This one may be cheating, since it’s three or four things in one.
First, there’s the rare Kyphoscoliosis-type EDS (Also known as Type 6, where Classical EDS is Type 1 or 2 depending on degree of skin involvement, Hypermobility type is Type 3, Vascular is Type 4 and there is no Type 5. Dermatospraxis and Arthrochalasia types are 7c and 7a/7b respectively, and even rarer) – Less than 60 reported cases worldwide, and characterised by, surprisingly, kyphosis and scoliosis which get progressively more pronounced over time, blue scleras, and extreme muscle weakness.
Then there’s kyphosis – Also known as having a hunched back, or a dowger’s hump, or even just a slouch. People with it will get either told off for not standing up straight (as teenagers) or will be correctly identified as having an unusual outward-curvature of the spine, usually at the top, which makes the shoulders seem hunched and the head seem to be pushed forward. It can cause pain in the shoulders and, sometimes, lung problems.
Next is scoliosis, which is where the spine is unusually kinked sideways, making it asymmetrical or s-shaped. Amazingly, this is sometimes only discovered upon x-ray, even though it causes uneven muscle development up and down the spine and across the shoulders and hips, often resulting in pain or compression of the ribs or lungs. Much like kyphosis, it can be compensated for slightly by deliberate strengthening of certain muscle groups, and there are surgical solutions available, with varying degrees of success, but largely speaking it’s just something that “is” rather than “is a problem in its own right”.
Then there’s lordosis, the third of the trip of spine mis-shapes; Where the lower spine is kinked in towards the belly, also known as “swayback” or “ewe back”. It looks inoffensive, in fact, it mimics the lordosis posture which is a pretty common “come hither” in… basically all mammals – But it puts horrendous strain on the lower back, thighs, and pelvis. And, well, hurts.
All, effectively, caused by the collagen in the connective tissue in the spine being too spongy and stretchy, and thus the bones forming slightly “squished” in the first place, and then the whole spine being supported basically by a combination of muscular tension and gravity. This is pretty much also why so many of us have sciatic nerve impingements, herniated or thinned discs, or just generic “back trouble” that follows us around like, well, a backbone.
Broadly speaking, this is why a lot of us don’t do very well in ordinary chairs – even allegedly “ergonomic” ones. No matter how tall or short we are, the lumbar support will be in the wrong place, the headrest will be at the wrong angle, and bits of either vertebrae or pelvis or scapula will dig into the upright of the chair. You can pretty much spot a zebra household by the amount of cushions, well-used chaise longues, unused dining chairs and foot-marks on settees. Most of us either squat, kneel up, sit asymmetrically, or sit with our feet up, if we’re forced to sit upright at all – Otherwise, we recline, lie flat, or (if we’ve got the leg strength) just stand up and pace around.
It’s bad enough when the limbs aren’t cooperating, but when even the torso is out to get you, it starts to get harder and harder to make the body do anything constructive.
(Notable runners-up to be “K is for…”; Kyphosis, keratin, ketamine)