Q:

Is Fructose the #1 source of obesity in the United States today?

Not my belief but one I ran into over at health SE. Interested to see what data there is beyond my (and tyler's) findings. 
Argument for: https://health.stackexchange.com/a/7648/5266 -dead link
Argument against: http://www.nature.com/ijo/journal/v37/n6/full/ijo2012157a.html
Argument made for amount of calories being the only factor:

Independent of your diet’s macronutrient ratios, a negative energy
  balance (consuming fewer calories than your body needs) is responsible
  for weight loss. (Examine.com)

Argument made against amount of calories being the only factor:

"Not all calories are created equally. The body has to expend
  different amounts of energy to digest different foods. For example,
  just looking at sugars alone, it is easier for the body to utilize
  glucose than fructose. Some foods are completely indigestible even
  though they have "calories". Also, some fat usually passes through the
  digestive tract and is not even digested at all"

Does the evidence support a strong causal relationship between (specifically) fructose consumption and obesity?

A:

Question: "Does the evidence support a strong causal relationship between (specifically) fructose consumption and obesity?"
My answer: NO.
Consideration 1: This question has arisen because of "simultaneous" increase of use of high-fructose corn syrup (HFCS) and obesity in the United States.
Consideration 2: There has been some debate if fructose increases appetite. According to American Journal of Clinical Nutrition, 2008, it may not:

Insufficient scientific evidence currently exists to indicate that
  HFCS...increases short-term appetite and energy intake more than do other
  tested sweeteners.

Evidence:
1. Effect of fructose on body weight in controlled feeding trials: a systematic review and meta-analysis (PubMed, 2012)

Fructose does not seem to cause weight gain when it is substituted for
  other carbohydrates in diets providing similar calories. Free fructose
  at high doses that provided excess calories modestly increased body
  weight, an effect that may be due to the extra calories rather than
  the fructose.

2. Misconceptions about fructose-containing sugars and their role in the obesity epidemic: literature review (PubMed, 2014)

Current evidence on the metabolic effects of fructose, as consumed by
  the majority of populations, is insufficient to demonstrate such a
  role in metabolic diseases and the global obesity epidemic.

3. Fructose Metabolism and Relation to Atherosclerosis, Type 2 Diabetes, and Obesity (PubMed Central, a review article 2015)

Current literature does not indicate that a normal consumption of
  fructose (approximately 50–60 g/day) increases the risk of ...obesity
  more than consumption of other sugars.

4. Calories listed on the Nutrition Facts labels represent "physiologically available energy, which is the energy value remaining after digestive and urinary losses" (USDA.gov, p. 14). If you additionally eliminate the part of energy used for heat, you get "net metabolic energy (NME)," which, if in excess of body needs, results in weight gain. According to the table 3 from EJCN, 2007; fructose has one of the lowest net metabolizable energies (NME) among digestible carbohydrates:

Glucose (dextrose) monohydrate: 14.1 kJ or 3.4 Cal/g
Fructose: 15.2 kJ  or3.6 Cal/g
Glucose: 15.7 kJ or 3.7 Cal/g
Lactose: 16.3 kJ or 3.9 Cal/g
Sucrose: 16.3 kJ or 3.9 Cal/g
Starch: 17.5 kJ or 4.2 Cal/g

Additional info:

Foods high in fructose: apples, pears, mangoes and several other fruits, agave, honey; HFCS sweetened beverages, sucrose (NutrientsReview)

