what's going on in the price action, but they're not the actual reason a stock is tradable. Instead, the "why" comes down to basic supply and demand for shares of the stock itself.

The $21 support level in Waddell & Reed is a place where there has previously been an excess of demand for shares; in other words, it's a spot where buyers have been more eager to step in and buy shares than sellers have been to take gains. That's what makes a breakdown below $21 so significant -- the move would mean that sellers are finally strong enough to absorb all of the excess demand at that price level.

The bottom line with WDR is that until this stock can start making some higher lows and highs, it makes sense to stay away from the long side.

First Solar

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Solar stocks have been staging somewhat of a rebound in 2016 -- and $6 billion solar system manufacturer First Solar(FSLR) - Get Report  has been leading the pack. Since last summer, First Solar's share price has been on fire, rallying some 43% at the same time the S&P 500 was barely above breakeven. But investors in First Solar might want to start thinking about taking some of those gains off the table at this point; the uptrend in this solar firm is beginning to roll over.

First Solar is currently forming a rounding top, a bearish reversal setup that looks just like it sounds. The rounding top in First Solar indicates a gradual shift in control of shares from buyers to sellers, and it triggers with a violation of the pattern's support level at $60. It's important to note that the well defined uptrend that's been propelling First Solar higher since October is already busted -- that uptrend got violated as soon as the calendar flipped to April.

It's important to be reactionary here. Technical analysis is a risk-management tool, not a crystal ball that predicts prices. So far, $60 appears to holding up in First Solar as buyers step in and pick up cheaper shares. That said, downside becomes the high-probability trade in this stock if $60 gets materially violated. Shares are testing that key breakdown level in today's session.

DexCom

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The price action in mid-cap medical device maker DexCom(DXCM) - Get Report  is under pressure this spring -- and it doesn't take an expert technical trader to figure out what's going on here. Instead, the price action on this chart is about as simple as it gets. Even though this stock has been rebounding for the last couple of months, the longer-term trajectory in DexCom is unmistakably down and to the right.

DexCom has been locked in a downtrend since last summer, making its way lower in a bearish price range formed by a pair of parallel trendlines that have corralled most of this stock's price action since last September. Every test of the top of the channel so far has provided shareholders with an optimal selling opportunity before the subsequent move lower. Shares are testing that trendline resistance level again this week. From here, it makes sense to sell the next move lower.

Waiting for this week's bounce lower before clicking "sell" is a critical part of risk management for two big reasons: It's the spot where prices are the highest within the channel, and alternatively it's the spot where you'll get the first indication that the downtrend is ending. Remember, all trend lines do eventually break, but by actually waiting for the bounce to happen first, you're confirming that sellers are still in control before you unload shares of DexCom.

Infosys

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Last up on our list of potentially toxic trades is India-based IT consulting giant Infosys(INFY) - Get Report . Infosys is having a great year so far in 2016, rallying more than 10% since the calendar flipped to January. But it looks like that upside move is finally starting to lose steam in April. Here's how to trade it.

Infosys is currently forming a head and shoulders top, a classic price pattern that signals exhaustion among buyers. The setup is formed by two swing highs that top out at approximately the same level (the shoulders), separated by a higher high (the head). The sell signal comes on a move through Infosys' neckline, currently at the $18 price level; if $18 gets violated, then Infosys becomes a sell.

Price momentum is an extra red flag to watch in Infosys right now. Our momentum gauge, 14-day RSI, has been making lower highs on each of the three peaks in the head and shoulders pattern. That's a bearish divergence from price that signals buyers have been quietly fading in this stock. $18 is the line in the sand that makes owning Infosys toxic for your portfolio.

Disclosure: This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.